Sam's Laser FAQ, Copyright © 1994-2001, Samuel M. Goldwasser, All Rights Reserved.
I may be contacted via the Sci.Electronics.Repair FAQ Email Links Page.

  • Back to Sam's Laser FAQ Table of Contents.

    Amateur Laser Construction

    Sub-Table of Contents



  • Back to Sam's Laser FAQ Table of Contents.
  • Back to Amateur Laser Construction Sub-Table of Contents.

    Introduction to Amateur Laser Construction

    So You Think You Really Want to Build a Laser

    While most lasers are extremely high tech devices which require the engineering and manufacturing expertise of corporations, universities, or Government agencies to design and construct, there are a few types that can be built from scratch by the very determined amateur. I'm not talking about wiring a helium-neon laser tube to a power supply or a laser diode to a driver circuit. A truly home-built laser may start out as 4 foot lengths of various sizes of glass tubing, mirrors, special gases and chemicals, scrap metal and hardware; electronic components like transformers, rectifiers, capacitors, and resistors - and laser and high voltage warning signs! Converting this collection of materials into a working laser will require many many hours of effort as well as blood, sweat, and possibly tears. :)

    Considering the enormous gap between your likely capabilities and those of a large corporation, the final result probably won't match a commercial laser in either performance or appearance (though there are a few exceptions). However, that shouldn't be the point of the exercise. Rather, a laser (or multiple lasers after you are hooked) should be built for the education, challenge, and opportunity for experimentation afforded by dealing with all aspects of laser construction. And, the realization that the laser you built is a complex precision device you were able to bring to fruition successfully.

    To the amateur scientist, experimenter, hobbyist, or even weekend tinkerer, there is something about the idea of actually creating a working laser that makes its construction from the ground up a very attractive and rewarding project with "first light" - the instant that your home-built laser first emits a beam - approaching the excitement of a religious or sexual experience!

    Although the laser is a device or instrument based on fundamental quantum mechanics which is very simple in principle - an excited medium between mirrors - building one successfully may require mastering several disciplines not normally found in even the high tech home. These include: glass working, vacuum systems, gas handling, high voltage electronics, and precision mechanical fabrication. Dealing with these can in itself be an excellent educational experience. Access to a university or industrial lab will make things a lot easier but isn't essential - it is possible to build a laser without outside assistance. Academic studies in laser physics or related subjects are also not necessary unless you want to attempt to do serious research as all the lasers that can be reasonably constructed at home are based on well established principles where rules-of-thumb and simple calculations will suffice. However, the cost of such an undertaking can be significant - experience in scrounging is a definite asset! And, the construction of home-built lasers can be quite addictive and may impact other activities like social interaction, eating, sleeping, and the timely performance of other bodily functions. :)

    In this chapter and the one that follows, we provide basic information on the construction of various types of lasers from scratch including: home-built laser safety, setting up a home laser lab, sources of supplies and chemicals, vacuum systems, glass working, structural materials, power supplies, and more.

    Then, a variety of specific types of home-built lasers are described in more detail. Much of this material is derived from the Scientific American collection "Light and its Uses" [5] and from the email, Web sites, articles, and experiences of those who have been successful in building their own lasers from basic components and getting them to work (not taking the easy way out and using commercial tubes or laser diodes!) - or have given it their best shot trying!

    While this will not substitute the hands-on of actually having built one of these lasers or detailed construction plans, it may provide the spark to get you started!

    Reasons NOT TO Build a Laser from Scratch

    First, let us consider some ill-posed justifications for attempting to build a laser from (almost) raw materials: If these are your only reasons for wanting to do this, you will rapidly tire of the endeavor and the parts will end up in a box alongside that dusty old partially ground telescope mirror you also never completed. :-(

    If you want a working laser for a particular application, save your pennies and buy one. The cost of a used laser appropriate for what you have in mind may not be as terrible as you may think. The result of building a laser from scratch isn't likely to be something you can use reliably day in and day out without constant maintenance, repairs, and the occasional disaster. (The one exception to this might be the axial flow CO2 laser which if properly constructed, is less finicky than the other types discussed in the following chapters.) A system that starts life on and under a workbench will also probably never be packaged in a nice self-contained cabinet and may have to coexist with the home washer-drier, family car, or kitchen table. :) Anything home-built is also going to have many potentially serious hazards associated with it unless significant effort has been made to provide the necessary beam blocks, electrical and thermal protection devices, and safety interlocks.

    Reasons TO Build a Laser from Scratch

    However, there are many justifications for embarking on an adventure of this type:

    Experiences of People Who Have Built Lasers From Scratch

    Here are a few of the (mostly) success stories:

    (From: Mark Wilson (wilson_mark@htc.honeywell.com).)

    I was born in a very small town in Idaho. I was fascinated by physics and laser technology. When I saw the Scientific American article on building your own Helium-Neon (HeNe) laser, I decided that I wanted to build it. The Scientific American HeNe laser was extremely difficult to build and I could not have done it without a lot of help. I got Spectra-Physics to donate a set of laser mirrors to me, a glass shop in my home town to help me cut and grind the Brewster window angles on the tube. The tube was made from lead glass from a sign company, and I also used neon sign electrodes. The optical rail was a 3 foot long piece of 2"x6" extruded aluminum that I got from a glass company which used this material to make doors for commercial buildings.

    I followed the directions in the Scientific American article to the letter. I sealed the microscope slides to the glass tube using flexible colodian that I got from a pharmacy. I filled the tube at a sign company which had He, Ne, Ar and other gases on a glass manifold. I assembled the laser and made the power supply using a mercury rectifier tube and a neon sign transformer. I got the tube to lase for a brief time, but since it was not a hard-sealed tube it quickly died probably due to helium diffusion. The tube would light up but not lase for a while then that too stopped.

    I then made a dye laser, again from a Scientific American article. This was much easier, and the materials were much easier to obtain. I did not need any wavelength selective mirrors, vacuum system, or high voltage supplies. I ordered samples of a couple of laser dyes (Sodium Fluorescene, Rodium 6G (possible spelled wrong)), and mixed up dye samples which were flowed through a piece of quartz tubing. A flash lamp was located at one focus of an elliptical reflector and the dye tube was at the other focus. The reflector was a juice can that I polished up as per the article. The flash lamp power supply was very simple which put several hundred volts across a large capacitor, and then tripped the flash with a tickler coil. This laser was easy to build, and actually worked for quite a while, but I couldn't set anything on fire with it.

    I then built a flowing gas CO2 laser again using glass and equipment from a neon sign company. The Brewster windows were 2 near perfect salt crystals that I got for a salt company in near by Salt Lake City. I had to keep the windows inside plastic bags a moisture absorbing material when the laser was not in use to keep the windows clear. I made my mirrors from round glass blanks that I got from a local eye doctor. He ground me a set with a -1/8th diopter (-8 meter focal length). I then drilled a small hole in one of the glass blanks to allow the output beam to escape. I coated the mirrors with gold using a sputtering machine that I built, again from a Scientific American article. I assembled the tube and mirrors onto a extruded aluminum base, and then connected it to a vacuum pump. This pump was two Fridgidare compressors connected in series with each other and a cold trap. Later I replaced the pump with a 2 stage Cenco Hyvac pump that my friends at the sign company donated to me. I made a gas manifold including a vacuum gauge, to mix gases for the laser. I got a cylinder of CO2 (used in pop machines), and also a small cylinder of Helium, bubbled the gases through water to add water vapor and then flowed this mixture through the tube. I used a center tapped neon transformer and a set of solid state rectifiers to make a DC power supply to run the tube, and with an input of 150 watts of power, I calculated that I got about 5 watts of IR power at 10.6 um. I could burn holes in things so I was finally happy.

    (From: Steve1W1 (steve1w1@aol.com).)

    I built my first C02 in 8th grade for our school's science fair with plans from "Roy Davis Laboratories" (if anyone is old enough to remember those days). It took about a year's worth of work and scrounging, but it worked so such a project CAN be done.

    (From: Steve Roberts (osteven@akrobiz.com).)

    I tried to build several lasers when I was a kid, dye worked, nitrogen worked, argon failed. But, I found making Brewster angle windows nearly impossible. The guy at the local refurb shop told me I went about it wrong, all I really needed was a belt sander and some abrasive, the brass bladed sawing technique in "Light and its Uses" doesn't work. I have also repumped a commercial laser tube or two with mediocre results.

    Flavio's Comments on Amateur Laser Construction

    (From: Flavio Spedalieri (flavios@ihug.com.au).)

    The reason for building a laser from scratch, is to learn how lasers work through physical hands on construction. Also, one major feature that you have with home-built lasers, is the unlimited freedom of experimentation, you can control many variables like; power (voltage and currents), gas, optics, materials etc, which otherwise is very difficult or impossible with commercial lasers.

    Another rule that I have set when it comes to building lasers: Build the lasers that are more expensive, exotic, and least obtainable like:

    In the list above, you may notice that I have NOT included typical lasers like Helium Neon, and Argon Lasers, the reason being that HeNe lasers are now too cheap to even consider building, and are all the too common. I have not limited myself to not experimenting at all; I do have a HeNe laser tube that I, one day would like to cut the vacuum nipple off, and connect the tube to a vacuum system, and back-fill with HeNe, but the chance of getting it to lase is quit small, due to the fact of the need of very critical gas mixture and purity, and a very good vacuum system.

    CW argon ion lasers, on the other hand, require huge amounts of current, and the necessary materials to build the tube are not easy to work, toxic, or both (e.g., beryllium oxide, tungsten). Also, glass work is a major component with the need of correctly angled Brewster windows. Today, like the HeNe lasers, argon lasers can be obtained with ease, and relatively cheaply. Argons, also require very good vacuum systems as well as very pure and critical gas pressures.

    So, as a summary, and as a rule of thumb please take the following in mind:

    1. If you are building a laser for experimentation, learning/educational purposes, then all the better, as your learning curve will be very steep.

    2. If you are intending to build a laser purely for a 'workhorse' application like wood or metal cutting, especially if for a commercial venture, it might be more economical and much easer to purchase a second-hand laser system from the surplus market.

    3. DO NOT expect great powerful beams of light outputting from your laser, and you will be shortly setting yourself up for a disappointing downfall. As mentioned, home-brewed lasers will require much experimenting around, you may be lucky enough to have the laser produce an output at first go, but also be prepared that you MAY NOT get an output at all.

    4. Use the 'KISS' Method - "Keep It Simple, Stupid". In doing so, you can reduce the number of areas that can cause problems. For example, the electrodes of a CO2 laser can just be copper pipe fittings (one at each end of the tube). There is no need for neon sign electrodes which require additional assembly and possible glass work - and can possibly fail.

    5. Keep you laser tube within reasonable lengths. At the very least for your first laser. Build your first laser, get this running and lasing, conduct experiments and chart results within a spreadsheet or something. voltage/current versus gas pressure/flow rate/mixture, etc. Once you have succeeded in building your first working laser, then you can move on, and try building a bigger laser, at least you will not be disappointed if the laser does not work as you have already built a nice small working model.
    I hope that this will bring some reality into your projects, but please don't interpret these comments as discouragement from building lasers. I'm actually trying to create more of a challenge for anyone who is or will be embarking on this wonderful area of laser technology, yet to keep in mind that you may not have a working product at first - a little like trying to build a tall building, expecting it to stay up without the foundations.

    Diane's Home-Built Laser Experiences - The Beam and I

    Only rarely do I receive email demonstrating true enthusiasm and determination for *anything*, let alone laser building, from an early age. Here is one I had to include here!

    (From/by: Diane Neisius (diane_va@yahoo.com).)

    The Beam and I

    Do you still remember the first Star Trek series? Ah, that was something for a child's heart. Ha, there *was* a woman in a starship (so don't tell me a girl can't fly to space silly boy you!!!), and they had these "phasers" raying around during their adventures. Nevertheless, I was a big girl (10) and knew, it was a TV series. A kind of technical fairy tale. The more I was surprised when I found a popular science book at the local city library. There were guys who made beams - really, not on TV. I didn't understand much of the stuff described, but got that there were some sort of mirrors and a ruby in it. They called it L.A.S.E.R., and it was real. For the fact my grandpa could make for himself *everything* (he repaired all the electric and mechanic stuff for our family, even hopeless cases), I believed he (and also me) could make our own laser if we just had one of these expensive rubys. I well knew a ruby was a high-priced gem. When I asked grandpa, he told me something about precision and that it is not quite easy to reach this in the living room of a hobbyist. Ok, I could not buy a ruby from my spare money. But from those days on I was convinced one day I *will* have my own laser. One day.

    Childhood dreams came to an end. No, to be honest, they only slept until I was in the final High School classes. It was the beginning 80s, and being a frequent reader of the German issue of Scientific American, one day I found the famous description of the mercury laser in it. Huh? A guy somewhere out there built his OWN laser? The fever came back again. When he could do it, I also can do it, I decided. However, I knew a lot more about physics than before, also liked to spend time with the school's little HeNe laser (on rare occasions). Got a basic knowledge of what is important for lasers to become working. A few telephone calls made me quite unhappy. I had learned you can get all the stuff you need if you really want - and can pay the prices. HIGH prices. Again there was the "ruby problem": lasers are *expensive*. "Silly, one cannot have one's own laser", a fellow laughed about me, "that's only for laboratories." I better not tell him about my desire. So I thought: "One cannot? Let's see about that."

    Don't ask me today why I started to study mathematics. We had a quite famous Department of Quantum Optics on our university, perhaps I should have gone to the laser business. But those days computers were still more exciting than lasers to me, and that's it. Now, being a student, I had access to real scientific literature, and I learned a lot about the theoretics of lasers (and, reading the business laser magazines, also about technical realisations). I studied various descriptions of laser types and decided to try about a flashlamp pumped dye laser. This one at least needed no expensive ruby. :) To make a sad story short, I learned a lot about how to blast stroboscope tubes using a voltage doubler and really BIG capacitors, and I guess the carpet in my room at the student's community will still have these nice pink rhodamine spots. :( I gave up on dyes. But I still wanted a laser, and some more telephone calls brought a fine small Siemens HeNe tube to me (it was a new 1.5 mW LGR7621). Shall I say I heard some of the well known "brzzzz's" from self-wound transformers until I looked for a used neon transformer? Still a descent of my grandpa I was... So I spent my time to build a casing for the transformer and a recitifier out of a box of 1N4007's (never tell this an engineer student. I did, the poor boy almost got a heart attack). By the way, the LGR7621 is quite robust. One day I caused a short which *detonated* the anode resistor (another lesson about BIG capacitors). After my eyes had recovered from the resulting supernova inside the tube, I anxiously looked if there's still anything alive. A visual inspection showed up a lot of very small cracks along the inside of the bore. Replaced the mortal remains of the blown resistor by a fresh one, it still started and lased! I used it for years after this accident without significant drop. Brave little HeNe. :)

    After some time, HeNe became boring to me. Using an internal mirror tube is one task, to build a device like Scientific's mercury completely on your own, quite another. So I started doing some experiments in that direction for the next time. I got a simple vacuum pump and an unsealed neon tube and began to work with glow discharges. To keep it short, I had to perform lots of experiments to learn about vacuum, outgassing and purity of gasses. Over the years, my little laboratory grew: a self-made voltage doubler for the neon transformer, a self-made mercury vacuum gauge, a better pump, noble gases in liter bottles, a hand-held spectroscope, longer discharge tubes. To work, all this took years of learning by doing. Then the next strike came, of course again by Scientific American. It was the copper vapor laser. How exciting... For I knew very well I had no experiences about laser optics until now - but the superradiant copper lines would need none. In words, I could start immediately. But unlike the old days, I decided to study a bit about superradiant laser before blindly begin to "hammer and saw". And by this I found out about the still simpler N2 laser. I decided to have one. And in 91, a self-made 10 cm test tube of acrylic with quartz windows lased! I danced in my room, for after 10 years I finally got it!

    For the fact the 10 cm tube lased quite weak I studied more about N2 laser design. The major problem was, I could reach only 10 kV with my equipment thus having poor energy densities in the discharge. Longer self-made tubes also lased weaker than expected (I tried several), even if attaching a metal mirror to one end. I had to pinch the discharge somewhat, but doing this in a tube made of acrylic would easily overheat and smoke the walls. Those days I already worked toward my Ph.D. thesis, and the research center where I did it had the most precious thing I ever saw in a library: the COMPLETE set of the Review of Scientific Instruments! Complete means complete: from issue #1 of 1929. And after some hours with it, I found the most useful paper about N2 type gas lasers I know. It is about the "strip line" type laser using a segmented discharge bore originally designed for the UV lines of the hydrogen laser [1]. I adopted the design a bit to my power supply (shorter strip-lines, shorter discharge segments) and pinched the discharge even more by adding short pyrex capillaries inside the segments. Believe me, it was a *lot* of work to drill holes for 56 electrodes and fill in the pyrex tubes successively from the end of the outer acrylic tube. But finally I had a 3 mm x 80 cm bore with a nice high energy density. For its strange appearance, I baptised it the "German Flute". The first thing I noticed after the first tests was, this baby would lase with every gas containing a bit nitrogen, even dirty air. :) Mirrors were good for nothing, and I easily got all three UV lines on a fluorescent screen using a "water prism" (triangle pot glued together from thin pyrex pieces and filled with water, which absorbs UV much less than a massive pyrex prism). Of course I also tried other gases, and the green superradiant Ne line at 540.1 nm was strong in this tube, too. Over the time it became my favourite. And on few occasions, after long times of green Ne, it was also possible to get the much weaker orange line at 614.3 nm at a lower pressure. But normally it disappeared after some time from outgassing impurities. The still weaker yellow line I never caught.

    Impurities were what finally drove me tired. It was common for my "German Flute" to be run with gas through flow. Otherwise lasing stayed only for seconds. On one occasion I tried a bore cleaning via He bombardment which took several hours. But after finish, what I call the "dirt spectrum" (N2 band, H alpha line plus Hg lines -- mercury from the vacuum gauge) reappeared in half an hour. Whenever I liked to start my laser, I had to spend days and hours in front just for basic cleaning the vacuum devices. And, the cost for the needed constant flow of Ne burned a hole in my pocket. Pure lab-grade neon isn't that cheap. And then, a few years later, I got a pen-sized red diode laser which made roughly twice the output power of my home-built in the green. It was depressing.

    Sometimes in life one has crises and has to separate from several things. So it was for me, and it was such a crisis which made me giving away a lot of things - including all my lasers. The university didn't take them (security reasons of course), but I found a physicist collecting strange devices, and I hope my baby still has a home there (even if it doesn't lase any more). So, off I were.

    Yes, until now. Some weeks ago I visited some friends, and I wondered to see a yellow HeNe in their rooms. They told me to use it for illumination of large naturally grown quartz crystals, for they like the "golden shattered glow" in them doing so. But they knew less to nothing about lasers, and I talked about lasers for an hour or so. And thought about I also would like illuminated crystals. But not red or yellow - green and blue, perhaps from an argon ion laser it would have to be. In the days of internet it is an easy task to feed "argon laser" to google.de and see what happens. Of course it leads to Sam's Laser FAQ and to lots of surplus advertisements. I have the chance to get an "all-included" ALC 60X (head, fan, cable, power supply, tube refilled) for roughly $1,000. Goddess, I have to scratch off those bucks somehow...

    The beam has me back! :)

    1. Kirkland, Dogett, Kim: Vacuum-UV H2-laser excited by a traveling-wave discharge, Rev. Sci. Inst. 52(1981) p.1338.

    I bet you're sorry you gave all that stuff away, even if you do know how to do much of it better now. A 60X is a nice laser but not quite the same as something built from scratch that one can fondle and tweak! :)

    (From: Diane.)

    What did you do... Talking about fondling and tweaking my home-built laser... Asking if I regret to give it away...?? Ah, sigh... :) Finally I phoned the guy I gave all the equipment and he told me I CAN HAVE IT BACK! Now, I guess I will have to do lots of maintenance work on my baby if it's home again (stored at a garage for the past 3 years).

    So I guess the beam really has me back! :)

    Building a Femtosecond Laser at Home?

    Well, probably not. But after you have constructed all the SciAm and other "common" home-built lasers, it could be something to keep you occupied. :)

    (From: Anonymous (localnet1@yahoo.com).)

    Info on how to build a femtosecond laser? I have had a rather strong academic interest in the field, if nothing else, on this subject for the last 10 years or so. never in that time have I seen a 'how to' build such a laser. if you would like to see what others have done the academic journal 'Optics Lasers' is your best bet, any major university library should subscribe to them, and certain articles are available via their on line archive at the Optical Society of America Web site.

    If on the other hand you are simply looking for background information on mode locked lasers, nowadays most systems (or at least the easiest to build systems) use a self starting mechanism and only really need:

    1. Gain medium with sufficiently high bandwidth to support the pulses you are interested in - Ti:Sapphire and dyes are the old standbys, but there are all sorts of different hosts that will produce sub-picosecond pulses.

    2. A system for achieving GVD compensation. For a long time, group velocity distribution has been accomplished by a pair of prisms tailor made with 'x' apex angle and 'y' refractive index. However, in recent years it ahs been possible to use multi stack dielectric mirrors for GVD compensation. If I'm not mistaken this was first done by Newport (or perhaps they had the first commercial mirrors, I don't remember which).

    Some Photos of Home-Built Lasers

    (From: Chris Chagaris (pyro@grolen.com).) (From: Laserist (laserist@geocities.com).) (From: Michael Andrus (andrus@ccountry.net).) Many additional photos can be found on Massild's Laser Project Page.

    (From: Daniel Ames (dlames3@msn.com).)

    Also check out the links in the sections: General Resources for Amateur Laser Construction or Amateur Laser Construction Sites.

    Diagrams Showing Major Components of Typical Home-Built Lasers

    These drawings show the structure and power supplies for some of the lasers built by amateurs. The first seven are based on the laser articles from Scientific American (including the book: "Light and its Uses" - see the section: On-Line Access to the Scientific American Laser Articles Their purpose is to give you a flavor of what this type of laser construction entails - but are NOT intended as dimensioned plans and are NOT drawn to scale. Refer to the more detailed descriptions in the chapters on each laser type following the introductory chapter: Home-Built Laser Types, Information, and Links and the relevant Scientific American articles. The final one is from the journal: Review of Scientific Instruments. See the chapter: Home-Built Pulsed Multiple Gas (PMG) Laser for more information. There may be additional diagrams in each of the chapters on specific home-built lasers, particularly those constructed by various contributors to this document.

    Comments on a Universal Experimenter's Gas Laser

    A question that comes up occasionally is: "How can I build a laser that I can use to try out various gases and other parameters?". Here are some suggestions for a gas laser testbed optimized for visible and near-IR operation. Actually, these are more like random thoughts to get you started:

  • Build your plasma tube with Brewster windows at both ends on stems that are at least 2 or 3 inches long to keep them away from the discharge. For a wide wavelength range, putting these on ball and socket or flexible mounts may be desirable to permit their angle to be varied slightly.

  • Provide electrodes suitable for your expected types of lasers. The positive (anode) electrode can usually just be a wire or sleeve (cooling and sputtering aren't significant). However, the negative electrode should be made of a suitable material (e.g., aluminum, heated tungsten, etc.) for the laser operation - pulsed or CW, low or high current, etc. Place the cathode(s) in a side-arm so prevent sputtered material from getting to the Brewster windows.

  • Build universal mirror mounts that can accept a variety of mirrors - you won't always be able to find the same size or thickness. I would suggest something like the mirror cell arrangement described in the section: Mounting Laser Mirrors.

  • Install a Helium-Neon (HeNe) alignment laser as a permanent part of your testbed. Mirror alignment is one of those things you will be doing constantly. It will be desirable to be able to do initial set up and checking without having to assemble the alignment jig every time!

  • Start with broadband HR mirrors having as high a reflectivity as you can get or afford over the wavelength range of interest. The HRs used in HeNe lasers may be much better than 99.9 percent. (If an 'other color' home-built HeNe laser is in your plans, particularly one that does green, even the OC reflectivity will have to approach these values!) The higher the reflectivity, the lower your lasing threshold. The radius of curvature (r) should be at least equal to the distance between the mirrors (L) but probably not more than 2*L since if the focal lengths (f = r/2) are too long, alignment becomes more difficult.

    As noted below, some HRs are not polished on their rear surface. It may be possible to attach an optical flat (e.g., piece of a good quality microscope slide) with optical cement or Epoxy to reduce scattering and reflections from that surface but this won't be ideal.

  • For initial experiments, take the output beam off the Brewster reflections or from the leakage through the (HR) mirrors. Once you know which line(s) you want, a specific OC mirror spectral curve and reflectivity can be selected. Reading through the chapters on each of the types of home-built lasers that follow should provide some of the details. Start with the chapter: Home-Built Pulsed Multiple Gas (PMG) Laser which deals with an approach along these lines.

    (From: Bob.)

    Also, take note that most HR mirrors are a lot better than 'just' 99% reflectivity, at least when you are talking about intracavity mirrors. Also a lot of commercially available HR mirrors are not designed for laser use - they only have their back surface fine ground, not polished. This means you can't get a HeNe laser beam through them for alignment purposes (particularly optics from Thor Labs and CVI). Another reason why generic broadband lasers may not be suitable, is that if you have any plans on making a high power pulsed laser (e.g., ruby or YAG), most off the shelf optics can not cope with the power/energy levels you would be exposing it to, and will quickly fail. Just a few things to watch out for!

    What About a Home-Built Solid State Laser?

    Note that there is currently no mention here (or as far as I know, in the Amateur Scientist articles of Scientific American) of ruby, YAG, vanadate, and other solid state lasers. However, there is a chapter: Home-Built Pulsed Solid State (PSS) Laser which is under construction and another one: Home-Built Diode Pumped Solid State (DPSS) Laser which is already fairly well along.

    Since there is no realistic possibility of actually growing, shaping, grinding, and polishing a raw laser crystal in your basement, there are, never were, and never will likely be any truly built-from-scratch SS lasers. You will have to buy the crystals ready-made. And, since there is less standardization on SS laser components than for many other types of lasers, it isn't even possible to suggest many sources for parts with particular specifications unless you are willing to pay new (and very high) prices. That's the bad news. The good news is that SS lasers are much easier to get working even with a less than optimal match between the lasing medium, pump source, and mirrors, than many other types of lasers. While the gain of a HeNe laser may be 10 percent per meter, the gain of a solid state laser rod with flashlamp pumping may be 10 percent per cm! (And when diode pumped, the gain is much higher still.)

    Pulsed (flashlamp pumped) SS lasers have been popular projects since the invention of the laser and with reasonable care, a successful outcome is likely. They are by far, the easiest lasers to construct capable of blasting holes in things. :) There are many surplus components and partial or complete systems available at reasonable cost. As companies switch over to Diode Pumped Solid State (DPSS) lasers from lamp pumped types, more and more pulsed SS laser components and systems are showing up on the surplus market.

    Building a DPSS laser, especially one with frequency doubling to produce green (532 nm) output is a more complex - likely much more expensive - undertaking, but one that can be accomplished successfully. And, because DPSS lasers are becoming more popular, components for these are coming down in price, at least somewhat. So, there is hope. :)

    Home-Built X-Ray Laser?

    The following is from a recent paper: "Generation of millijoule-level soft-x-ray laser pulses at a 4-Hz repetition rate in a highly saturated tabletop capillary discharge amplifier", C. D. Macchietto, B. R. Renware, and J. J. Rocca, Optics Letters, vol. 24, no. 16, pp, 1115-1117, August, 1999.

    (Portions from: Bob.)

    There are a good number of people who have built or are currently building their own lasers. from simple systems, to the extravagant. I was going through some current journals today, as things were slow here, and I saw something that caught my eye, making me instantly think of this group: A table top soft X-ray laser.

    Basically, this system was quite simple. It had a 0.32 mm ID aluminum oxide capillary evacuated to roughly half a Torr filled with argon gas, pre-ionize by a discharge. It was pulsed with a high current pulse (approximately 26 kiloamps!) with a fast (40 ns) rise time using a water capacitor and series spark gap switch. The water served as both the dielectric of the capacitor and as cooling for the capillary. The capacitor was charged by a 4 stage Marx generator located in a separate box. The laser itself occupied an area of only about .4 x 1 m (16 x 40 inches). Since the laser operates in a highly saturated regime, no cavity optics are required. The output beam profile had a ring shape (due to plasma density gradients in the plasma column) with a half-angle divergence of about 4.6 mR. The output energy averaged about .88 mJ at 4 Hertz.

    I (Bob) kinda like the idea of having an X-ray laser in the corner of my lab. So I'm gonna build one, though maybe others might like to as well. For your information, when the NOVA laser was used to pump a soft X-ray laser, they got out 8 mJ pulsed, with a repetition rate of about 1 pulse per 30 minutes (!!) or so. It would be kinda cool to have a 1 mJ X-ray laser, especially if it could operate at 4 or 5 pps. And, with higher average power than the NOVA laser pumped X-ray laser!!!!! (Although I must admit that one generated shorter wavelength X-rays: 15 nm instead of 47 nm. Well, you can't have everything.)

    General Resources for Amateur Laser Construction

    There are actually a larger number of places than you might think to find information on home-built lasers as well as some ways of interacting with like-minded individuals on-line. Check out the amateur laser construction Web sites for examples of lasers others have built, or are in the processing of building, as well as much related information. Many of these sites have descriptions, diagrams, and photos of their home-built lasers. There are also a number of companies that may sell complete plans, parts, and other items related to home-built lasers:

    Scrounger of the Month Award

    Here is a success story on obtaining help in glasswork, AND inexpensive neon sign transformers and vacuum pumps from the same source - you guessed it - a neon sign shop! It seems that many neon sign types are also interested in lasers (or at least fantasize about building one) and are therefore sympathetic to the needs of amateur laser constructors! For more info, see the section: Tips for Dealing with a Neon Sign Shop.

    (From: Tom Miller (tmiller@umaryland.edu).)

    Ok, today was GOOD! I visited a local sign shop, one of the larger ones, and got to talk to the owner. He was the one who initially nixed selling any transformers to the general public. After a brief discussion explaining what I wanted to do, he took me on a tour of the whole facility. It seems he may be interested in constructing a laser also. He was checking prices for CO2 lasers for use in cutting plastic sign material and for a 50 watt unit, was seeing prices in the $50,000 range. Now granted, this included the mechanical positioning equipment, but still, he thought it to be too high.

    Anyway, I left him a copy of Sam's Laser FAQ and my business card. He wants me to give him a drawing of the glasswork and he will put it all together. Says it will take less than an hour, so I told him I would pay for his time. He suggested that I use a "tubated" electrode on each end and connect the vacuum pump to one end and the gas supply to the other. This way, the flowing gas will cool the electrodes.

    We got around to talking transformers and I ask what a 15 kV, 30 mA unit would cost. He asks if I could use a 15 kV, 60 mA transformer if it was used. I was completely surprised when he said it would only cost $10. This transformer was sitting on his glasswork bench and he was using it to test tubes. The guy showed me a pile of neon sign transformers under a very large workbench. Must have been 20 to 30 there. Also, the owner told me he had MANY old vacuum pumps just sitting around. I saw at least 5 of them.

    Next, he asked what I would mount the laser on. I said I would like to use an aluminum I-beam about 3 to 4 inches wide and about 4 feet long. He took me to a different area of the shop and found a scrap piece of the stuff. I figured that I would stop using all of his time, and told him I would go and make a drawing for the tube and get it to him sometime in the next few weeks.

    So, anyone having problems, just load up with Sam's Laser FAQ, go find a large old neon shop and talk to the guys who actually do the work. You will be surprised how much interest they will have in a good powerful laser.

    So today was a good one. :)

    Acknowledgements

    Information from many sources has been used to compile the chapters on amateur laser construction. Wherever possible, I have attempted to identify the individual contributor. However, if you feel that there is something here you wrote without an acknowledgement, please let me know.



  • Back to Amateur Laser Construction Sub-Table of Contents.

    Setting up a Home Laser Lab

    Safety Issues in a Lab for Home-Built Lasers

    There are a variety of issues that are important for any sort of home lab or workshop but the following, in particular, apply directly to lasers and laser construction: There didn't appear to be a critical mass of lawyers present at the time most of the articles in "Light and its Uses" were written. Therefore, they tend not to deal with the safety issues as emphatically as might be desired. Most of these projects have aspects (most often the high voltage power supplies) that are potentially dangerous or lethal. Safety must be at the top of your list of priorities when undertaking such an endeavor!

    Work Area - Setting up a Laser Lab

    Since any of these lasers represents a long term comittment, it is essential that an area be set aside for your laser lab. Therefore, the kitchen or dining room table is NOT an appropriate place to be constructing a laser. It is possible to do without the sort of setup depicted in the section: Possible Laser Lab Layout but there are some basic requirements for a safe, functional, and convenient space:

    Possible Laser Lab Layout

    I wish I had this! Note: Two means of exit and two fire extinguishers!

    Also note the chair - most important - and the bench for your guest (though probably should be s eleep-sofa so they can snooze while you spend the afternoon adjusting your gas mixture or performing mirror alignment. :-)

    
        |<------------------------------- 12' ------------------------------>|
     ___|____________________________________________________________________|
      ^ |    |                                                         |     |
      | |    |        Storage Cabinets/Shelves (above work area)       |     |
      | |    '---------------------------------------------------------'     |
      | |          Electrical Outlets (two circuits) all along wall          |
      | |                                                                    |
      | |            Work Surface - thick hard-plywood (3' x 12')            |
      | |____________________________________________________________________|
      | |           |                                                        |
      | |           |  Vacuum System on floor (beneath work area)     Gas    | 
      | |           |                                              Cylinders |
      | | Test      |                                              __________|
      | | Equip.,   |                    ________                 |          |
      | | Power     |                   |        |                | Wet area |
        | Supplies, |                   | Office |                | Glass-   |
     10'| Misc.     |                  (| Chair  |)               |  working |
        |           |                   |________|                | Ventila- |
      | |           |                   '--------'                |  tion    |
      | |___________|                                             |__________|
      | |                                                                    |
      | |S Power Switch            _ _        __________                     |
      | |(on Wall)          .-======'======-.|          |                    |
      | |        /          |               ||  Bench   |           \        |
      | |      /       Fire |    Storage    ||          | Fire        \      |
      | |    /         Ext. |               ||==========| Ext.          \    |
     _v_|__/         _______|_______________|____________________         \__|
    
    

    Sources of Special Parts and Supplies

    This section deals mostly with the items to equip your lab, small parts, chemicals, and so forth. Also see the sections on vacuum equipment, optics, and power supply components, in this and the chapter that follows, as well as the chapter Laser and Parts Sources. And note that many of these items appear regularly on eBay and other auction sites as well as high-tech flea markets. Apparently, at least one person is even selling small quantities of chemicals to the public on eBay!

    Develop a relationship with a teacher/instructor/professor/researcher at a high school/technical school/college/university/industrial lab. Some people will be more than eager to help and mentor you - even to the extent of loaning equipment or donating small quantities of chemicals, electronic components, hard to find optics, etc., to your cause. Use of their lab may even be possible. And, universities sometimes toss out the most amazing things - like complete vacuum systems - when a grant runs out and they need the space! There are various programs as well to encourage students to go into science and technology fields. Who knows, they may even pay you to do this!

    Call up laser and optics manufacturers. Sure, many won't give you the time of day unless they think you will be ordering $1,000,000 worth of equipment. But, all you need is one to say yes! There are always such things as cosmetic rejects or seconds - that are useless to them because they cannot sell the parts - but fine for your needs. The trick is to hold their attention long enough - or be such a (polite) pain in the neck that the easy way out is for the company to provide what you want! I have heard of people obtaining all sorts of material, parts, equipment - some of it worth quote a lot of money - in this manner.

    In summary - possible places to find useful stuff:

    (From: Chris Chagaris (pyro@grolen.com).)

    Here are some resources that I have not seen mentioned anywhere on the Net:

    For chemicals used in various aspects of laser construction and laboratory glassware at unbeatable prices, a fine source is:

    For quartz tubing and quartz windows of all sizes, at very good prices: I would be glad in assisting other individuals in locating some of the more difficult to procure items needed in some aspects of constructing these various lasers.

    (Portions from: Steve Roberts (osteven@akrobiz.com).)

    While Sargent-Welch, Edmund Scientific and Dunniway might be what come to mind when thinking scientific suppliers, they are most expensive, expensive, and not cheap, in that order. :-)

    The ideal thing to have is the Laser Focus World (LFW) Buyers Guide, a phone book sized list of suppliers put out by Laser Focus World for their subscribers. Subscriptions are free to qualified individuals, so you need a company name to subscribe. If somebody is in the laser business, they are in LFW. Photonics Spectra is also a good freebie if you can qualify.

    If you are into building your own HeNe (or other) laser from the ground up, these suppliers may come in handy:

    Glass and glass working equipment suppliers: Flanges, glass-to-metal seals, electrode material:

    For cathode material, just ask for what the machinists call "gummy" aluminum, the really hard to work soft stuff that gums up tooling, and you've got it. Not 905 or 2025, maybe 6061, but 6061 has a lot of weird stuff in it like silicon monoxide. One guy I watched once sputtered the cathode in a oxygen discharge, then cleaned it with hydrogen in a soft glow. Nickel is the metal of choice for anode pins and wiring inside the tube, but its going to take some work to spot weld it to the kovar or dumet lead throughs, you need to find or make a "thermocouple" welder designed for cap discharge welding of small wires.

    You can get pure aluminum, nickel wire, tungsten wire, titanium sheet, etc. from Small Parts, Inc. (Miami Lakes, Florida, 1-800-220-4242). They sell it chopped up in small quantities at decent prices, with no minimum order. They specialize in what they call "Engineering Findings", in other words, small quantities of all the goofy parts you need to make industrial prototypes, and they tolerate us hobbyists. Reading the Small Parts catalog will keep you spellbound for a few hours, it's worth the call.

    The other source for cathode type aluminum and nickel tubing/wire is MDC, Inc, also in Florida. They also sell high vacuum parts such as flanges and seals and have the tubing and the low temp. brazing materials you need to make tubes. They don't stock the aluminum as cathode material, but it's the right type. Surprisingly the stuff the hobby shops sell in the K&S displays is pure aluminum, otherwise they couldn't extrude it that cheaply. Both Florida companies have Web sites but I (Steve) don't recall what they are.

    Miscellaneous parts:

    (From: Joe or JoEllen (joenjo@pacbell.net).)

    A good resource for components found in "Light and it's Uses" is:

    (From: John De Armond (johngd@bellsouth.net).)

    Duniway Stockroom for items like Dow corning Silicone High Vacuum Grease. Less than $10 for a lifetime supply. Also a nice stock of Viton O-rings (what you want to use instead of Buna-N for vacuum), fittings and other vacuum goodies. Might also take a look at their "vacuum epoxy". Really a common industrial epoxy made by Hysol that has a sufficiently low vapor pressure to be used at the low end of high vacuum. Same stuff Varian sells as "TorrSeal" for many bux.

    More on Obtaining Gasses

    British Oxygen Company (BOC) Gasses has a variety of technical and safety information on-line as well as handy units conversion tables on-line. They have request forms for an extensive catalog and other technical info.

    Spectra Gases supplies all sorts of gases and support equipment for lasers and related applications including a variety of mixes for CO2 lasers, pure gases for helium-neon, argon and krypton ion, and excimer lasers. See their Laser Gases and Equipment Page. They have been recommended and will sell in small quantities to the private individual (more below). They also are planning some technical and reference pages for their Web site but they are not presently complete.

    The following comments deal with a variety of gasses required for laser construction.

    (From: Cass (cassegrainian@galaxycorp.com).)

    The CO2 laser-mix gas is typically sold in "H" bottles for $85.00 per fill. The company that I checked with will allow one to specify their own blend of CO2, He, N2. Your local welding/medical supply company may vary. Try to find the largest gas company in your area as many of the smaller ones simply use them to fill your bottle(s) and tack on an additional charge.

    (From: Steve Roberts (osteven@akrobiz.com).)

    I went to purchase a tank of nitrogen today. I own the tank, so a fill was $10.80 + tax for a size "P" tank (It seems no two gas places use the same letters) because I own the tank. I needed N2 to test a nitrogen laser for a customer. As I was walking out the door it dawned on me to ask what the price for a CO2 mix would be. The fellow couldn't give me the mix percentages, but said that for my tank, the fill of UltraMix by AGA would be $16.85. My tank is about 14" tall by 5.5" in diameter cost $89.00 when I bought it last summer. When I need a fill, they just swap tanks. So I guess I'm getting a tank of mix next week when I'm done with the N2.

    I actually think this is good news because when I talked to the rep on the phone and a leased tank of gas in the same size was quoted at about $200. The tanks all have the same CGA 580 fitting on them so I've been able to get oxygen, N2 and Ar in the same size with the same regulator.

    If your operation is set up like our local gas shop, you have an industry side and a retail side of the business. I'm not a industrial customer, but I make it a habit of going to the industrial dock in person for my gasses, its much cheaper then calling in and getting the sales person who seems to jack up the price when you pick it up at the retail end of things. It's easier and cheaper to go buy it from the dock clerk. BTW, if you're hunting for something pure like krypton, Try Spectra Gases. Or one of the others that cater to the laser industry. Even after the hazmat shipping fee, buying a bottle from Spectra was much cheaper then getting it locally. The large company around here varied their krypton price on a day to day basis like it was on the stock market. Spectra's quote was about 1/3rd their price even after shipping.

    (From: Tom Miller (tmiller@umaryland.edu).)

    The place I checked gave a price of $160 to purchase a 40 cu. ft. tank with a fill. Refills are $60. I have access to the "R" tanks used for medical O2 and these are 20 cu ft (tank is ~2 ft tall and 4 in diameter) and have a tank valve like scuba type gear. I guess the valve could be changed to some standard but it would be nice to use an O2 regulator as they are designed for low pressure and low flow. I wonder if the O2 regulator would work with the high He mix? When I talked to the gas supplier, they said to use a He regulator.

    (From: John De Armond (johngd@bellsouth.net).)

    You can change the valve on the "R" tank, as almost all tanks use 3/4" NPT threads. This is a job, though, because the valves are put in REAL tight. when I owned a welding gas supply company, we had a clamp rig that would hold the tank while we pulled on a closed-box wrench attached to a 6 ft cheater. still usually required some heating. One thing to watch for is the thread sealant. If the gas co used pipe dope, you'll never get it clean enough (short of vacuum baking) for high purity gas.

    You really do need to use a helium regulator for He. The reason is the He atom is so small that it'll through the rubber diaphragm of most regulators like a dose of salts! He regulators typically have a 316 SS diaphragm and a copper crush gasket. Helium's kind of tough to hold onto for any length of time. At the welding gas co, we kept close track of the code dates on our tanks because if we let one sit around for a long time, the He would diffuse through the walls and lower the pressure. Got complaints from customers. :-) Also be aware that ordinary He sold for balloons has a goodly chunk of air in it. You can get an inexpensive catalytic gas scrubber from Matheson Gas that will clean up the stream but probably not worth it. Just get clean gas.

    I buy my high purity gas from Spectra Gases. They sell gas in disposable cylinders. A 20l cylinder that looks like a propane torch cylinder but with a needle valve filled with Neon is about $50. A 100l cylinder is about $140. This is spectroscopy grade gas. And when the tank is empty, it is useful for a number of things plus the 100l tank has a standard CGA valve so you can put it on your "E" tank :-) I use a 20l tank as a "day tank" on my bench. I refill it from the 100l tank. That way if I leave a valve open I don't trash the whole 100l tank. These are really good guys to work with and they have all kinds of gas. Need 20l of Xenon? Got a BIG pocketbook? They got it. :)

    Tips for Dealing with a Neon Sign Shop

    Neon sign shops can be a fabulous potential source for inexpensive neon sign transformers, glass tubing, electrodes, vacuum equipment, and glass working services. Gas lasers and neon signs share a lot in common so the people who work on neon may be very cooperative if you convince them you are serious about building a laser. I have even heard of someone not only getting his HeNe glass work done by a friendly glass bender, but having the tube filled with helium and neon at the proper pressure (along with the required bake/back-fills) as well. However, you do need to use at least some common sense when you walk in (and maybe a bit more) so as not to just be shown the exit:

    (From: John De Armond (johngd@bellsouth.net).)

    When someone contacts me with a request for a transformer, for example, I'm pretty cooperative once I determine that they probably won't hurt themselves and aren't planning on using it for practical jokes. I don't need the liabilities. I buy used transformers in bulk from a large regional shop, test and refurb them and either use them for my neon or resell them to experimenters. I get a buck a kilovolt for as-is units. If you're lucky, a shop may give you transformers but it generally greases the skids for future requests to offer to pay a bit.

    A few other things to keep in mind when visiting a neon shop:



  • Back to Amateur Laser Construction Sub-Table of Contents.

    Introduction to Vacuum Systems and Technology

    Vacuum Systems for Home-Built Gas Lasers

    All but one of the gas lasers described in the chapters on specific home-built lasers require a decent vacuum system to remove air from the laser tube so that it can be back-filled with the required lasing gasses at a low pressure. These include the HeNe, Ar/Kr, CO2, HeHg, CuCl/CuBr, and multiple gas (PMG) lasers. The N2 laser requires only a 'low' vacuum since it runs at a substantial fraction (e.g., perhaps 20%) of atmospheric pressure and some versions can run ambient pressure (1 atm).

    The vacuum system serves three functions:

    By the standards of the vacuum industry, our requirements are modest and are not really termed a 'high' vacuum but they are still not the sort of thing you come across in daily life.

    I am gradually putting together a vacuum system (or at least acquiring parts!) and may have an interest in your cast-off or excess vacuum equipment (small items, not complete ion beam machines!) or accessories. Please see: Sam's Classified Page, "Wanted to Acquire" near the end, if you have anything available.

    Vacuum Systems References, Links, Forums

    The sections that follow represent the barest introduction to vacuum technology. The following resources may be useful:

    What does a Pressure of Such-and-Such Really Mean?

    We always hear about the barometric pressure - or the level of a vacuum - in terms of 'mm or mercury' or 'inches'. 1 atmosphere (at sea level under some unidentified ideal conditions) is also said to be 14.7 pounds per square inch. Why?

    The earth is covered with a vast ocean of air. Despite common experiences, even air has mass and mass implies weight. We know it has volume or else your automobile would have a real problem with flat tires. Most of the volume (the contribution from the volume of the the protons, neutrons, and electrons in the atoms are negligible but not precisely zero) results from the constant motion of the molecules (in air or other gas) bouncing against each-another due to their thermal motion. This also keeps the air in a gaseous state. At really low temperatures, the motion is reduced resulting in liquid and solid phases of even air. At exactly absolute zero (-459 °F or -273 °C) all motion ceases. However, even then most of the volume of the frozen air is still empty space - but that is another story.

    At sea level under average conditions, the column (actually an inverted truncated pyramid if you want to be strictly correct) of air above 1 square inch of area would weigh 14.7 pounds if you could capture, compress, and package them and plop them down on a delicatessen scale! As you move away from the earth, this 'column' of air becomes increasingly rarified approaching a prefect vacuum at 50 miles or so - else low earth orbit satellites would not stay up very long due to air friction.

    It turns out that a column of mercury with an area of 1 square inch and 29.92 inches (760 mm) high weighs exactly 14.7 pounds as well (what a coincidence, huh?). So, if you take a closed-end tube a little more than 30 inches long, fill it with mercury, and invert it in a pool of mercury, the pressure of the surrounding air will be able to support a column of mercury 30 inches high. The space above the mercury will be a decent vacuum. You have made a mercury barometer. (Strictly speaking, there will be mercury vapor in that space but it won't affect the height by much.)

    If you were to take this barometer and place it inside a vacuum vessel and start up the pump, the column would go down until at the point of a perfect vacuum (not achievable but close), it would be precisely level with the surrounding pool of mercury.

    Note that the diameter of the tube doesn't matter - wider implies a heavier column of mercury but the area of the air acting on the column changes by the same factor. In fact, it can have pretty much any convoluted shape you want (except that if portions are too thin, surface tension becomes a factor) as long as it is sealed and totally filled with mercury. Why this is so is left as an exercise for the student!

    The corresponding height of 1 atmosphere for water is about 34 feet - a column of water with a cross sectional area of 1 inch and height of 34 feet weighs 14.7 pounds. This also means that for a diver, the water pressure increases by 1 atm for each 34 feet of depth. Thus it is not surprising that there are significant problems in deep sea diving! You have to go up by MILES in air for the pressure to decrease by a substantial fraction of 1 atm but need only go down 34 feet in water to increase pressure by 1 atm!

    Note that the most likely form of a pressure you are familiar with is the reading on the gauge you use when checking or filling your automobile or bicycle tires. However, this is calibrated relative to the surrounding pressure of around 1 atm. Thus, the actual pressure inside a tire will actually be 1 atm + the reading on the gauge. And you thought you had a perfect vacuum inside that flat tire when the reading was 00.0! :-)

    Similarly, a vacuum can be measured relative to atmospheric pressure and this is often done for the sort of vacuum you find in an automobile engine intake manifold, vacuum hold down plate, and other familiar applications. However, these readings represent the difference between one large number (local atmospheric pressure) and another large number (your vacuum). Since weather conditions (i.e., high and low barometric pressure) can result in a variation of 1/2 inch of mercury or more, such measurements will fluctuate and aren't very useful when the absolute level of vacuum is what's important.

    Here are some units and relationships commonly found in dealing with vacuums:

    If you are totally confused at this point, for a wonderful description of these units and their history, see: The Electronic Bell Jar - Pressure Units.

    Or, here is an instant conversion chart. To convert from the units in a row on the left, multiply by the entry in the appropriate column.

                 Micron   Pascal  Millibar   Torr    Inches
     ----------------------------------------------------------
      Micron       1      0.133    0.0013   0.001   1/25,000
      Pascal      7.5       1       0.01    0.0075   1/3,387
      Millibar    750      100       1       0.75    1/33.87
      Torr       1,000     133     1.33       1      1/25.4
      Inches     25,400   3,387    33.87     25.4       1
    

    What is a Low, Medium, or High Vacuum?

    Vacuums come in all shapes and sizes - and I am not referring to vacuum cleaners! Any local reduction in air pressure significantly below standard atmospheric pressure (760 mm of mercury, 14.7 pounds per square inch) is termed a vacuum (except by your local weather person who talks about 'low pressure areas'). For convenience (and because there must have been a meeting of elder statesman with nothing better to do), the Torr in honor of some Italian named Torrecelli is used to designate a pressure of 1 mm of mercury I guess referring to 'Torrecellis' all the time would be too confusing. :-)

    The Vacuum Chart provides a nice instant summary of pump types, gauges, and applications, as a function of the level of vacuum.

    The following dividing lines between low, medium, high, and ultra-high vacuums are somewhat arbitrary but will be convenient for discussion:

    You may also hear the term 'hard vacuum'. I don't know if there is a precise definition for this either but I would assume that anything with a low enough pressure to behave similarly to a perfect vacuum from the normal experiences point of view would qualify. Also note that in terms of the strength required of a vacuum vessel, the difference between a vacuum of 1 Torr and 10-19 Torr is irrelevant. Why? :)

    Handy-Dandy Vacuum Chart

    The following chart shows at a glance the relation between inches of mercury (relative to 1 atmosphere and to a perfect vacuum), mm of mercury (mm of Hg or Torr), Pounds per Square Inch (PSI), percent vacuum, and microns (1x10-3 Torr).

    (Original chart from: Chris Chagaris (pyro@grolen.com).)

               Inches of Hg         Torr or
          Rel to 1 atm  Absolute    mm of Hg   PSI      % Vacuum     Microns
        ----------------------------------------------------------------------
             0.0          29.92      760      14.696      0.0           -
             0.40         29.52      750      14.5        1.3           -
            10.24         19.68      500       9.7       34.0           -
            18.11         11.81      300       5.8       61.0           -
            25.98          3.94      100       1.93      87.0           -
            27.95          1.97       50       0.97      93.5           -
            28.92          1.00       25.4     0.4912    96.6           -
            29.52          0.40       10.0     0.193     98.7           -
            29.88          0.04        1.0     0.0193    99.9       1000.0
            29.916       3.94*10-3    10-1    1.93*10-3   99.99       100.0
            29.9196      3.94*10-4    10-2    1.93*10-4   99.999       10.0
            29.91996     3.94*10-5    10-3    1.93*10-5   99.9999       1.0
            29.919996    3.94*10-6    10-4    1.93*10-6   99.99999      0.1    
            29.9199996   3.94*10-7    10-5    1.93*10-7   99.999999     0.01
    

    How Good a Vacuum System is Really Needed?

    None of the gas lasers we will be discussing requires a vacuum better than about 0.1 Torr when operating. However, in order to clear them of contaminants in a timely and economical manner (without a semi-inifinite number of purge and back-fill cycles), it is desirable to be able to pump down to a much lower pressure than this. The better your vacuum capability - to a point - the easier it will be to obtain a pure gas fill. Less gas will be needed (due to fewer pump-down and back-fill cycles) and time will be saved. However, there is no need to go overboard. My rule-of-thumb (read: wild guess) is that a vacuum system capable of reliably pumping down to 1/100th of the lowest operating pressure is adequate for dealing with a laser tube that has a single vacuum/gas fill port. Pumping to 1/10th the desired final pressure may even be good enough if the laser tube is fabricated to have a gas-fill port at one end and a vacuum port at the other. For a flowing gas design (e.g., CO2 laser), the requirements are even less stringent and just being able to maintain the desired operating pressure may be good enough. If you think you will be building more than one type of gas laser, make sure this applies to the one with the lowest operating pressure. Also keep in mind that some types of lasers (like the HeNe) are particularly sensitive to the slightest traces of unwanted gasses and a better vacuum system would definitely be advantageous for these.

    Unless you have worked with a decent vacuum system in the past, own a HVAC service business, or just happened to pick up something that looked like a pump of some kind at a garage sale (but you weren't really sure and got lucky), you don't have the needed equipment! However, an adequate 'medium' vacuum system can be put together for less than $400 - possibly a lot less if you are determined and somewhat resourceful.

    Types of Vacuum Pumps

    Various kinds of vacuum pumps are needed to pump down to different levels of vacuum. Generally, rotary vane mechanical pumps are used for low to medium vacuums and other types are needed to go below this range. Note that I often drop the "vane" from "rotary vane mechanical pump" but most are of this type. There are also rotary piston and rotary scroll pumps but these are less common for vacuum applications.

    See Vacuum Pumps Suitable for Various Home-Built Lasers for diagrams of the types of vacuum pumps described below that are relevant for our purposes.

    There are many types of mechanical pumps but they are usually based on one of two basic principles: positive displacement (perhaps these should be called negative displacement in dealing with vacuums!) and turbo-molecular:

    Positive displacement pumps can be further classified as to the number of stages:

    While not generally thought of as pumps, the following perform related functions helping to rid the system of moisture and other unwanted volatile materials:

    In addition to helping to achieve a high vacuum, a dryers and cold traps may also help to prevent contamination to the oil in the vacuum pumps.

    (From: Steve Roberts (osteven@akrobiz.com).)

    You really want an ion pump or a getter sublimation pump a.k.a. Titanium Pump (TP), or if you're really lucky, a turbo pump, although turbos don't hold up to abuse too well.

    Diffusion pumps are old news and outdated, they run hot, and some are huge! Depending on what is being pumped, they may need to be torn down and cleaned frequently. Small ion pumps are about $1,200 rebuilt. maybe $250 for the pump and $1,000 for the controller after rebuild, which means they are dirt cheap if you find a used one. I've seen a few diffusion pumps given away in the past few years but they really were clunkers and sensitive to contamination. Ion pumps do run out as they have a consumable material inside them that reacts with remaining gas, but they also get you a cleaner vacuum then a diffusion pump.

    Ion pumps also act as a gauge by themselves, although not as accurate as a dedicated gauge. What you want is an appendage ion pump. They are small and fast for your purposes, and small enough you can lift them.

    TPs use a hot titanium filament to bury gas molecules under a thin film of metal. The only thing they use up are rods which are presently about $160 for a lifetime supply. TP are more suited for large vacuum chambers, but they create a fast clean vacuum and are resistant to an amateur's mistakes.

    The gauges are only about $250 for a new digital one, which is almost cheaper then buying or scrounging a used ThermoCouple (TC) gauge and these do wear out. You're better off looking for a digital capacitance manometer then an old beat up TC.

    Specifications of Welch Rotary Mechanical Vacuum Pumps

    Welch Vacuum (or "Welch" for short) vacuum pumps are one of the most popular brands - at least to me from my cyclotron days at my high school! (See the section: The Central High School Cyclotron.) Welch used to be "Sargent-Welch" and has been in the vacuum business since 1880 - over 120 years! (I may use the two names interchangeably. However, the current Sargent-Welch company deals only in science education supplies and furniture.) Welch vacuum pumps show up on eBay, in high school and college physics labs, in scientific and engineering research and production facilities, in use and in the storage of neon sign shops, inside commercial flowing gas CO2 lasers, in (bad) Sci-Fi movies, and many other places.

    The specifications below are for their belt-driven ("DuoSeal") pumps. Newer ones will always have a belt guard (some OSHA requirement I assume) but many of the older pumps do not (adding a belt guard is definitely advised in any case). For a given pumping speed, belt-driven pumps run slower and cooler and this increases reliability and life expectancy compared to their direct-drive counterparts. They are also quieter. And, should the motor need replacing for some reason, a standard model from any number of sources or your junk corner will do. The motor in direct-drive pumps (which Welch also manufactures) may only be available from the original supplier, if at all.) I also don't think direct-drive pumps have quite the same aesthetic appeal as belt-driven pumps. :) A testament to Welch's pumps longevity is that these same models have been around for at least 50 years and probably a lot longer. And, those 50 year old pumps are still serviceable, requiring at most a cleaning and relatively inexpensive rebuild kit to meet original factory specifications.

    Here is a chart of the most important specs for a variety of Welch vacuum pumps. More detailed specifications for these as well as Welch's direct drive pumps may be found on the Welch Vacuum Web site. They also have repair parts and kits for all their pumps as well as exploded diagrams (with repair manuals and more coming soon) should your acquisition turn out to need a bit of maintenance or a miracle. :) (See: Basics of Vacuum Pump Repair for a really quick summary of an overhaul.) These pumps and pumps from other manufacturers can also be found in the catalogs or on the Web sites of vacuum equipment suppliers like Duniway Supply or Lesker Vacuum Systems and Components.

                         Ultimate     Pumping     Motor
               Number     Vacuum       Speed      Power
       Model  of Stages   (Torr)    cf/m    l/m    HP
     ----------------------------------------------------
       1399      1       1.5x10-2     1.2    35    1/3
       1404      1        2x10-2      1.2    35    1/3
       1380      1       1.5x10-2     5.6   160    1/2
    
       1400      2        1x10-4      0.9    25    1/3
       1405      2        1x10-4      2.1    60    1/2
       1402      2        1x10-4      5.6   160    1/2
       1376      2        1x10-4     10.6   300     1
       1397      2        1x10-4     17.7   500     1
       1374      2        1x10-4     23.0   650   1-1/2
       1375      2        1x10-4     35.4  1000     2*
       1398      2        1x10-4     53.1  1500     3*
       1395      2        1x10-4     71.0  2000     5*
       1396      2        1x10-4     99.1  2800   7-1/2*
    
       * Motor requires three-phase power.
    

    Notes

    1. The "Pumping Speed" values used here are the manufacturer's free air displacement ratings. Actual performance may be slightly lower. The specs for the 1405 on Welch's Web site show 3.1 cfm (91 l/m) but I list those I've seen everywhere else.

    2. The single stage 1404 and two stage 1375 and larger pumps are not listed on Welch's Web site and are presumably no longer in production but may be available from surplus and rebuilding companies. Various other older belt-driven Welch models appear from time-to-time on eBay and elsewhere but I could not find any information on them.

    3. Even the smallest of these pumps would be more than adequate for most home-built lasers. (The typical commercial flowing gas 50 W-class CO2 laser used a 1399.) The 1402 (and 1380) are really total overkill in terms of pumping speed but the 1402 tends to turn up surplus quite frequently so a good deal on one in decent condition shouldn't be passed up unless you don't have someone to help drag it home! Anything larger than a 1402 is just plain silly for use with home-built lasers. And the 1402 isn't exactly small, weighing in at over 100 pounds! I have listed the big ones here so you will know not to get too excited about those models should you find them in a catalog or auction unless, of course, you just want to have the biggest pump on your block! :)

    4. Each of these pumps is available in several versions depending on factors like the ability to handle corrosive gases - any will be fine for gas lasers (except excimers!). Given the choice between a single or two stage pump with otherwise similar specs, get the two stage model as it should be able to deal with all of the home-built lasers without requiring a diffusion or turbo pump if it is in good condition.

    5. Single and two stage Welch pumps should not be run continuously with inlet pressures above 50 and 10 Torr, respectively. This really shouldn't be a problem since there will be a metering valve between the pump and laser tube. Even for the CO2 laser with the smallest pump, the inlet pressure should be quite low. But, don't let the pump run for hours with nothing connected just because you like the way it gurgles. :)

    6. If some of these pumps are idle for a long time, especially with the inlet left at vacuum when powered down, oil may fill the pump chambers and the first rotation could then have tough spots as the incompressible oil is cleared. But, there is nothing really wrong and they will then run fine. Where this situation is found to be the case, it's a good idea to rotate the pulley first by hand to reduce starting stress on the motor (and slip clutch, if present).

    Rotary Vacuum Pump Maintenance

    So, you picked up a fabulous Sargent-Welch two stage pump at a garage sale for $5 but are now wondering whether it was worth the price and (grunt!) effort to get it home since it won't even suck the air out of a paper bag! OK, this is perhaps a slight exaggeration. :) However, if the pump won't get anywhere near the vacuum expected (less than a Torr for a single stage pump or a few microns for a two stage pump is realistic even if this is not quite as good as what the manufacturer's specs call for), all it may need is a little tender loving care. OK, maybe a lot of tender loving care and possibly a few selected chants and some four letter words! :)

    WARNING: Confirm that any second-hand pump hasn't been used with hazardous or toxic materials without having been decontaminated. If it has, the services of a HASMAT/DETOX center will be required - that's not something you should do in your basement.

    (From: John De Armond (johngd@bellsouth.net).)

    Go to Duniway Stockroom and buy a couple of gallons of flushing oil and a gallon of vacuum oil. Drain all the old oil out, fill with flushing oil and run the pump for a couple of hours deadheaded (e.g., with the vacuum inlet capped) until it's good and hot. Drain. repeat until the flushing oil comes out clear. Fill with regular pump oil and run for several hours deadheaded until the oil is good and hot. If you have a thermocouple vacuum gauge, check the deadheaded vacuum. A used 1402 or 1405 should achieve 3 to 5 microns after the oil has completely outgassed. Outgassing might take a day or more.

    If several flushings won't clear the oil or you can't achieve that vacuum, the pump chambers may need cleaning.

    I also suggest buying Duniway's shaft seal kit. This kit replaces the original crappy seal with a modern tensioned rubber seal. It completely stops shaft weeping.

    These pumps are so rugged that I believe that the very first one, which provided vacuum on the Mayflower, is still running. :-) Everything is rebuildable.

    (From: Ed Phillips (evp@pacbell.net).)

    I recently replaced the seal in a NIB but never used Welch 1400 pump which had been sitting in my attic since I bought it new (for about $150 delivered) back in 1960. The new seal was around $50 delivered. Nice little pump.

    I have a couple of 1405's (or is it 1402? Anyhow, next size up from the 1400) here which have a note painted on the outside indicating that they were overhauled back in May, 1954. They sat around in the back of a CRT rebuilding shop for many years before I picked them up a few years back. They still had a full charge of (very dirty) oil in them. Fired one up, flushed it out a couple of times, and refilled with DuoSeal oil. Using my little old McLeod I indicate a pressure of around 1 micron after a few hours of running. (Gauge connected to the pump through a short section of 1/2" ID rubber tube.) They must have many thousands of hours on them since overhaul. By the way, the shaft seals don't leak on either pump.

    Replacing the Shaft Seal on a Sargent-Welch Rotary Vacuum Pump

    I had to do this for my Welch 1402 so here's the step-by-step (this may vary slightly for other models but not by much):

    Order the appropriate shaft seal from Duniway Stockroom. It's about $25 including shipping which is more than you might think it should cost but the machined cover into which the actual seal is pressed is probably a custom made part. And, this is about 1/3rd of the price direct from Welch! However, if you can machine your own cover or an adapter for the old cover, a seal of the appropriate diameter from an auto-parts store should work just fine. :) I couldn't find a similar seal from the other vacuum suppliers except as part of a rebuild kit which was much more expensive. If anyone knows of one, please let me know via the Sci.Electronics.Repair FAQ Email Links Page. The item listed as "shaft seal" actually includes the seal, gasket, three (3) screws, and instructions but they are not really as comprehensive as what follows. :)

    1. Remove the belt, pump pulley, and Woodruff (half moon) key on the pump shaft. Clean any oil off of the pulleys. If the belt is oil-soaked, it should be replaced. Note: Both pulleys can be pulled off at the same time without the need to move the motor, or just the pump pulley and belt can be removed by loosening the motor mount clamps and lifting the motor off its cradle. But don't try to remove only the pump pulley with the belt still tight.

    2. Drain the oil into a clean container (if it's to be reused) by unscrewing the plug at the bottom of the pump. If oil doesn't come gushing out immediately when the plug is removed, the rubber sealing gasket may not have come off with the plug - just gently pry it out with something pointy. The 1402 should contain about two quarts of oil but draining only a quart is enough to prevent the shaft from weeping while you are working on it. Now's probably a good time to change the oil if you haven't done it in about thousand years. :) Of course, if your shaft seal is as leaky as mine was, it probably already drained itself completely onto the lab floor. :( The Duniway instructions recommend tipping the pump on its back at this point but I don't really think it is necessary as long as enough oil drained. Not doing this also reduces the possibility of getting debris in the weep hole - I'd suggest plugging it up while cleaning the shaft/gasket area in any case.

    3. Remove the 3 screws holding the seal cover in place and remove it and the old gasket. Depending on the original type of seal and how stuck to the shaft parts of it are, a small gear puller may be needed to free it from the pump. Everything you remove is to be discarded anyway (unless you're installing your own non-Welch seal into the old cover in which case it will need to be in good condition and thoroughly cleaned of seal and gasket debris) so don't worry about damaging the pieces but take care not to scratch or dent the shaft! Some parts may still be stuck to the shaft. On mine, the old seal assembly came apart but there was an aluminum ring which had to be freed using pliers in a twisting motion before most of the parts of the old shaft seal would come off. It probably had gummed up oil holding it in place. At this point, only the shaft should be sticking out of the main pump casting.

    4. Inspect the shaft for damage in the vicinity of the pump casting where the seal will need to contact the shaft. Use degreaser to remove any baked on oil and other debris. If this doesn't result in a perfectly smooth shaft, it may be necessary to use 600 grit or finer sandpaper followed by ultra-fine steel wool, very gently polishing in a direction around the shaft, NOT parallel to it. The shaft doesn't need to be mirror shiny but there should be no scratches or dings that will catch a fingernail. Also remove any remaining pieces of the old gasket stuck to the pump so that the seating surface is also nice and smooth.

    5. Inspect the rest of the shaft for damage and carefully sand or file it so there are no sharp edges or protrusions to catch the new seal as it is slid onto the shaft. Slightly bevel the end if it isn't already. Put a piece of cellophane "Scotch" tape over the keyway as a precaution.

    6. Make sure any sandpaper or steel wool particles are removed and take special care that they don't make it into the weep hole above the shaft that enters the pump. Use a cotton swab or something similar to remove debris from the three seal cover mounting holes.

    7. Wipe the new seal clean inside and out. Note that the new seal has two sides which differ. The one with the spring surrounding the rubber lip goes toward the pump; the one with the nylon cover goes toward the pulley.

    8. Add a few drops of pump oil to the area between the two rubber lips. (The Duniway instructions warn against putting in more than 5 drops - I'm not sure if the Universe explodes if this rule is violated but I wasn't about to find out!).

    9. Slip the new foam gasket onto the shaft. There is no need to put oil on it and do not use any sealer.

    10. Put a couple drops of oil on the shaft and smear it around. Carefully slide the new seal onto the shaft using a rocking motion. Remove the Scotch tape. :)

    11. Install the 3 new screws through the holes in the seal cover and gasket just so the seal is snug against the pump.

    12. Check that the seal is centered on the shaft and then tighten the 3 screws equally in a rotating pattern in small increments. Wipe up any spilled oil on the pump and base.

    13. Replace the shaft key(s), pulleys, and belt. Make sure the two pulleys line up! Check the belt tension - it should deflect about 1/2 inch when pressed midway between the pulleys. Adjust it if necessary by changing the motor position.

    14. Let the seal set in for at least 1/2 hour to allow it to conform to the shaft. Duniway doesn't say this but I've read it from other suppliers who warn of early failure if the pump is run immediately. It probably doesn't matter but what's your hurry? :)

    15. Replace or add new oil. DON'T FORGET THIS STEP!!!! Note: The oil level on these pumps needs to be between the two marks in the windows when the pump is running. When stopped, the pump chambers may fill up with oil and the level will be somewhat lower. So, add oil only until the level is at the bottom of the window. Top it off only after the pump is running.

    16. Start up the pump and check for leaks. With even the smallest amount of luck, your pump will be happy for another thousand years. :)

    Salvaged Refrigeration Compressors as Vacuum Pumps

    How many refrigerators, window air conditioners, freezers, and dehumidifiers, have you hauled to the dump or passed up on the curb???? The compressor (using its suction inlet) in these systems may be pressed into service as a vacuum pump where a low (and in some cases, medium) vacuum is acceptable. However, before you get too excited, realize that finding a suitable refrigeration compressor (or more than one to use in series) may be a crap-shoot. Many types are at best marginal even for the minimal requirements of the nitrogen (N2) laser. It's not only a matter of the condition of the the compressor, since they are designed for compression, the vacuum performance may simply be mediocre. Personally, I would go for a real vacuum pump unless cost is the most important consideration and you're just going to run the laser only long enough to see that it works and then move onto some other hobby. :) Almost any real vacuum pump (including refrigeration service types) - even one that has been neglected and abused - will pull a better vacuum than most refrigeration compressors, and will be less hassle and more reliable. See the section: Sources for Vacuum Equipment.

    Having said all that, a used refrigeration compressor will probably be very close to free and that's often hard to pass up! :)

    A detailed discussion of using refrigeration compressors as vacuum pumps is provided in the hard copy version of the Bell Jar. (The Electronic Bell Jar being the subset of these articles that are on-line. Check that site for contact and subscription info.)

    Speaking of hooking two pumps in series, I've even heard of this being done with those pathetic excuses for vacuum pumps used on solder rework stations. Apparently, this approach was adequate to reach the 100 Torr level required by the N2 laser!

    Salvaged Refrigeration Compressor Wiring

    The following applies to a typical GE refrigerator compressor. YOURS MAY BE DIFFERENT! Don't rip out the compressor without making a wiring diagram and saving all the relevant parts!

    The sealed unit has 3 pins usually marked: S (Start), R or M (Run or Main), and C (Common). The starting relay is usually mounted over these pins in a clip-on box. The original circuit is likely similar to the following:

                            |<- Starting Relay ->|<---- Compressor Motor ---->|
               
                     ___            L      
           AC H o----o o--------------+--o/   S    S
                  "Guardette"         |    o----<<-------------+
                   (Thermal           +-+                      |
                   Protector)            )||                   +-+
                              Relay Coil )||                      )||
                                         )||                      )|| Start
                                      +-+                         )|| Winding
                                      |                           )||
                                      |      M    R/M          +-+
                                      +--------<<------+       |
                                                        )||    |
                                               Run/Main )||    |
                                                Winding )||    |
                                                        )||    |
                                                     +-+       |
                                                  C  |         |
           AC N o------------------------------<<----+---------+
    
    

    The Starting Relay engages when power is applied due to the high current through the Run winding (and thus the relay coil) since the compressor rotor is stationary. This applies power to the Start winding. Once the compressor comes up to speed, the current goes down and the Starting Relay drops out. (Some models may use other starting schemes but this is the most common.) You can always use a heavy duty pushbutton switch in place of the starter if you like or if you lost the original starting relay. :-(

    Leave the the Thermal Protector (often called a "Guardette" which I presume is a brand name) in place - it may save your compressor by shutting it down if the temperature rises too high due to lack of proper cooling or an overload (blocked exhaust port or low line voltage).

    Chris's Comments on Refrigeration Compressors

    I have used both refrigerator compressors as vacuum pumps and commercial vacuum pumps in operating my home-built CO2 lasers. Some of the compressors that you come upon will work much better than others in this application. It is advisable to collect a number of these for testing. They can usually be salvaged for free, so this should not be a problem. When you find the 'best' one, I would suggest flushing out the original compressor oil and refilling with a good quality mechanical vacuum pump oil for improved performance. There are a couple of drawbacks to using these types of 'pumps'. One is their tendency to become very hot if operated for extended periods and the other is their tendency to allow large quantities of oil to backflow into the vacuum vessel when shut down. Always clamp off the vacuum line to the laser before turning off the power to the 'pump'.

    However, I would strongly suggest that you purchase a refrigeration service vacuum pump if you can afford the $300.00 or so price tag. These inexpensive vacuum pumps will be much superior for anything except perhaps the N2 laser since they are able to pump to much lower pressures - and with fewer hassles.

    John's Comments on Refrigeration Compressors

    Some of the comments below may depend on compressor type - rotary compressors found in some systems behave differently and have differing requirements than piston compressors.

    (From: John De Armond (johngd@bellsouth.net).)

    Chest or upright freezer compressor - yes, refrigerator compressor - maybe, AC compressor - no.

    Refrigeration compressors are marginal because they depend on the freon flow for part of their cooling. Little to no flow in vacuum service. The ambient pressure around the compressor motor is vastly lower so convection cooling is not available. AC compressors are even more dependent on refrigerant cooling. Besides that, they have a larger clearance volume (space above the piston) in order to limit exhaust valve temperature and this limits the ultimate vacuum achieved to less than what we need.

    Freezer compressors, because they are designed to run at low pressure to low vacuum depending on temperature set-point, will work fine for vacuum service. They have small clearance volumes and they can cool themselves just fine without freon flow.

    Two such compressors in series will provide enough vacuum to backup a small diffusion pump for high vacuum service. I used just such a setup with a homemade linear accelerator I built back in high school.

    Don't use PVC tubing. The plasticizer incorporated to make the PVC flexible has a fairly high vapor pressure and will contaminate everything. The red rubber vacuum hose is fairly inexpensive in small diameter. Surgical tubing can be used if a spring is inserted to prevent collapse. Some automotive vacuum hoses should work but may require a solvent wash to eliminate process lubricants that may out-gas.

    I haven't seen a rotary type refrigeration compressor in so long that I think they're extinct. Most of the tall skinny cans now contain scroll compressors and they suck for vacuum pumps. Or, I guess they don't suck enough. :-)

    I have a couple of compressors I pulled from equipment around the restaurant. I intend to cut the cans open and see if there is anything a home hacker can do to improve the vacuum. Something like perhaps replacing the intake valve with shim stock or grind the head down to reduce clearance volume, etc.

    Vacuum Valves

    Two types of valves are required. Fancy expensive types may not be needed so you may find some of this at your local hardware store or home center. However, since common valves are designed to operate in a positive pressure environment, they may not hold up under vacuum conditions - or they may be fine! In addition, the sealing grease used may outgas at low pressure. Some testing will be necessary to be sure.

    Vacuum Gauges

    Some means of determining the precise level of vacuum is perhaps not totally essential but certainly highly desirable. Otherwise, whatever you do is like a shot in the dark. The old 'thumb over the hole' trick really isn't precise enough!

    Here are some of the types in common use:

    There are many others including: cold cathode, convection, diaphragm manometer, and Penning. Some of the vacuum supplier Web sites like that of Lesker Vacuum Systems and Components have brief descriptions of the various vacuum gauge technologies.

    Electronic vacuum gauges may have either analog (e.g., meter needle) or digital readouts. Digital gauges aren't necessarily more precise as they are still limited by the sensing technique. It's likely that many of them use the same front-end circuitry but add an A/D converter and lookup table before the display. What's the point of 4 digit readout if the accuracy of the underlying measurement is only 10 or 20 percent, as with a thermocouple vacuum gauge?! However, they do excel in the coolness factor. :)

    CAUTION: Any serious arc or discharge that reaches the a vacuum gauge sensor will likely ruin it and blow out portions of the control unit as well as the operator if he/she happens to provide a convenient path to ground! Means should be provided to prevent this from occurring. Metal plumbing in the vicinity of the sensor(s) should be grounded! Locating the sensors in regions of the vacuum system away from electrical pyrotechnics would be highly recommended!

    All types of vacuum gauges are readily available, new, surplus, eBay, and elsewhere. However, before you grab the first one available, make sure it comes with the necessary sensing head(s) - some of these are either expensive or hard-to-impossible to obtain.

    Specifications of Teledyne Hastings Thermocouple Vacuum Gauge Tubes

    Teledyne Hastings manufactures one of the most popular line of thermocouple (TC) vacuum gauges and ontrollers. See Teledyne Hastings Vacuum Instruments. (A gauge is just a readout; a controller has adjustable set-points to activate other equipment, alarms, etc., based on pressure limits. Note that in what follows, I use the term "control unit" to refer to any electronics that goes with the TC gauge tube to implement a basic TC vacuum gauge, usually an analog meter movement or digital display, and a small amount of circuitry.)

    Here is a chart of the functional specifications for many Hastings models. Duniway Stockroom sells some of these under their own model numbers. Other manufacturers like Varian and Veeco may have equivalents. There are two basic types: Those using an AC heater with a single thermocouple (the majority) requiring a bridge circuit for readout and those with the heater and outputs combined. Some older samples of the DV-3M, DV-4D, DV-5M, DV-6M, and DV-8M may not have the letter suffixes but are electrically equivalent (e.g., DV-8 instead of DV-8M).

    Model:
     Metal-case       DV-3M   DV-4AM  DV-4D   DV-5M   DV-6M   DV-8M  DV-23   DV-24
     Ruggedized         -       -     DV-4R     -     DV-6R     -      -       -
     SS/Ceramic         -             DV-34     -     DV-36     -      -       -
     Pyrex-case       DV-17   DV-16   DV-16D  DV-18   DV-20   DV-31  DV-43   DV-44
    
    Pressures (mT):
     Best Sens Range  20-200 200-5k  200-5k    2-20   10-200 0.1-10   5-1k  100-5k
     Useful Range      1-1k  100-20k 100-20k 0.2-100   1-1k  0.1-10   5-5k  100-20k
    
    Volume (cu.in.):   1/2     1/20    1/20    1/2     1/2     1/2    1/2     1/2
    
    Heater:
     Current (AC mA)   125      40      29      30      21      53   40/40   30/40
     Voltage (AC mV)   300     370     320     200     380     320  200/200 190/190
     Power (mW)         37      15       9       6       8      17     16      11
     Resistance (ohms)   2.2     8      11       6      18       6    5/6   6.5/7.5
    
    TC Temperature (°C):
     At high vacuum    260     275     250      48     300     120    400     400
     At atmosphere      15      30      30     1.5       6      10     10      35
    
    TC Output at High
     Vacuum (DC mV):    10      10      10       2      10       2     13      13
    
    Response Time (s):
     Atm->high vacuum    3.3   0.16    0.16     25      2.9     25      3     0.2
     High vacuum->atm    0.12  0.04    0.04    0.8      0.05    0.8    0.07   0.05
    
    Connections:
     Heater           <------------------- 3-5 -------------------->  <- 2-4/6-8 ->
     TC Output        <-------------------- 7 --------------------->    -      -
    
    Base color:        Black   Blue   Purple    Red    Yellow  Green  Orange  White
    

    Note: mT = mTorr = 10-3 Torr = 1 micron of Hg.

    I have made meter face templates for the DV-4, DV-5, and DV-6 TC gauge tubes based on graphics from the Hastings Web site for the VT-4, VT-5, and VT-6 gauges, respectively, but somewhat improved. For the DV-8, I modified one of these based on my NV-8B as best I could by eyeball. :) These may be useful for home-built TC gauges (see the next two sections):

    Modify the size of these as required for your meter, print out on high quality opaque paper stock, and attach to your existing meter. For normal meters without a reversal circuit, they will first have to be mirrored left-to-right with the text adjusted appropriately.

    I don't have calibration curves or conversion factors for any of these tubes (for air or other gasses). If anyone can provide these, please contact me via the Sci.Electronics.Repair FAQ Email Links Page. Select a tube which puts the operating pressure of your intended home-built laser(s) within its "Best Sensitivity" range if possible. Wide range tubes like the DV-4D cover most gas lasers but may not be as accurate or repeatable at any given pressure point compared to a tube with a more limited range like the DV-3M. However, almost any of these models will be better than wild guesses! :)

    Sam's Home-Built Thermocouple Vacuum Gauge 1 (SG-TC1)

    I decided to construct my own ThermoCouple (TC) vacuum gauge after seeing the prices of new units and the final bid prices eBay of used ones. However, in retrospect, I'd really recommend buying a used gauge if the price is less than half of the price for an equivalent new one. It simply wasn't worth the time and effort even though my total cost was under $25. That was entirely for the never used Teledyne Hastings DV-3M TC gauge tube I did find on eBay for less than half the price of the same thing from Duniway Stockroom (DST-01M). In addition, I still need to calibrate the gauge and paint the meter face to reflect its new life as a vacuum gauge instead of a multimeter. I have a template for the meter for the DV-6 tube (Face for VT-6 TC Gauge Meter Using DV-6 Tube) but I don't know if the calibration curve for the DV-3 is the same.

    The first control unit I built is based loosely on the Electronic Bell Jar article Building a Thermocouple Vacuum Gauge. However, for the type of tube I used, that design has to be modified slightly for two reasons: (1) the Hastings TC gauge tube only provides a single sense output for the thermocouple rather than two (which I didn't realize at first, see below) and (2) I wanted to use a single supply (9 V battery) for the op-amp. For the latter, I used a pair of resistors to create a virtual ground - not terribly stable as the ground shifts about 1 V between 0 and full scale on the meter but adequate for this not very demanding application. Almost any garbage op-amp can be used.

    The tube is attached to the meter circuit via an 8 conductor cable (only 3 are actually needed). A separate wire is used to connect the metal case of the tube and plumbing to earth ground minimizing any effects of from RF or other electrical discharges in the vacuum system on the gauge electronics. (Neither the sensor nor the electronics would likely survive any serious arc!)

    The system includes a select switch for vacuum or TC heater (filament) current setting, AC null and offset adjustments for the op-amp and thermocouple, and internal gain adjustments for heater current and TC output.

    While I was able to obtain the basic specifications for the DV-3M, it wasn't clear how to sense the output as only a single pin was listed for the "DC connection" (unlike the pair of pins for both tubes described in the Bell Jar article). Also, the heater is spec'd as AC rather than DC.

    I found another pin that has continuity to it and assumed that was the return but when wired that way, only got about 2 percent of the expected output voltage. After contemplating the meter's utter and total lack of cooperation for awhile, I concluded that the thermocouple junction was between the centertap of the heater and the listed pin, and the return would have to be to both ends of the the heater. So I added a 4.7 ohm resistor to each end of the heater with their common point as the return forming a bridge circuit which will pass the millivolt level DC thermocouple output while cancelling out the AC heater voltage:

                          Ra           Rb
                    +----/\/\----+----/\/\----+
                    |            |            |
                    |            o -          |
                    |        TC Output        |
                    |            o +          |
            AC o----+            |            +----o AC
                    |            |+           |
                    |       Thermocouple      |
                    |            |-           |
                    |         Rc | Rd         |
                    +-------/\/\/\/\/\/-------+
                        TC Gauge Tube Heater
    
    

    If the ratio Ra:Rb is exactly equal to the ratio Rc:Rd, the TC Output will have no AC component; if the ratios are close, there will be some AC but as long as the peaks don't result in the op-amp exceeding its linear range, they will be ignored by the DC meter. Apparently, it was close enough because even without an AC null adjustment (as shown in the schematic), the gauge started to work as expected. Note that this could also have been done by removing the 56 ohm series resistor to the op-amp (-) input and using a 250 ohm pot instead of the 10 ohm pot shown. I decided to leave it alone since with the low value resistors or pot, if a TC gauge tube requiring a much lower heater current (as most other models do) were accidentally installed, it would probably survive, at least at atmospheric pressure since much of the current would be diverted. With the higher value pot, virtually all the current would go through the tube likely burning out its heater almost instantly.

    The diodes across the meter protect it from overload; the capacitor slows down its response so the needle doesn't go banging back and forth when flipping "Meter Select" (S2).

    As in keeping with my "if I have it in my junk box, why buy it" approach to project construction, the power supply was from a dead electronically controlled coffee maker (partly because it already had the relay used to control power to the op-amp) and the meter was from an ancient RCA VTVM which I picked up for $1 at a garage sale (which is nice and big but of course, not labeled in any useful units). It's installed in a nearly pristine aluminum Minibox(tm) from an unidentified past project. The only thing I had to actually buy was the TC gauge tube. Everything else was, well, just laying around taking up space. :)

    The adjustment procedure (below) is the same for both SG-TC1 and SG-TC2 (in the next section) though the setting of the "I Adj." pot should never need to be touched after initial setup for SG-TC2.

    The same basic design will work for all Hastings TC gauge tubes (and other manufacturers equivalents) except the DV-23/24/43/44 which appear to have a different internal configuration (presently unknown). However, changes in resistor values will be needed to accommodate the differences in heater current and thermocouple sensitivity.

    A suitable vacuum pump will obviously be needed. A Bordon tube or similar coarse vacuum gauge and TC (or other) gauge covering the relevant range (especially at the low end) that is calibrated are highly desirable. My test setup consisted of a Pfeiffer Duo 1.5A two stage rotary vane vacuum pump, Bordon tube vacuum dial gauge indicating 0 to 30 inches of mercury, the DV-3M TC gauge tube, and a glass tube RF discharge widget as described below. Flexible thick wall PVC tubing connected the pump to a pair of brass "Ts" for the dial gauge and TC gauge tube. A hose pinch-off clamp enabled the pump to be isolated from everything else. A needle valve to admit air at a controlled rate would have been highly desirable but I didn't have one.

    In order to set some of the adjustments, the system should be pumped down to below 1 micron (or as low as your vacuum pump will achieve). Without a calibrated vacuum gauge, an adequate vacuum may be indicated by using a low power RF source (e.g., small transmitter, hand-held Tesla/Oudin coil, flyback oscillator, etc.) to excite the residual gas inside a glass tube. For air, the glow discharge will go through a range of colors including bright red, pink, white, blue-white, darker blue-white, and eventually disappear once the pressure is below a few microns. A well maintained two stage pump should be able to achieve this. See the section: Sam's RF Discharge Color and Intensity Vacuum Test. The glow discharge also aids in clearing the tube of contaminants and reducing the time to achieve a decent vacuum.

    The controls should be adjusted as follows for both SG-TC1 (all settings assume the use of the DV-3M; modify as appropriate for your TC gauge tube). Any differences in the setup procedure are noted for the SG-TC2. CAUTION: To avoid unsightly burnouts, before attaching the TC gauge tube, set the front panel "I adj." to min and use an AC mA meter (true RMS if available) to confirm that the heater current is below the rated value for the tube.

    Set the Meter Select switch to the "Vacuum" position. The system should be pumped down to a pressure no less than about 100 Torr (about 4 inches of mercury) so that the TC gauge tube is essentially measuring no vacuum. CAUTION: I have heard that running some TC gauge tubes near atmospheric pressure may be bad for them. Thus, having the system pumped down a bit is recommended whenever the tube is powered. I doubt this matters for any of the Hastings TC gauge tubes as their heater temperature at 1 atm is not very much (e.g., only 30 °C above ambient for the DV-3M, even less for some other models). I selected 100 Torr, above, to be high enough that the output of the gauge will be virtually identical to that at 1 atm, but it's not at all critical.

    1. Offset: Set to exactly 0 current through the meter - the reading where the pressure is higher than the useful range of the gauge.
    2. AC Null: Set for minimum AC ripple at the output of the op-amp (SG-TC1 only).

    If these controls were initially far from their correct settings, it may be necessary to go back and forth a couple times between them.

    Set the Meter Select switch to the "Iset" position and allow the system to pump down to below 1 micron (or as low as it will go):

    1. I Adj. pot (front panel): Set so there is exactly 300 mVAC across the TC gauge tube heater. This should correspond to 125 mA through the heater under vacuum. (Both spec'd in the DV-3M datasheet.) It will take a minute or two for the voltage/current to stabilize, be patient.

    2. I Cal. pot: Set for exactly a full scale reading on the meter.

    Set the Meter Select switch to the "Vacuum" position (with the system remaining pumped down to below 1 micron):

    1. TC Cal.: Set for full scale on the meter corresponding to the ultimate vacuum of your system.

      (SG-TC1 only) Since there is no regulation on the heater current, it will fluctuate slightly with changes in power line voltage. Flip S2 between the "Iset" and "Vacuum" and confirm that it hasn't shifted - tweak "I Adj." and/or "TC Cal." if necessary.

    Now go back to step (1) and check and touch up the adjustments if necessary.

    For SG-TC1 as noted above, since the heater current is not regulated, before taking a measurement, flip S2 to "Iset" and check that the meter reads full scale, then touch up "I Adj." if necessary. Changes in heater current will mostly impact readings at the high vacuum-end of the range - those in the middle (100s of microns) won't be affected that much.

    The response of these tubes going from higher pressure to lower pressure (as well as when first turned on or with changes in heater current) is quite sluggish - allow 10 or 20 seconds for the reading to settle. Going from lower pressure to higher pressure is much faster - by a factor of 10 or more.

    Keep in mind that TC vacuum gauges are NOT precision instruments - even with everything adjusted perfectly, accuracies of 10 or 20 percent at best are to be expected. Although the DV-3M can be used between 1 and 1000 microns, its best accuracy will be in the 20 to 200 micron range. (Other model TC gauge tubes cover ranges from 0.1 micron to 10 or 20 Torr.) And, totally different calibration will be needed if what is being pumped isn't plain old air.

    I later hooked up my Welch 1402 to the test setup and confirmed that it achieved nearly as good a vacuum as the Pfeiffer Duo 1.5A, quite good considering it is using a combination of its old oil and the oil from a well worn refrigeration service pump. I haven't yet gotten around to changing the oil. :)

    Note that if a proper low impedance milliammeter had been available, the control circuit could be much simpler, without any active devices at all. Such a circuit is shown in the Duniway TCG Instruction Manual. However, I wanted to use a meter movement I already had and it wouldn't have responded more than 1/25th of full scale at the maximum output of the TC gauge tube. :) The circuit in the manual also includes some regulation on heater current - not perfect, but better than nothing - which I may add to mine in the future.

    I have since acquired a battery powered control unit for the Hastings DV-4D TC gauge tube. The manufacturer of this system uses a Hastings meter but has apparently modified the 115 VAC powered control unit by substituting their own electronics which includes a zener regulator and a 500 Hz oscillator (two transistors, toroidal transformer) to drive the tube from a 9 V battery (pushbutton activated so as not to run the battery down too quickly!) - I added an input for an AC adapter since portability isn't a major concern for home-built lasers! :) An IC regulator (LT1084) provides stable 9 VDC from an 11 to 15 V source. But just in case I need a mobile rig, the battery is still in place - a relay automatically switches to the adapter if AC power is present. (The simpler approach of using a pair of blocking diodes would have dropped the battery voltage by 0.7 V which would greatly reduce its useful life; using the switched contact on the adapter jack would not permit selecting between the regulated 9 VDC and battery.)

    I have also acquired a control unit for the Hastings DV-8 TC gauge tube. This one (genuine Hastings throughout) also uses an oscillator to drive the tube heater with a regulated AC voltage.

    I'm looking for an inexpensive Hastings DV-4D (Duniway DST-04D, Veeco DV-4M), DV-4R, DV-34, or DV-16D tube; and a DV-8 TC gauge tube (or a DV-31 which is compatible) to match up with each of these control units (used if OK as long as it works and doesn't require HASMAT decontamination). The combination of the DV-4, DV-8, and my home-built using the DV-3, would provide nice coverage of the pressure range from 20 Torr to 0.1 milliTorr. Please contact me via the Sci.Electronics.Repair FAQ Email Links Page if you have one available.

    Sam's Home-Built Thermocouple Vacuum Gauge 2 (SG-TC2)

    After becoming annoyed by the fluctuations in heater current resulting from power line voltage variations, and inspired by the two commercial TC gauges I've acquired, I modified SG-TC1 to use an oscillator powered from a regulated source.

    An astable multivibrator drives a step-down transformer to power the TC gauge tube filament. I rewound the vertical output transformer from a mono video display terminal. Any small E-I or toroidal core should be suitable (this was an E-I core about 1" square) and at the operating frequency of around 500 Hz, either laminated iron or ferrite will be acceptable - a test of both types showed virtually no difference in waveforms. The primary and secondary were each wound "bifilar style" (wires in pairs) so the two halves of each winding have exactly the same number of turns. Insulating tape was put between them (mainly for appearance - this is low voltage so there should be no problem with insulation breakdown!).

    The initial setup procedure for SG-TC2 is the almost the same as for SG-TC1 except that "Symmetry" should be set for a 50 percent duty cycle if an oscilloscope is available. If not, just set it in the middle and that will be close enough. There is no "AC Null" adjustment - if the number of turns on each half of the windings of T2 are equal, the only ripple at the output of the op-amp will be due to the unequal resistances in the two halves of the TC gauge heater. This can be checked with an oscilloscope or AC meter that blocks DC. If less than 1 volt p-p, don't worry about it. (For my DV-3M, it was only 0.1 V p-p.) Nothing is perfect and the bit of dither may help a meter with sticky bearings. :) The op-amp has plenty of head-room. Of course, the "Iset" position of the Meter Select switch is just for confirmation as the TC gauge heater current should never need to be adjusted after this for the remainder of the lifetime of the existing Universe. :)

    Home-built Closed-End Manometer Gauge

    (From: John De Armond (johngd@bellsouth.net).)

    Many vacuum systems use U-tube manometers. While the U-tube is accurate, it requires a vacuum cock to allow zeroing and if one accidentally pumps down the system with the cock closed, it will likely blow the manometer oil into the vacuum manifold. The closed-end manometer requires no such manipulation because its reference is the (near) perfect vacuum in the closed arm.

    One such design is shown in Closed-End Manometer Gauge.

    The fluid movement in each leg is proportional to the ratio of inside diameters. This manometer and its scale is designed to be built with standard neon sign lead glass tubing. If you change tubing, you will have to recalculate the scale division. Ditto if you use a different fluid.

    This manometer is designed to be used with Dow DC-704, a diffusion pump fluid available from Duniway Stockroom. This fluid has a specific gravity of 0.975. Octoil and other similar fluids will also work but may be more hygroscopic and more viscous. I added a drop of butyl oil to give it some color. Any analine dye would also work but it would outgas a bit.

    My scale is etched on a piece of steel. A hunk of flexible magnetic plastic is glued to the mount. This lets me slide the scale as needed to zero the manometer.

    To fill the manometer, fill the 15 mm leg about half-full of fluid. Then connect it to a vacuum of about 100 micron or better. Tilt the manometer so that the small leg is up. Make sure there is an open path from the small leg to the vacuum. The better vacuum at this point, the more stable the zero. Rotate the manometer upright. Release the vacuum. The fluid should rise completely to the top of the closed end. If it doesn't, apply as high a vacuum as you have and heat the closed end bulb with a heat gun. This should drive out most of the remaining air.

    Theoretically the two legs should balance with a high vacuum applied but it rarely happens that way. The manometer fluid will outgas some moisture and there will be some trapped gas in the closed leg.

    My manometer is attached to my brass plumbing fitting manifold using a modified compression fitting. Simply take an appropriately sized compression fitting (7/8", I think), remove the ferrule and replace it with a couple of Viton O-rings. Grease the O-rings with vacuum grease. Then simply slide the 15 mm leg of the manometer into the fitting and finger tighten.

    To operate, pull as high a vacuum as possible on the system. Slide the scale so that the zero line aligns with the fluid level. Then measure the pressure.

    Measuring a Vacuum Without a Vacuum Gauge

    If you are dealing with a decent vacuum, there is no real choice - a proper vacuum gauge will be needed. See the section: Vacuum Gauges. However, to check the capabilities of a compressor hooked up backwards using its suction input or some unidentified stationary object that looks sort of like a vacuum pump, there may be an alternative.

    The easiest way is to obtain a cheap Bordon tube type vacuum gauge. An auto parts store should have something suitable for $15 or less. Or, a pressure gauge used backwards. (Though these won't do much good below 5 or 10 Torr - which is where the important action is for many of the gas lasers.) See the section: Vacuum Gauges.

    However, if it is 3:00 AM Sunday morning and you just have to get that N2 laser going, there are other options:

    While I don't know of any convenient household gadget to act as a vacuum gauge, you can determine the level of the vacuum indirectly. A vacuum-safe vessel (e.g., your laser tube if you don't mind getting it full of water, a small thick-walled flask, or even a pickle jar), length of vacuum hose, and shutoff valve, will be needed:

    WARNING: See the section: Safety Issues in a Lab for Home-Built Lasers for comments on vacuum safety especially if using a container not designed for vacuum service!

    1. Connect everything together with the valve as close to pump as possible.
    2. Pump down the vacuum-safe vessel as best you can.
    3. Close the valve to the pump.
    4. Disconnect the vacuum hose and put it in a sink full of water.
    5. Open the valve and let the vacuum vessel suck in the water until it stops.
    6. Measure the relative amount of air and water inside (including the hose).

    The ratio of (air:total volume) is equal to the ratio of the (vacuum:1 atm). Of course, you if the vacuum-safe vessel was your laser tube, it is now (mostly) full of water! :(

    This isn't going to be very accurate for vacuums near 1 Torr or below, but for the N2 laser or dye laser flashlamp, it may be an adequate test to determine if you need to go find a better pump!

    Somewhat similar approaches can be used to determine (destructively) the pressure inside things like light bulbs and flashlamps - break them in oil under an inverted container to catch the gas bubbles and compare the bubble volume to the total volume of the (former) lamp or whatever.

    Here is another simple alternative. Just make sure the water can't be sucked into the pump!:

    (From: James Sweet (jamessweet@hotmail.com).)

    What about taking a long tube stuck in a bucket of water and dangle it out a high window. Connect the end to the vacuum and measure how high the water rises in the tube, that should measure fairly accurately. Mercury would make it more practical, but a lot more dangerous.

    Discussion on Cheap and Dirty Vacuum Measurement

    I was thinking that for a quick test of a vacuum pump or system, a DC or RF source to excite a discharge inside a glass tube could be used to provide an indication of the pressure based on brightness, color, and pattern of the glow.

    (From: John De Armond (johngd@bellsouth.net).)

    I don't think that can be done reliably because the various glow points are so highly dependent on electrode shape and electron emissive coating (if present). In the same batch of neon electrodes, I've seen some that would go out at 7 or 8 micron and others that would still be glowing faintly when my micron gauge zeros. Even a little tramp radioactive material on the electrodes or near them will greatly affect the characteristics of the tube.

    FOR A GIVEN TUBE I believe that the glow points would be repeatable. That's why many old neon guys got away with using that for the vacuum gauge. That plus neon will take a lot of abuse as long as there's no hydrocarbons in the tube.

    A TC gauge tube is fairly cheap - under $40 from Duniway. Even if someone has to build a homemade readout or even use a DVM, that seems like the best solution for the money challenged. For the REALLY cheap, I think a better solution would be a Pirani gauge built from an industry standard lamp, say, an automobile tail light bulb. The Pirani gauge is a thermal conductivity gauge. Just put the filament in a power Wheatstone bridge so that it is lit a dull red. Preferably with a duplicate bulb with a high vacuum in the other leg for ambient temperature compensation. Seal some tubulation tubing to the bulb envelope to connect to the vacuum system. A Pirani gauge won't go quite as low as a TC gauge without fancy electronics but it will go a little higher, handy for some laser projects.

    The old book (Lindsay reprint) "A Manual of Vacuum Practice" by L. H. Martin and R. D. Hill outlines briefly making Pirani gauges. They used platinum wire, as I recall, but a lamp should work well. You can get the book from the Society of Amateur Scientists.

    (From: Sam.)

    If the tube dimensions and electrode material were standardized, then while not a substitute for any sort of real vacuum gauge, this should be able to indicate the pressure to within an order of magnitude or better at least. :)

    For example, how about a 1/2 inch diameter glass tube 18 inches long excited by a flyback based RF source such as that described in: Simple High Voltage Generator with 2 inch wide aluminum foil electrodes wrapped around the tube near each end. (Note that a flyback transformer without a built in HV rectifier must be used to produce an AC field.) A low power (e.g., 5 W) true RF source would also work. Such a scheme eliminates the need for glass working to fuse internal electrodes into the tube and also eliminates the issue of the type and shape of internal electrodes. The tube could be installed on the pump's inlet to test just the pump or in any convenient port of a complete system.

    Sam's RF Discharge Color and Intensity Vacuum Test

    I put together a chart of discharge appearance versus estimated pressure using a two stage rotary vane vacuum pump (Pfeiffer Duo 1.5A), my home-built TC vacuum gauge, and a lot of wild guesses. See: Appearance of RF Discharge in Air at Various Pressures. These are based on an 18" length of 1/2" O.D. glass tubing sealed at one and attached via a short length of PVC tubing to a brass "T" with the TC gauge tube, and then via more PVC tubing to the pump inlet. Aluminum foil electrodes about 2" long are wrapped around the glass tube near each end and excited by my "Simple High Voltage Generator" found in the document: Various Schematics and Diagrams (with the HV rectifier removed so the output is RF AC). It is the same circuit shown in Flyback Based RF Source. None of the tube dimensions or the specific RF source or its output power fundamentally affect the general appearance of the glow discharge. CAUTION: At higher pressures, there may be leakage of the RF into the TC (or other) gauge. This doesn't seem to cause any immediate damage but will confuse the electronics and may be bad if left on continuously. After noticing the meter needle moving even with the gauge turned off, I attached the metal case of the gauge tube and its plumbing to earth ground which mostly eliminates any noticeable effects.

    CAUTION: Any serious arc or discharge that reaches the TC gauge tube elements will likely ruin the tube and control unit. Thus, the need to ground the plumbing! Locating the sensor in regions of the vacuum system away from electrical pyrotechnics would be highly recommended!

    CAUTION: This chart is very approximate and may be off by an order of magnitude or more in pressure! At intermediate pressures, there may also be striations moving back and forth along the tube. At very high and low pressures, there may be variation of color along the tube and near the electrodes. Residual water vapor or other compounds will affect the appearance, possibly quite significantly. These are not photographs but my attempts at rendering the appearance in LVIEWP and MSPAINT. :) If anyone has additions or corrections, or has a vacuum system with a calibrated vacuum gauge and can take some decent photos of an RF discharge at various pressures, please contact me via the Sci.Electronics.Repair FAQ Email Links Page.

    Vacuum Tubing

    The flexible tubing that is used to interconnect various parts of the vacuum system must satisfy several requirements:

    When in doubt, test a length (e.g., a meter) by comparing the lowest pressure achievable with your pump(s) capped by the vacuum gauge and with the tubing in place. The final pressure should be identical.

    Vacuum Seals and Sealers - Red Glyptal

    Three types of material are used depending on the particular needs: Scientific and vacuum supply companies should carry all of these and other suitable products. Your needs are quite modest compared to say, the CRT industry, so there is no need to go overboard with ultra high vacuum sealers. None of these lasers require anything beyond 10-3 or 10-4 Torr anywhere in the vacuum system so the stuff that is guaranteed to 10-10 Torr is probably a bit of overkill (but won't hurt except in terms of cost).

    (From: John De Armond (johngd@bellsouth.net).)

    Teflon's OK to about 50 microns (.05 Torr). Below that, Teflon outgases and permeates too much to be useful. I use high vacuum epoxy. You can buy it from Varian for about $30 per tube. Or you can buy Hysol Engineering's Epoxi-Patch from any industrial supply store and many hardware stores for about $4 a package. Same stuff. Once cured, it's good to to least 10-6 Torr and probably lower.

    For lasering and neon, you really don't need fancy vacuum fittings unless they're free. Soldered copper pipe works well down to about 10-5 Torr or so where the tin in the solder starts to show its vapor pressure. Cadmium free silver solder will go another decade or so. I was recently at the Smithsonian and saw the first cyclotrons. Interesting to see them silver soldered together for pressures in the 10-8 Torr range or better. All that stainless is pretty and stone cold reliable but not necessary for experimenting.

    Leaks and Leak Testing

    For our simple vacuum system, leak testing is usually self evident - there are only a few places where leaks can develop. Using new decent quality components (not garden hose or 30 year old rubber stoppers!) and care in assembly with proper sealers will avoid most leaks. However, note that very small leaks (by the macroscopic standards most people are familiar with) can result in mediocre performance even for these low to medium vacuum systems even if your pump can go down to 0.1 micron.

    Note that failure of a vacuum system to reach an acceptable pressure even with a pump in most excellent condition may be a result of many factors, only one of which are actual leaks (i.e., holes in the system somewhere). Adsorbed and absorbed materials, organic contamination (e.g., fingerprints), and trapped gases in screw threads or other crevices, can all result in slow pump-down but will go away if you wait long enough. Thus, if the pressure continues to decrease on average (there could be times where pockets of "stuff" are released and pressure may rise slightly), one or more of these fake leaks may be present. :) Once successfully pumped down, if the system is allowed to lose vacuum, immediate pump-down will probably be much faster. However, let it sit at atmosphere for any length of time, and all those undesirable nooks and crannies will fill up again. Even a short length of tubing or a single threaded fitting can trap a lot of contaminants which will become a pregressively greater problem at pressures below a few hundred microns. Use of inappropriate materials for seals and tubing can of course also result in poor performance.

    The following deals mostly actual hole-type leaks. :) There are a number of approaches to leak testing:

    And, not all leaks are undesirable. Controlled leaks are used to admit precise quantities of gases into vacuum systems and in particular, gas lasers. For more than you probably wanted to know about leaks, see the Electronic Bell Jar article: Leaks: The Good, the Bad, and the Ugly.

    A Typical Vacuum System

    For gas laser work, a suitable 'minimalist' vacuum system might consist of: Also see: A Simple Medium Vacuum System for some additional ideas on a low cost approach to a setup that may be adequate for laser construction.

    A Small High Vacuum System

    I found a more or less complete self contained high vacuum system (well, at least the pumps) in a pile of trash headed for the dumpster. (There was also a Sargent-Welch 1402 two stage rotary pump in somewhat bedraggled condition which needed a replacement shaft seal but that's a separate story.)

    The system consists of:

    Everything is mounted on an angle bracket frame. Unfortunately, there was no vacuum gauge which is what I really need to evaluate the condition of this equipment. I did find a nice brand-new Varian ion gauge tube in the same pile of trash but no controller. :(

    The complete manual for the DUO-1.5A can be found at the Pfeiffer Vacuum Web site. However, I have not been able to locate documentation on the TPU-050 though there is a manual for a TPU-062 which appears very similar.

    Sources for Vacuum Equipment

    As noted elsewhere, your local home-center, hardware store - and scrap yard, can provide everything from plumbing fittings and tubing to refrigeration compressors. However, to find serious vacuum pump equipment, you may have to buy from a scientific or industrial supply house or surplus outlet. There you will find vacuum pumps of all types and sizes, vacuum gauges, vacuum fittings, valves, sealers, etc.

    However, there is no need to buy vacuum pumps or accessories like vacuum gauges new (vacuum pump oil is another story!). Here are some options:

    But, how do you determine the condition of a pump from a photo or description, or even looking at it without testing its operation? The quick answer is: It's difficult. A pump in a totally a dirty, greasy, sorry state could work perfectly. One that looks brand-new my have some major mechanical problem which caused it to be replaced. Obviously, if the owner or seller has tested the pump for ultimate vacuum and offers a guarantee, that is better than one that is sold as-is. Only rarely will there be an indication of a problem like a tag which reads: "This pump leaks oil all over the place". And that in itself isn't really bad - oil seals are replaceable. Even without a tag, a pump that has had its oil drained may operate perfectly having been taken out of service only because it leaked oil all over the place!

    Note that I have picked up vacuum pumps at garage sales including a somewhat bedraggled but very usable refrigeration service pump for $15 (it looks like a Precision Scientific D25 - the model number isn't legible, but the name on it is Madden Brass Company, a division of Robinaire) and a Sargent-Welch 1399 in mint condition for $8. But, I passed up a larger two stage rotary pump at a garage sale (Darn!) probably because it was too heavy to move (it would really have been only $5!). I've also found a high vacuum rig including a Pfeiffer Duo 1.5 rotary vane pump and TPU-050 turbo pump in the (University) trash (see the section: A Small High Vacuum System) along with a Sargent-Welch 1402 (which just required a new shaft seal). I've confirmed that all the rotary mechanical pumps operate close to their new specifications (even with old worn out oil except for the Duo 1.5A for which I changed the oil because it looked dreadful). I haven't yet tested the turbo pump other than to confirm that it starts to spin. And, I just found another 1402 in the university trash, very dusty but apparently in otherwise good condition! I suspect my basement would be full of vacuum pumps if I lived near a monthly high tech flea market. Come to think of it, my basement is already full of vacuum pumps! :)

    Considerations for Gas Laser Filling/Regassing

    To a large extent, the life expectancy of a HeNe or other low pressure gas laser will be heavily dependent on the cleanliness of the interior of the tube and all its constituent parts and the purity of the final gas fill. Therefore, while it may be possible to use a marginal vacuum system and less than super pure gasses with common basement workshop conditions to get a home-built laser to work for a short time, don't expect optimum output power or stability and a useful lifetime for such a tube if sealed off to be more than a few dozen hours, if that!

    The following comments come from someone who has experience with both HeNe and ion laser refurbishing:

    Letting a HeNe laser tube up to air outside of a inert gas glove box is not a good idea. A class 1000 or better clean room environment with HEPA filtered inert gas is really needed.

    The main things that determine your power are: correct Brewster window angle and material (for external mirror tubes), ultra clean optics, and clean gas fill. We're talking cleaner then cleanroom clean - better than the best surgical suite - semiconductor manufacturing type clean!

    For an argon ion laser the same cathode is often used on a 1 watt tube as in a 20 watt one - same size made by the same company. A cathode isn't just a helix of pure tungstan wire. It has a lot of different compounds sintered into it when its made and is more like a hollow sponge if you look at it under an electron microscope. What's in the sponge is a secret and what you have to do to get the sponge in the right shape for use is another secret. Remember, chemicals and elements can undergo changes in state or composition with changes in temperature and pressure.

    Its not as easy as the Scientific American stuff says it is, Vacuum processing is an art for a SEALED OFF tube that is going to last awhile. In addition to the actual laser gas or gas mix, there is often xenon, oxygen, and hydrogen on the pump station and each has its own use in cleaning up a tube. For a tube that never leaves the pump, like most home-built lasers, you can put in a fresh fill and flush away dirt. But that sponge in a commercial tube is not so willing to give up all its dirt so certain things have to be done with different gasses to make it clean up and emit electrons. How that is done is the real heart of cleaning up a tube. And, the tube gets temperature cycled up and down almost a thousand degrees several times so what you can't pump out you burn off.

    The Scientific American Lasers can be built for around perhaps $1,000 each using scrounged equipment. However, their lifetime is measured in hours and they can tolerate leaks, impure gas, etc. But you can never turn your back on them or leave them alone as they require constant attention or they will fail. A basic vacuum/gas fill/bakeoff station to do a commercial tube might cost $300,000 if bought new but the resulting laser tube will last years. That's a BIG difference.

    If you were ever to crack the seal on a commercial tube, you probably never could seal it off again, so at best it's an experiment. That's the difference between research/play and work. Thus, don't try this if you only have one tube to risk!

    Mark's Notes on Vacuum Systems

    (From: Mark Dinsmore (dinsmore@ma.ultranet.com).)

    Just thought I would throw in a bit of my experience with vacuum systems. My job involves making miniature thermionic cathode X-ray tubes, so I've had my share of vacuum traumas.

    First note - be aware that the high pressure electronic vacuum gauges may have a verrrry different calibration when you get helium involved. If this is the same type I'm familiar with, they work by sensing heat lost by convection and conduction through the gas. Helium has a phenomenal heat conductivity, and can throw of the calibration by orders of magnitude. Second, be aware that all it takes is ONE arc into your gauge, and there goes the controller and probably the sensor (voice of experience).

    If you want to use the electronic gauge, you need to calibrate it against a closed-end manometer using the laser mix gas. Helium is a very good heat conductor, while carbon dioxide is a better insulator than air. With so much helium, I think that it's conductivity will dominate.

    You can use this heat conductive quality of helium, and its much greater conductivity through small orifices, to trace leaks. Just pump your system down as low as it will go with your electronic vacuum gauge hooked up, and spray a little helium at all the suspicious points in your system. If you see the needle on your vacuum gauge twitch, you're close to the leak. If you see a slow rise, it may be permeation through rubber or some elastomer. Helium has orders of magnitude more permeation through these substances than air.

    You also can CAREFULLY (no flames or high voltage on!!!) dribble a little pure isopropyl alcohol over the suspect joints. The alcohol will penetrate the fissure, freeze on expansion, and temporarily seal the joint. Wait a few minutes, and the leak will reappear.

    Inexpensive Filters for Home-Built Gas Lasers

    The suggestion below is perhaps a bit of overkill and definitely messy. Alternatives include the absolute sub-micron filters in (dead or obsolete) hard drives and I would think that cotton or filter paper should actually be more than adequate.

    (From Daniel Ames (Dlames2@msn.com).)

    Since most of us amateur laser constructors do not have a class-5 clean room to build our lasers in, here is a tip or option for helping to keep those home built (gas) lasers cleaner inside the cavity.

    What is really needed is a very fine filter just before the gas inlet or port to the cavity, but the commercial ones are very expensive, so if you need one on a budget, look towards yours or your neighbor's ink jet printer cartridges - the empty ones.

    So, what's inside an ink jet cartridge that can be used for a home brewed - gas laser? Glad you ask, the answer is ultra fine (round) screen like filter, one large one in the black, and 3 or 4 in the color cartridges. My Lexmark printer's cartridges contain one 11 mm diameter filter in the bottom, and the color cartridge contains three smaller ones, about 6 mm diameter. The mesh size of these ink filters are so incredibly fine, that the beam from a 7 mw HeNe will not pass through it. Their appearance looks like stainless steel. Note: many ink cartridges use the water-soluble ink, this works well for cleaning off the old ink.

    They come out of the cartridge pretty easy, just be sure to wear some disposable gloves and do this when either your wife, husband or mother is not home. :)

    First, you'll need to pull out the ink sponges by popping the top cap off the cartridge.

    Rinse out the remaining ink.

    Now cut the bottom of the cartridge off, below the bottom of the ink well to expose the hole below the ink filter.

    Then, insert a blunt object into that hole in the bottom and gently push the filter out. It seams like they are slightly fused onto their mounting post. There will probably be a little residue where it was secured to the plastic mounting, this can easily be cut off, nice and clean by sharpening one end a round metal tube that has the ID that you need. Then, just lay the filter on some hard wood, or plastic, (not the kitchen counter) center the tube punch coaxially with the filter and tap the tube with a hammer to cut the filter.

    As for how to mount this newly found ultra fine filter, you could push it into a metal tube, then using a tubing cutter, like the ones used for refrigeration & air conditioning, make two genital indented rings on the tube, one on each side of the filter and as close as possible to it. I'm sure your creativity can come up with more ways to install this filter into your lasers.



  • Back to Amateur Laser Construction Sub-Table of Contents

    Introduction to Glass Working

    Laser Tube Fabrication

    The following mainly applies to the traditional gas lasers like the HeNe, Ar/Kr ion, HeHg, and possibly CO2 where the entire laser discharge tube may be a single glass structure - it is made in one piece from various individual pieces that are fused together. The N2 laser does not require glass working of this type.

    As laboratory apparatus goes, what you need for any of these lasers is pretty mundane: A few tubes joined together with butt or tee joints, a few dimples or bumps, some angled cuts, and pieces attached with glue.

    Note that at least in principle, it is possible to construct these lasers without actually fusing glass pieces together as Epoxy or other adhesive and/or vacuum rated flexible tubes and clamps can be used. However, such a structure is not nearly as stable and are not recommended. In addition, the added nooks and crannies of clamped pieces and places with glue that can outgas mean that achieving the required level of vacuum and maintaining it is much more difficult.

    There are basically two ways to go about obtaining the needed assembly of tubes, electrodes, Brewster windows, and so forth.

    1. Have someone else do it! Assuming you can find a cooperative individual or pay a neon sign shop or laboratory equipment fabricator, this is by far the easiest especially if you have to start from ground-zero. For someone at all experienced in this sort of stuff, the assembly of the main portion of a typical laser tube (not including the Brewster windows) is a 20 minute job if all the bits and pieces are available. If you can arrange that your design uses the same sizes and types of glass tubing that they commonly deal with, so much the better.

    2. Learn enough of the skill of glass working to do it yourself. This is by far more fun and who knows, maybe you have a talent for the sort of glass art exhibited in museums. This really isn't as difficult as it might seem at first. Glass working is a skill and you will no doubt create some pretty interesting failures at first. But with a little practice (OK, maybe a lot of practice!), butt and tee joints, and dimples and bumps will become second nature. We aren't talking about fancy decorative glass blowing, mostly just basic cutting and joining.
    Glass working as it relates to laboratory apparatus fabrication has been covered in the Amateur Scientist columns as well. See: Glass blowing, technique explained, Scientific American, May, 1964, pg. 129.

    A recommended book on this topic is:

    In sections below provide the briefest of introductions to the glass working skills that are needed for laser tube frabrication. But, here's a Web site with extensive material on scientific glassblowing or glass working (same thing): It includes information on safety, equipment, materials, and terminology; an extensive illustrated tutorial on basic techniques, tips, glass recipes, useful data and tables, and more.

    Types of Glass

    What was call glass is made from silicon dioxide (SiO2) and other additives to produce the wide range of properties of various glasses that we are familiar: from window glass to Pyrex cooking and labware; colored glass bottles and stained glass windows, light bulbs of all shapes and sizes; and optical glass for lenses, mirrors, and prisms. SiO2 is the same stuff that constitutes beach sand and the insulating layers of integrated circuit chips.

    Glass is an amorphous material - it has no crystalline structure and is really a liquid at room temperature. A liquid, you say???? Well, a slow moving liquid at least. As its temperature is increased, glass becomes softer but has no distinct melting point (compared to water, salt, or any other material that forms a crystalline structure where there is a distinct phase transition from a solid to liquid state).

    The two types glass which will be of most interest for laser construction are:

    Fused silica (Vycor) and pure quartz are two highly heat resistant materials that you hopefully won't have to shape since they have even higher softening (or in the case of quartz, a crystal, melting) points as well!

    If you order common laboratory glass tubing, it will likely be made of S-L glass though other types are also available - make sure you specify what you want since for some of the laser parts, heat resistance is an issue. Most beakers, flasks, and anything else that may be heated are made of Pyrex or the equivalent B-S formulation of another manufacturer. However much other labware is of the S-L variety. Since the coefficient of thermal expansion also differs for the two types of glass, there may be problems in attempting to mix them in a given structure.

    Cutting Glass Tubing

    For small tubes - say less than 1/2" in diameter, cutting is, well, a snap!

    All you need is a small triangular file (new or in excellent condition, not rusty and clogged with something disgusting) and perhaps some spit. :-)

    Sometimes, wetting the filed location with a bit of spit or tap water will aid in the process.

    Practice on some scrap pieces of tubing. In on time you will be turning all the neon tubing in your neighborhood to small bits suitable for making beaded necklaces!

    This also works for larger diameter tubing (like CRT necks) but a longer crevice may be needed - try to keep it straight. In some cases, one pass all the way around will be needed.

    There are also hot wire cutters (the heated wire produces local stress which fractures the glass). For large or irregularly shaped objects, the best tool is a power driven diamond grit glass cutting wet wheel - a water cooled miter saw for glass and ceramics! For small pieces, a Dremel tool (compact high speed multipurpose hand-held grinder/saw/sander/drill) can be use though you may go through (non-diamond coated) cutting wheels rather quickly. :( Dentists have nice high speed drills with diamond impregnated bits, cutters, sanding disks, buffers, and other widgets - and they are water cooled! Perhaps, you know a friendly dentist? "Please be patient Ms. Jones, we'll get to your root canal as soon as we finish cutting these glass pieces for Jerry's laser". :)

    Any sharp edges left by the cutting operation should be smoothed with fine sandpaper or in the flame of your glass working torch.

    Glass Working 101

    All glass working consists of four steps:
    1. Heating. This is going to be done with a flame of some kind:

      • A common propane torch or natural gas burner using air is just hot enough to soften S-L glass. A bunsen burner works - barely. Other types of lab burners are better.

      • With the use of pure oxygen, the flames from these all run much hotter and that is what you is really needed to be able to do any sort of glass work easily and consistently (or borosillicate glass at all).

      • An oxy-acetylene or oxy-hydrogen torch will be needed to easily deal with some types of heat resistant glass and fused quartz. (CAUTION: Hydrogen flames tend to be invisible!)

      At the proper temperature, glass has the consistency of soft taffy - easy to bend and shape but not so soft that it runs or drips. Part of the skill (and fun) is keeping the glass at just the right temperature as it is worked. As the glass approaches the proper temperature, the flame will take on a yellow tinge from the sodium ions in the glass (the soda part) and the glass itself may appear red or orange-hot itself.

    2. Working: Bending, joining, pulling, dimpling, blowing, etc. is done while the glass is maintained at a relatively constant temperature in the flame. Glass cools quickly so repeated or constant heating is needed. Some positive pressure in the glass parts may be needed to prevent them from collapsing - or to blow bubbles! The surface tension of the soft glass is going to be both our friend (since it will help smooth out much of the damage you will inflict) and foe (since it will tend to want to cause tube ends to collapse or other holes to expand). Usually two hands and a mouth (safely at the end of a length of rubber tubing!) are enough but at times you might wish to be an octopus!

    3. Once the particular joint or whatever is formed to your satisfaction, the piece must be cooled so that it solidifies. However, you cannot just dunk the whole affair in a bucket of water as the sudden temperature change will cause your hard work to shatter into a million pieces (sometimes it will do this even without such help!) It must go through a process of annealing where a lower temperature flame is run back and forth over a large area of the glass - beyond that which was dealt with originally. The cooler flame can be obtained by reducing the air or oxygen supply to the torch. Fortunately, this takes only a couple of minutes for anything we are interested in constructing (unlike the 17 foot diameter Palomar telescope mirror which required over a year of annealing). Note that there is no real way of knowing how much annealing is enough - it is just something that one does based on recommendation or experience.

      Surrounding larger pieces with warm vermiculite after flame annealing will further slow the cooling process. Vermiculite is ground up mica and is sold in garden shops. :) This is probably not necessary for the sorts of things needed for home-built lasers but one never can tell where your activities will lead!

    4. Cooling. The worked and annealed area will still be very hot. Set it down on a non-flammable material or better yet, in such a way that the hot parts do not touch anything until it is cool enough to touch. This allows it to cool slowly and uniformaly, further minimizing the chances of stress cracks.

    Gas Flames

    A gas flame (natural gas, propane, etc.) adjusted for hottest temperature (optimum fuel:air ratio) is divided into several parts:
    
             Tip---> /\ (Dark Blue)
                    /  \
          Cone --> / /\ \ (Light Blue)
                  | |  | |
                 _|_|__|_|_
         Burner |          |
    
    
    Great diagram, huh?

    Note that it is mostly shades of blue - there should be minimal yellow or orange (indicating that there is adequate air/oxygen) but the flame should not begin to separate from the burner (indicating too much). There should be no smoke or soot from such a flame.

    The hottest location is just above the inner cone.

    With soda-lime glass, once the glass is hot enough to work, the flame will take on a yellow color due to the sodium ions in the glass.

    With the air/oxygen supply cut off, the flame will be long and yellow and may produce black smoke and soot. This will be the proper temperature for the annealing step.

    Note: Where you have control of the air/oxygen supply as with a professional glass working torch (or Oxy-Acetyline welder, for that matter), light it up by first opening just the gas supply a small amount and then adding air/oxygen and adjusting gas flow after the flame is lit. Shut down in the reverse sequence. This avoids unsightly pops, bangs, and other explosive behavior. In other words, always make sure the gas is turned on first and shut down last!

    Glass Working Examples

    The only way to really be come proficient at this is to practice. You will create many many interesting disasters at first but glass is cheap. After a while, these sorts of 'simple' procedures will become automatic and second nature. Who knows, even your failures may find a place in the Museum of Modern Art!

    Home-Built Glass Working Lathe

    Even with a lot of practice, it is difficult to make consistently high quality joints between various size glass tubing and other parts free-hand. A glass working lathe is a fixture that permits the parts to be mounted in such a way that they can be rotated together in the gas flame(s) while maintaining precise alignment and leaving at least one hand free (the other would do the rotation - this needn't be motorized) to do other things. It probably doesn't make sense to acquire or build a glass working lathe for the fabrication of a single laser tube but if several more complex lasers are in your future, it might be worth considering.

    A commercial glass-working lathe is a fairly expensive device that can't be justified for sporadic and limited hobby work. However, A Home-Built Glass Working Lathe provides a short description of a piece of equipment which can be used in the fabrication of the plasma tube, water jacket, and other glass components for a variety of lasers as well as for other scientific glassware applications.

    Learning Glass Working Skills

    (From: John De Armond (johngd@bellsouth.net).)

    One way to start is to subscribe to a couple of email discussion groups related to glass working. There are 2 lists I'm involved with:

    See the section: Laser (Email) Listservers for other possibilities.

    The list discussion tends to be esoteric. There's no substitute in learning to blow than to do it! I taught myself to blow glass so it can be done. First, get the books mentioned in my article on the web page. Then find a neon shop and arrange to spend some time in the shop. Most shops will let you watch for a bit but it's only proper to compensate the benders for their time. most benders work on piecework so time = money. When I first got started, I slipped a bender in Atlanta a C-note to let me tag along behind for a day. I had his undivided attention. This is VERY important. The early motor skills you learn will be the foundation that you build on with practice and if you learn incorrect basics, it will be hard as hell to correct it. It is also important to understand that there are some people whose motor skills will not let them learn to blow glass. I've seen people who went through $6k, 9 week neon bending classes and not be able to do a 90 without it flattening out. You can usually tell within a few hours' work. If you end up being one of those people, just live with it. You'll waste a lot of money and make no progress. Avail yourself of someone who can.

    After you progress to one level, don't be afraid to spend a little money for a day of tutoring from a master. I've been bending for several years and I still try to go a couple of times to someone better than me and study under them. Now you can usually get someone to do it for free but things are a lot more clean if you simply pay the expert for his expertise. You'll profit in the long run.

    Here's Someone Who Has Offered Glass Working Services

    Here is a possible way of getting your custom glass work done. I do not know what he charges but it sounds like if you supply the proper specs and drawings, there will be high quality results:

    (From: Georges Koff (kopp@chemistry.mcgill.ca).)

    Although the bulk of my work is for industry and research, I do work for individuals.

    I am a professional scientific glassblower, a member of "The American Scientific Glassblower Society". For the last 27 years, I have been in charge of the glassblowing shop of McGill University in Montreal, Canada. I have been working in research for the last 33 years in Europe and Canada and have a lot of experience in the design of scientific glassware, high vacuum technology, glass-to-metal seals, special glasses, etc.

    I design and work from blue prints so any drawings could be submitted including those in electronic form (most formats are supported).

    I do not consider the type of laser but the kind of glassware to be blown. So as long as the design of the laser tube is fully specified, it doesn't matter whether it is for a CO2, HeNe, argon ion, or other type of laser. I most likely should be able to make them.



  • Back to Amateur Laser Construction Sub-Table of Contents

    Electrodes and Getters

    Types of Electrodes Used With Home-Built Lasers

    The electrodes used with home-built gas lasers are of two types, both of the 'cold' variety: A third type may be found in some lasers and is, of course, is commonly used in vacuum types including CRTs and microwave magnetrons: The cathode (negative electrode) is usually where the most heat dissipation occurs due to ion bombardment. Thus, it is generally made large and of a material and shape to minimize heating. The anode can often just be a little wire through a glass-to-metal seal.

    Getters in Home-Built Lasers

    A getter provides a means of ridding a permanently sealed system of the last traces of unwanted gas molecules by chemically combining with them to create a stable, non-gaseous compound. Common getter materials are designed to react with as many gasses as possible. This includes O2, N2, CO2, H2, and many others but NOT noble gasses since they won't react with much of anything. So these getters can be used with noble gas lasers like the HeNe and Ar/Kr ion type, but not with most others. The getter is that silvery or black metallic spot visible in a vacuum receiving tube (if you remember those) or CRT (though it may be hidden by the external or internal coating). Some HeNe laser tubes have getters but this doesn't seem that common today and those that do often have not had them activated (the getter electrode is present but the getter spot is missing. I suppose that modern vacuum systems and processing methods are so good and hard-seal tubes don't really leak, so there is no need for a getter). Argon ion laser tubes may also have getters but they are more likely to be hidden behind metal and beryllia.

    The getter is a one-time use device and is ruined by exposure to any significant amount of gasses in air like oxygen and nitrogen (more than a dozen molecules or so - OK, just a slight exaggeration!). Thus, their use doesn't make sense with a flowing gas or continuously pumped system even if all it uses are noble gasses. And, the residue from the getter reaction may be a powdery substance that will contaminate the laser tube.

    Therefore, none of the home-built lasers requires or should include a getter unless the intention is to permanently seal off the tube - which isn't recommended unless your gasses are ultra-pure, your vacuum system is superb, and your attention to minimizing contamination is equally superb. If the tube isn't hermetically sealed, air is almost certain to enter the tube between uses at the very least.

    So, you ask, "How does one install a getter if it is ruined by air?". Good question! The answer is that it is manufactured in an inactive form as a compound that is inert and contained in a small U-cross-section metal structure (the getter electrode). This must be activated by heating (once the tube has been pumped down and sealed off) to decompose the inert compound driving off a reactive metal that forms the getter spot. Should that metallic spot turn milky white or red (depending on the actual compound used), the getter has outlived its usefulness and the tube is probably leaky and no longer functional.

    Apparently, depleted uranium (U235, which is not radioactive - or at least not very radioactive depending on purity) makes an excellent getter after being processed to convert it to the so-called 'active' form. For obvious reasons, I'm not going to go into any more detail here but a search of the relevant scientific literature of the 1960s will turn up the complete recipe if you really want to build a nuclear laser and can get a few grams of the stuff. :)

    Methods to Activate Neon Sign Electrodes and Getters

    All neon sign (and most other types of) electrodes need to be heated via an external power source to prepare their surface for use ('activation'). Getters in these lasers as well as some commercial helium-neon and other gas laser tubes may need to be activated or reactivated to clean up contamination due to poor manufacturing or air leakage.

    During the manufacture of commercial tubes, they may be made red hot. My solar heater (see below) probably doesn't achieve this (for HeNe laser tube getters) but it also takes awhile. The minimum temperature for activation probably depends on the type of material used but I expect that it is at least several hundred °C. There are a variety of ways of providing the source to heat the electrode or getter:

    CAUTION: Make sure the glass-to-metal seals do not overheat while attempting any of these procedures! Also, any procedure you use should restrict heating to only the electrode itself or its contents. For getters in particular, heating the 'white cloud of death' (which might be inevitable with the solar heater) will likely result in an increase in unwanted gasses as it releases the contamination that had been collected over the eons. :(

    (From: John De Armond (johngd@bellsouth.net).)

    All regular electrodes must be activated. Running an un-activated electrode will crap up a tube in short order. There is one company that will supply uncoated electrodes on special order (maybe "Tecnolux" but I'm not sure). In any event the coating can be removed with a nitric acid wash. The electrode will still need to be heated to outgas it though. I'm not sure how one would attach an electrode with epoxy and still get it hot enough to properly outgas.

    Also note that barium carbonate (reduces to barium metal when activated) is one of the common activation ingredients. The metallic barium that results functions well as a getter. I've experimented to see just how bad a vacuum I could get away with. Using my usual brand electrodes, EGL, I can leave up to about 50 microns of air in the neon tube and still have the electrodes clean it up fairly rapidly.

    Basic Induction Heater Circuit

    An induction heater is just a source of high frequency driving a coupling coil. This acts as the primary of a transformer where the secondary is the object to be heated. The low voltage but very high current induced thus heats the object resistively. Induction cooktops may require multiple kW. For our purposes, power is much lower, perhaps 50 to 200 watts (though the schematics aren't all that different).

    The typical circuit is a line powered high frequency driver providing about 300 V p-p to the coupling coil of the induction heating head. It consists of a voltage doubler and filter, a half-bridge controller chip like the IR2151 or IR3M02, a pair of N channel MOSFETS, snubber, and return capacitor network. A half-bridge consists of a pair of switches (MOSFETs in this case, though bipolar transistors or IGBTs are often used) in series, connected between the positive and negative DC voltages. The output is taken from their common point. Clearly, only one had better ever on at a given instant!

    The key part of the typical circuit is shown below:

    
         +150VDC o--------+-----------------+               Coupling
                          |                _|_ C1             Coil
                      .|--+ Q1             --- 1uF      +------+
               P1 o---||<-. IRF850          |  250V     |       )
               _      '|--+                 +-----------+       )   
             _| |____     |                 |                   ) O Item to be
                  _       +----+------------|-----------+       )     heated
             ____| |_     |    |   Rs   Cs  |           |       )
                      .|--+    +--/\/\--||--+           +------+
               P2 o---||<<-. Q2             _|_ C2
                      '|--+ IRF850         --- 1uF
                          |                 |  250V
         -150VDC o--------+-----------------+
    
    
    The schematic of a commercial unit that I've seen is based very closely on the IR2151 Half-Bridge Driver Datasheet.

    A typical coupling coil consists of about 200 turns of #18 to #20 magnet wire. At this time I do not know if it has a ferrite core - I intend to find out.

    Amateur science and hobby outfits like Information Unlimited have plans, kits, and assembled induction heater drivers and heads suitable for this application. Commercial versions may be available from scientific and neon sign parts and equipment suppliers. Or, perhaps, you could borrow that old diethermy machine sitting in your doctor's back room. :)

    For an alternative to a special induction heater driver, see the section: Sam's Recycled PC Power Supply Induction Heater.

    Sam's Recycled PC Power Supply Induction Heater

    The induction heater driver circuits for firing getters and activating neon sign electrodes that I have seen produce 300 V p-p at around 50 kHz.

    Well, you know how lazy I am and always wanting to build stuff with recycled parts. :) So, rather than going to the hassle of constructing a line powered half-bridge induction heater driver, guess what's inside a typical PC switching power supply? Figure it out yet? A dual MOSFET 300 V p-p driver capable of 100 to 200 W! Cool, huh? :)

    So, I had this really bedraggled slightly water-logged no-name PC power supply sitting sort of in pieces (minus its case) in a box minding its own business and decided to see what it could do. I located the common point of the MOSFET pair and took a look at that with a scope (grounded to the source of the lower MOSFET, everything on an isolation transformer for safety) with the power supply driving my head light load. Almost perfect! 300 V p-p but the frequency is a bit lower than I would like - 20 kHz rather than the 50 kHz used in the commercial induction heater driver. The lower frequency would require a larger number of turns on the coupling coil and a lower single-turn voltage on its secondary (the getter or neon sign electrode), neither of which is desirable. Ah! No problem, just change a capacitor or resistor! The controller is an IR3M02. Its timing capacitor looked like a high quality job so I left that alone. The resistor was 30K. I substituted 12K and presto! It now runs at 50 kHz, apparently none the worse for the experience. The MOSFETS, transformer, and rectifiers still seem to be cool enough. The waveform still looks clean. I suppose the switching losses are now about 2.5 times what they were before, but I won't be running this continuously at full power. I don't know whether the range of load regulation will be the same as before but I really don't care as long as nothing blows up. The supply even had the same configuration of split caps and snubber as the sample induction heater circuit so I didn't even need to add those components! I then attached a cable and connector for the drive and return connection to the coupling coil in the induction heating head.

    Note that due to the low duty cycle at the light (no pun...) load, the waveform looks more like a modified sine-wave than an ideal squarewave but should be acceptable. With a larger load, it should more closely approach a squarewave shape.

    WARNING: Consider doing this at your own risk. PC power supplies are directly connected to the power line - use an isolation transformer for safety. The output drive is 300 V p-p with a few hundred WATTS available, at least momentarily. This is an extremely dangerous setup in any case - make sure everything is well insulated. Never change connections with the power on. The main filter capacitors in the power supply can store a lethal charge for quite some time - always confirm that they have discharged and/or discharge them if necessary before touching anything - a couple of 47K, 2 W resistors should be added as bleeders if there aren't any originally (though it will still take a minute or two for the caps to fully discharge). It is also quite possible that any attempt at changing the power supply operating frequency will result in instant smoke. Although it worked once, this doesn't necessarily generalize to other PC power supplies and not all designs are based on the dual MOSFET drive circuit.

    To calculate the inductance needed for the coupling coil, I assume that the maximum available power is about 100 W (to err on the safe side, hopefully!) which for the 100 VRMS drive (which is about what this is) would have an impedance of 100 ohms. The inductance then follows from: L = Z/(2 * pi * f). For f = 50 kHz, the result is about 300 microHenries.

    According to an ancient Allied Electronics Data Handbook, the following formula will give the inductance of a multi-layer coil "to within approximately 1% for nearly all small air-core coils".

                                         0.8 * r * N2
                            L = -------------------------------
                                 (6 * r) + ( 9 * l) + (10 * b)
    
    Where: Don't you just love the mixed units - this was before the forced conversion to everything Metric! :)

    Anyhow, from this equation, it looks like about 100 turns on a 1 inch form should do it. Stay tuned. Next exciting installment: Winding the coupling coil.

    Simple Solar Heater

    I built this contraption in order to activate the getter inside a contaminated helium-neon laser tube (see the section: HeNe Tube Lases but Color of Discharge Changes Along Length of Bore. Solar Heater for Activating Tube Getters shows a HeNe tube in position ready for a treatment. (I haven't actually tried it on anything else so far.)

    The solar concentrator is just a $1 7" x 10" plastic Fresnel lens which is supposed to be used as a reading magnifier. It is taped to a wood frame with two degrees of freedom so that both height and angle can be adjusted depending on the location of the Sun and height and orientation of the structure of interest inside the tube. Despite being rejected as really terrible for its intended application, this cheap lens is capable of producing a nicely focused spot less than 1/4" in diameter. Based on an estimated 600 W per square meter of noonday Sun where I live, this works out to about 25 W of light energy in that area. Sounds like a nice size laser, huh? A larger lens can be used but don't go to extremes - even this one can be nasty (especially for any unsuspecting bug that might wonder too close!).

    It might be a good idea to cover the base with some sort of non-flammable or fire resistant material to prevent unfortunate 'accidents. Whatever the focus of the beam hits will char instantly and 6 foot flames won't be too far behind!

    I expect it could be used to make some pretty decent hot dogs as well. :)

    So, you ask: "Why isn't this considered as dangerous as a 25 W laser?". The main reason is that the Fresnel lens does not produce a collimated beam and it is virtually impossible to convert it to one. Yes, putting anything at its focus will cause the item to be charred, cooked, vaporized, incinerated, or otherwise damaged. However, 2 or 3 inches away, it is just a very bright source of light. Nonetheless, DON'T ever stare into the light from the Fresnel lens!



  • Back to Amateur Laser Construction Sub-Table of Contents

    The Central High School Cyclotron

    These sections summarize my experiences in high school many years ago with an atom smasher, sort of. :)

    Why is This Here?

    OK, so why is this included in a Laser FAQ? Well, the simple answer is that it would seem pretty silly in the IR Remote or VCR repair guides, wouldn't it?! :) Seriously, there is a great deal of commonality between atom smashers and lasers - at least in terms of the technologies involved.

    First, the major players (complete names are not included since I am unable to contact them for permission to make this public):

    Now, Central High School (CHS) wasn't a technical high school but was and still is to some extent, one of the principle academic high schools in Philadelphia, PA, USA. At that time, it was also an all boys school, with Girls' High, a long block away. Now, it is co-ed.

    There were the usual debating, rock climbing, sports, cycling, and all the other associations, clubs, societies, whatever, typical of any high school.

    There were also the technical ones. We had an amateur radio society and photo society, of course. There were also probably organization for chemistry and biology but I don't really recall as I wasn't enthusiastic about chemistry (despite having acquired a cabinet full of beakers, flasks, and such) and biology was even lower on my list of fun activities.

    I also managed to get involved in the Advanced Physics Lab (APL) where I did inherit a ruby laser based on a mid-60s Popular Science design (that never worked or maybe I was too chicken to turn up the capacitor charging circuit to reach threshold). Mostly, what was done in the APL is that Fred and I abused burned out light bulbs by among other things, driving a 110 VAC to 2.5 kV transformer on 220 VAC and I would swear that for several seconds, achieved a plasma jet through the bulb in open air. But it also looked good on my academic record!

    There was also the... Cyclotron Society. Myself, Doug, and Fred were the principle members. There was also a guy named Gary but he used the place more for political reasons and always seemed to find excuses NOT to do anything technical. Fred and I eventually got him thrown out of the Cyclotron Society on the basis of some infraction which to this day I don't know was real or not. Douglas was in some ways similar and the room (to be described below) DID make a good book repository and hangout! He, at least, didn't interfere with progress.

    Now, lest you think this is the sort of accelerator you have seen on documentaries or even bad Sci-Fi movies, you would be disappointed. It was small - the maximum energy was theoretically if everything was optimal, maybe 1 million electron volts (1 MEV). The diameter of the magnet pole pieces were 7 inches! However, everything was there - magnets, RF source, vacuum system, gauges, the works.

    What is a Cyclotron?

    The cyclotron is the earliest type of atom smasher which uses the combination of a magnetic field to confine the (usually) protons and an RF field to accelerate them. It was invented by Ernest O. Lawrence in 1930. In a cyclotron, both the magnetic field and RF frequency are constant. A charged particle traveling perpendicular to the magnetic field lines will follow a circular trajectory with the diameter determined by its energy (speed). For velocities much less than the speed of light, the period of each orbit and thus the frequency of rotation is a constant. So, if a little boost if given to the particles as they circle in the magnetic field, their energy will increase along with the orbit diameter. Cyclotrons can be of various sizes. The smallest one was probably the original invention, about 5 inches. So, ours at least wasn't the smallest! The upper limit on size is imposed by relativity. Once the energy of the particles approaches their rest mass (from E=mc2 or equivalently, their speed approaches the speed of light, the constant magnetic field/RF frequency behavior doesn't work anymore and they would lose synchronism with the RF. I've seen a cyclotron with magnet pole pieces about 2 meters in diameter but even this is probably pushing things. There were atom smashers called "syncro-cyclotrons" which were physically similar but varied the RF frequency as the particles spiraled outward to maintain synchronism. Nowadays, the modern versions are mostly "synchrotrons" which do away with all but the outer ring and may be miles in diameter producing energies of 100s of Giga electron-volts (GEV). The "Super Conducting Super Collider" was supposed to be over 20 miles in diameter (perhaps more, I forget) but that died when BIG science fell out of favor in Congress. The largest one was at Betavia, IL for awhile but that has probably been surpassed by something at CERN in Europe by now.

    The CHS Cyclotron Facility

    I don't really know all the details about how a public high school ended up with an atom smasher except that it was cobbled together by students in the years prior to my high school days. The main mover and shaker being Donald who if I recall correctly wrote a paper of sorts on "The effects of High Energy Protons on Semiconductors" - he stuck transistors with their covers removed inside the cyclotron and measured some combination of parameters and how they changed with exposure to high energy protons. Now, this was most likely totally bogus for reasons that will be come clear below, but I think it did win him a National Merit Scholarship.

    The Cyclotron Room was on the main corridor in the school basement just around the corner from the Bookstore and opposite one of the storage areas for those Nuclear fallout rations the Government was so fond of stockpiling around the country (I think the mice benefitted mostly). This was sort of fitting and I suppose there were some teachers who wouldn't rule out a nuclear accident from our activities! We shared the approximately 15 x 20 foot room with one of two huge water chillers for the school. (I never did quite figure out if these were for the fountains or something else.) However, the 5 gallon container of refrigeration oil left by HVAC techs next to the chiller came in handy from time-to-time.

    The power supplies, RF source, and instrumentation were mounted in a 6 foot and 3 foot rack. The magnet with the vacuum chamber was behind it with the vacuum system underneath. The "instrumentation" was a microamp meter for measuring beam current later upgraded to one with a tube based preamp. We had a collection of probably non-functional radiation meters most likely "liberated" from that storage room across the hall. There may have also been an actual working Geiger counter as well. Our test equipment consisted of a very abused Triplet 1K ohm/volt VOM with a bent needle, having been unwound from the right-hand stop more than once.

    The magnet:

    This was a HUGE 7 inch resistive (hey, no one knew about superconducting magnets in those days) magnet wound with a lot of #20 wire. (Once, we found the coil with an open connection - possibly cut by a saboteur! Maybe that Gary fellow I mentioned - really never found out. Or, it may have just been a natural failure. Needless to say, there was a minor panic until the break was found!). The magnet was on a 99 year lease for $1 from some company, maybe GE, like they'd ever want it back! I don't know how they got it into the place, pieces I guess.

    The magnet was horizontal. To install or remove the vacuum chamber required unscrewing some huge bolts and then prying apart the yoke. Now, keep in mind that this thing weighed in at about 6 tons even though it is a rather small magnet. So, replacing anything inside the vacuum chamber was always an interesting exercise. Underneath was a very fragile glass diffusion pump that somehow survived.

    Originally, power for the magnet came from a 200 VDC or so power supply. We eventually concluded that this was grossly underpowered (another reason to suspect the cyclotron never really worked until the prior management) so we changed it to the 2,400 VDC electric utility pole transformer based power supply that was originally used for the RF source (mercury vapor rectifiers and all that). It was controlled by a BIG Variac. With this improvement, it probably was running near the 20,000 (2 Tesla) limit of an iron core magnet. It was quite impossible to extract any ferrous/magnetic objects from between the pole pieces with the magnet fully energized.

    The vacuum system:

    We had Sargent-Welch rotary vane mechanical pump, maybe a 1405 but could have been the "economy version", and a glass oil diffusion pump. There must have been a box fan or something to cool the diffusion pump since it did not use tap water. The diffusion pump was about 3 inches in diameter with a glass O-ring flange-flange reducer for the 1 inch coupling to the vacuum chamber squashed between the magnet pole-piece right above it. There were no baffles, cold traps, or dryers.

    An ion gauge provided our only real indication of vacuum level other than that wonderful clacking sound the mechanical pump made when it has drawn down below 0.1 Torr or so. We had the obligatory hand-held Tesla ('Oudin') coil for leak testing though it was used much more often to chase the unwanted visitors from the cyclotron room. :) Anyhow, once the whole affair was drenched in Red Glyptal, leaks really weren't an issue!

    I seem to recall a scavenged thermocouple gauge as well but that may have been for the "Linear Time-of-Flight Resonance Mass Spectrometer" I started to build. It at least got me a free cruise on a Navy destroyer escort and my first plane flight to the Newport Navy Base as a consolation price at the local science fair (since it was not completed and never really worked - but the charts and front panel were impressive!). Oops, that's another story.....

    The vacuum system would do at least 10-6 Torr when it was cooperating. Usually, it didn't take long to get there but since we really didn't trust the ion gauge all that much, we usually left the thing running overnight.

    The vacuum chamber:

    For our huge 7 inch magnet, we needed a vacuum chamber large enough for the Ds (the electrodes that actually accelerate the protons - shaped like the letter 'D') and a little clearance. In all, it was about 12 inches in diameter and two inches thick. The 'D's (actually only one, see below) was mounted on a metal stud fixed in Epoxy passing through a glass insulator.

    Most cyclotrons have a pair of 'D's and drive them with a balanced RF source. Someone decided that this wasn't necessary, so ours had a single 'D' with the RF between it and the ground of the vacuum chamber. In principle, this would work though I suspect there would be less of a focusing effect. What did we know?

    The RF source:

    Originally, a 200 W amplifier driven by a 50 W exciter generated the RF (about 20 MHz if I recall correctly) for the 'Ds'. This was based on a pair of 811 tubes (remember those?). I later built a 1 kW linear amp (from the AARL Handbook) using an EIMAC 3-400Z or something like that. The original power supply (mercury rectifier based) was moved to the magnet and a new HV power supply was built for the amp.

    True R and D

    I consider the years I spent working on that machine to be more closely akin to true scientific research than anything since - designing giga instruction per second high performance 3-D visualization/graphics accelerators or microchip lasers just doesn't have the same feel!

    I had to learn - on my own - about high vacuum technology, high power (well, relatively speaking) RF, instrumentation, at least a little E/M and high energy physics, and much more. Keep in mind that no one else - including any of the teachers at Central High - had a clue about ANY of this!

    I also learned a lot about locksmithing (including all about making master keys using solder-fills and lock picking) - one has to be resourceful to succeed in these endeavors. The teachers were aware of this and kind of accepted it (as well as benefitting at times), realizing the limitations of an environment where advanced science was more along the lines of dissecting a worm. :)

    To be continued as I think of more tid-bits...



  • Back to Sam's Laser FAQ Table of Contents.
  • Back to Amateur Laser Construction Sub-Table of Contents.
  • Forward to Home-Built Laser Assembly and Power Supply Considerations.


    Sam's Laser FAQ, Copyright © 1994-2001, Samuel M. Goldwasser, All Rights Reserved.
    I may be contacted via the Sci.Electronics.Repair FAQ Email Links Page.