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!
If you find that you are serious about any of this, obtaining a copy of this material is essential. See the section: Light and its Uses - Table of Contents for a list of all the articles that constitute this valuable collection and an explanation of why I cannot provide on-line access to it.
Furthermore, you will be CONSTANTLY fiddling with adjustments like gas fill, mirror alignment, power supply voltage/current. In many cases, total laser lifetime is often short (a few hours) before a total rebuild is needed. These are not generally set-it-and-forget-it type equipment! If you just want a working laser, this is definitely NOT the way to go.
A surplus $25, 1 mW helium-neon laser head and power supply, or even a $9.95 laser pointer may be more than adequate for your needs.
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.
(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.
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:
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:
(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! :)
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! :)
(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:
(From: Daniel Ames (dlames3@msn.com).)
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.
(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!
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. :)
(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.)
Laser specific traffic on this forum is quite small but the high chance of finding someone with similar interests balances this out to some extent!
(Note: During the early months of its existence, there was a lot of activity including a group purchase of CO2 laser mirrors. At least one person did achieve "first light". Now, as far as I can tell, it is dead, or at least in a coma. There have been no postings in months. However, much of the information obtained from these discussions has made its way into the chapters on home-built lasers so you can benefit from the Laser Growing Mailing List even if it is no longer active.)
"Hopefully, this list will comprise of people who wish to build their own CO2 laser and/or other lasers (later on) and are willing to share their expertise, experience and resources for the benefit of others in the building process. Scroungers are of particular benefit to us as some of us may need what you have found.This is a non-profit endeavor. From time to time, group purchases MAY be suggested/made but, only at cost plus postage."
Everyone is welcome to monitor the list but please refrain from sending off-topic messages. To join, send an email message with the words "subscribe" (without the "") on the subject line to buzz_ard2@bigfoot.com. You will receive a confirmation message via return email. (Note: This listserver has recently moved - it used to be at buzz_ard@bigfoot.com.)
There are also some Web pages associated with the Laser Growing Mailing List with information and photos which will be updated as the project progresses:
(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. :)
See the chapter: Laser Safety for more information.
Read and understand the information in the document: "Safety Guidelines for High Voltage and/or Line Powered Equipment" BEFORE constructing and powering any of these systems.
However, should you actually be powering a tube (for whatever reason) pumped down to perhaps 10-6 Torr or better with a heated cathode (since you can then easily get significant current even in a perfect vacuum - not really practical or likely by accident with a cold cathode) attached to a HV power supply, X-rays can become a real safety issue! There is no actual threshold where X-ray energy starts to become a hazard but below about 15 kV, X-rays won't penetrate the glass of the tube. X-ray machines for mammography which run at *only* 30 to 50 kV have tubes with beryllium windows to allow the radiation to pass through the tube walls. However, taking precautions when using voltages higher than 15 kV with high vacuum systems would be prudent.
Commercial X-ray machines are based on ultra-high vacuum tubes consisting of a heated tungsten filament (cathode) and a target (anode) made from tungsten, tungsten-rhodium or some other exotic material enclosed in a glass or ceramic envelope. The typical dental X-ray unit (about the smallest common type of X-ray system for patient imaging) may use 10 mA and 70 kV. At the other end of a major X-ray equipment manufacturer's product line would be the X-ray generator for a high performance spiral CT scanner which may go up to 400 mA at 130 kV or more. :-). (If you are really curious, the way they get rid of the approximately 50 kW of waste heat in the latter case (most of which is generated at the anode and concentrated at a spot a mm or so across) is to make the anode in the form of a massive disk spun at high speed via an induction motor magnetically coupled through the tube envelope with the tube bathed in oil which is circulated through a large heat exchanger. Even all this isn't really adequate to keep up with heat production - these things can only run for a couple minutes at a time at full power before they need to be powered down to allow the tube to cool. And you thought it was difficult to cool an ion laser!)
Having said all that, it never hurts to err on the side of caution. Once you start powering your laser, get one of those X-ray films you love so much from your friendly dentist and cover the front (smooth side - the back has a lead foil shield inside) of it with a metal mask (e.g., a wire formed into your initials) so you will know if it actually gets exposed. Place it front-side facing and near your laser tube (but not so close that there may be arcing to it) and let it cook for a few minutes with power on. Have the film developed and inspect it carefully. If you see ANY shadow of the mask, there has been some X-ray exposure and further testing and possible shielding would be adviced. If there is nothing visible, try again with an exposure time of an hour. Of course, if the film is totally blackened, you have serious problems (or have somehow built an X-ray laser). For a more quantitative test, get a radiation film badge with specific instructions on its use, and have it processed and evaluated.
(From: Terry Greene (xray@cstel.net).)
"There is no practical problem with X-rays at the typical voltage used in most gas type lasers. All mammography tubes use special beryllium windows for the radiation output because X-rays at those low energies (20 to 32 kVP) won't penetrate the glass envelope of a standard tube. I've tried it. A glass envelope (standard) X-ray tube won't produce any exposure at all at 15 kVP according to my test equipment. (Keithly ion chamber). Even with complete evacuation, I seriously doubt you could find measurable X-ray output from a glass bore laser."
Note that it is the difference between atmospheric pressure and that of your vacuum that determines the stress on the container - whether you are pumping down to 10 Torr or 10-14 Torr is for all practical purposes irrelevant with respect to implosion risk!
If you insist on trying these bombs, cover the whole affair with some sort of shatter-proof outer cover like a thick solidly constructed Plexiglas shield but I don't recommend this in any case.
Forget about smoking around precision optics. Aside from slowly killing yourself, a miniscule amount of tobacco smoke residue will play havoc with mirrors and lenses - especially inside the laser resonator. You will be wasting your time or worse. Just because you saw a demo where someone blew smoke in the path of a laser beam to make it visible is no excuse as that was just a one-time demo. There are special means of generating smoke for this purpose which are non-toxic and do not condense on optical surfaces should a real need arise.
These are typically 18" to 24" deep by 36" wide and 60" to 78" tall with multiple adjustable shelves. The larger the better if you have the space. I have gotten mine from garage and tag sales in good condition for between $10 and $40 (they retail for over $200).
Of course, other kinds of cabinets are fine as well. However, anything you chose should have doors to minimize dust on optical and electronic components and assemblies.
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_|__/ _______|_______________|____________________ \__|
For borosilicate (Pyrex) tubing in particular any of the large scientific supply companies are probably the best sources. Tubing comes in 4 foot lengths and the typical price is $10 to $15 per pound. They will also carry fused silica (Vycor) and fused quartz tubing (though neither of these really should be needed for any of the home-built lasers unless you are doing something like attempting to push the envelope on the argon ion laser power output.)
(Portions from: Cass (cassegrainian@galaxycorp.com).)
"Friedrich & Dimmock, Inc. seem to have the most buying options and at the best prices that I have seen at their Web site, which is new although, they have been in business since 1919. For selected items, they have what they call a "shelf pack" and that seems to be the best bang for the buck for standard wall Pyrex tubing ($25 to $50 for 2 to 50 lengths (approximately 4 foot) depending on diameter. This isn't bad if especially if you can find a few buddies to share expenses. Unfortunately, they don't have this option for medium or thick wall tubing which is what may be needed for some of the home-built lasers. However, although small diameter tubing is sold by the 100, 4 foot pieces (typically $200 to $400, ouch), larger diameter tubing may be available by the pound - which is probably even better than the shelf pack. So, check out their on-line catalog. Note: Minimum order is $75."
However, Tim Goldstein of the Laser Growing group (see the section: General Resources for Amateur Laser Construction) has found that Glasscraft, Inc. (nothing much at their Web site yet) will sell in small quantities (even a single piece of tubing) to individuals. They accept major credit cards and there is no minimum order (though a $5 handling fee will added for orders under $50). More info is also available at: Tim's Glasscraft Borosilicate Tubing Info Page.
Places that deal with plastics and metal (e.g., sign shops, metal fabricators, machine shops, etc.) usually have scrap bins with all the stuff that is too small or irregularly shaped to put back into inventory. They will probably sell pieces from there very cheaply (maybe by the pound) or just let you take what you want free of charge. With just a little effort, you may be able to obtain all the little structural bits and pieces needed to construct your laser (except probably the main support beam - for that you will likely have to pay something) as well as the raw materials for the mirror mounts at little or no cost.
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:
This company has been in business since 1953 and does welcome orders from individuals. Some of the materials that may be of interest to the laser constructor are: Methyl and ethyl alcohols for dye laser solvents, copper chloride for the CuCl2 laser, mercury metal for vacuum work ($7.25/4 oz.!), and West type, glass, condenser tubes with water jackets up to 600 mm in length, for use as plasma tubes in CO2 laser construction.
I have dealt with them personally and found them to be very receptive and helpful. They, of course, also do precision glasswork of all kinds and specialize in working with quartz.
On a side note, they are the only manufacturers in the world of Ben Franklin's invention..... The glass harmonica!
(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:
Wale's catalog is mostly artistic glass now, but if you call and ask they still stock or can get most of the scientific stuff and they have a good line of low cost torches, cutting wheels, didymium glasses that protect your eyes from the IR and UV coming off the hot glass and remove the yellow sodium flare light so you can see what the glass is doing. I'd recommend the glasses for beginning glass blowers, it's like X-ray vision into the flame.
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:
A good resource for components found in "Light and it's Uses" is:
I just spoke to the owner-nice fellow. He says he still has inventory of some nitrogen, argon, dye, and Hg Vapor laser components but interest is dwindling so I don't know how much longer he will be in business. His prices are very good also.
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.
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. :)
(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:
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.
Its parent site, The Bell Jar has an index to many additional articles available only in hard copy and/or by subscription, vacuum equipment suppliers and links. and a Users' page which includes vacuum equipment "For Sale and Wanted" but doesn't appear to have been updated in a couple of years.
And, if you are into more esoteric aspects of vacuum systems and technology, there is a discussion group for these topics:
These include:
Try to locate "Procedures in Experimental Physics" by John Strong (who wrote many of the SciAm Amateur Scientist articles). In particular, see Chapter 3: Technique of High Vacuum.
"I highly recommend a really nifty book also from the Society of Amateur Scientists. It's one of the old reprints that Lindsay books does in cooperation with SAS called Procedures in Experimental Physics.
The vacuum section is EXCELLENT for the science hacker. Everything from building vacuum gauges from vacuum tubes to desktop thin film sputtering to CVD coatings to making your own diffusion pumps from available materials. I highly recommend it."
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
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:
Mountain climbers have to endure reduced pressure and above about 10,000 feet, require breathing equipment. Anyone who has traveled by air knows the standard speech at the beginning of each flight "....should oxygen be needed, the compartments overhead....". This would also happen above about 10,000 feet.
Astronauts on American spacecraft (at least they used to), breath unaided at a pressure of perhaps 1/5 of an atm because they breath nearly pure oxygen. Since in the normal atmosphere, oxygen is only about 18 percent of the total mixture (most of the rest is nitrogen with a little CO2 and inert gasses thrown in), the resulting biological activity (and the flammability of common materials, for that matter) is about the same but there is no need to carry the approximately 80% of useless other gasses and the stesses on the spacecraft structure (from the difference between the internal pressure and the vacuum outside) are reduced by 80% as well.
A low vacuum can be obtained by any number of simple mechanical means including fans and centrifugal blowers, piston and rotary pumps, aspirators, siphons, chemical combustion and other reactions (which use up the air), etc. Liquids boil at reduced temperature - often room temperature - in a modest vacuum but minimal or no precautions are needed to prepare surfaces and equipment since any outgassing is small compared to the remaining air.
I did a few very scientific experiments to determine values of two types of vacuums with which everyone is familiar:
So, next time the friendly vacuum cleaner salesperson calls, forget the oatmeal test, let them try a Bordon tube gauge instead! :)
However, note that normal straw sucking would result in hardly any vacuum at all, only needing to raise a fluid by a few inches; 15 inches of mercury is like drinking through a straw from a container about 16 feet below you!
A medium vacuum can be achieved with a high quality mechanical pump.
A high vacuum usually typically requires a multi-step pumping scheme with a rotary mechanical pump going down to a fraction of a Torr followed by a diffusion, turbo-molecular, or other true high vacuum pump.
Even if the final vacuum is modest (e.g., 1 to 10 Torr), being able to pump down to an ultra-high vacuum may be needed to purge the tube or whatever of contaminants with a small number of pump down/backfill cycles and minimal use of expensive gasses.
In addition to mechanical and diffusion pumps, additional means are required to achieve an ultra-high vacuum including exotic ion pumps, cold traps, and chemical getters. Surfaces exposed to the vacuum must be immaculate - a single fingerprint can mess things up for days!
To put a 10-9 Torr vacuum into perspective: If all of the gas molecules remaining inside a typical 17 inch monitor CRT that had been manufactured at this level of vacuum were rounded up, captured, and returned to normal atmospheric pressure, they would occupy a volume of space less than 25 um on a side - roughly 1/10th the diameter of the dot in the explanation point at the end of this sentence or half the diameter of a human hair! Yet, inside the CRT, there would still be approximately 1,000,000,000,000 gas molecules remaining for unsuspecting electrons to run into!
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? :)
(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
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.
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:
A well maintained two-stage rotary mechanical pump (the sort of thing you find in high school physics departments. OK, perhaps except for the 'well maintained' part) can achieve a vacuum of less than 10 milliTorr if conditions are perfect. However, with use, age, oil contamination, and somewhat worn seals, even 1 Torr may be optimistic. So, if you find one of these at a garage sale (as I have), it may be necessary to do an overhaul or at least to totally drain the old oil, flush and drain again, and fill with fresh vacuum oil of the proper type (available from scientific or refrigeration service supply houses, not motor oil or 3-in-one!). Refrigeration service pump oil isn't what you want for a true high vacuum but is good as a flush and is certainly adequate for the CO2 and N2 lasers. It may even be available from a place like Pep Boys - it might cost all of $10 for a gallon! (But real vacuum pump oil is also available for about $10/gallon from some suppliers like Duniway Supply. Flushing the pump and replacing its oil may be the single most important thing to achieve acceptable performance. Unless the pump was abused, it will probably be all that is needed.
Moisture is also a killer of oil, so using such a pump as a wet-dry vac isn't a good idea either!
See the section: Rotary Vacuum Pump Mainenance for more details on reviving a pump that doesn't pump very well. :)
If you do pick up one of these used, replacing the oil should greatly improve its performance as all sorts of contamination can be sucked in when used to evacuate refrigeration systems and there was probably little or no maintenance ever performed on the pump itself!
The Electronic Bell Jar has a detailed article on these types of pumps.
A diffusion pump has no moving parts (at least at the macroscopic level). An electric heating element in its base boils a small quantity of a special 'diffusion pump oil' inside a sort of tower or percolator structure which has vents to direct the jets of oil vapor downward toward the higher pressure region (to the mechanical pump) where it condenses on the cool surfaces of the pump housing and is recycled. In in the process, air and other gas molecules are dragged along with the oil vapor. When the oil vapor condenses near the bottom (higher pressure) part of the diffusion pump, the trapped molecules are released and sucked up by the roughing pump. The actual pressure differential between the top and bottom is miniscule - only a fraction of a Torr (and the diffusion pump cannot be fired up until the roughing pump has brought the vacuum down to this level). But this is adequate to suck out most of the remaining air or other gas molecules and once it gets going, the pumping speed of a diffusion pump is quite impressive despite its passing resemblance to a coffee percolator. I know what you're thinking. :)
Diffusion pumps require cooling of their own. This is usually tap water through a coil wrapped around their exterior though some use forced air cooling.
For the cyclotron at my high school (right, how many high schools have atom smashers - but that is another story), we had an air-cooled glass oil diffusion pump (probably because no one else wanted it or even knew what it was). Somehow, this fragile glass structure survived all sorts of catastrophies despite being located under the main vacuum chamber situated between the pole pieces of a magnet weighing several tons and joined by a clamp-type glass and O-ring seal..........
An alternative to a diffusion pump that appears distinctly low-tech (but no doubt requires very high-tech manufacturing) is the turbo-molecular pump. It is basically just a close-fitting turbine spun by an electric motor at an incredibly high speed (something like 90,000 rpm!!). Even a very small turbo-molecular pump will have an impressive pumping speed, but like the diffusion pump, can only operate against a relatively low pressure - 20 milliTorr is typical.
Though no self respecting high vacuum system would be without at least one of these high vacuum pumps, this is not really essential for most of the gas lasers under discussion especially if you have a well maintained 2 stage (or better) mechanical pump. However, if you come across a small one (almost any size would be adequate for pumping laser tubes) in good condition at a decent price, grab it. You can never tell when your interests might wonder in directions where a true high vacuum system would be needed.
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.
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
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.
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. :)
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.)
WARNING: The escaping Freon will be COLD - enough to cause frostbite. Let it alone until some time after the hissing stops!
WARNING: While Freon itself is non-flammable, poisonous gasses will result from contact with an open flame. Do this outside!
Note: It is currently against EPA regulations to release CFCs (e.g, Freon) into the atmosphere and therefore cutting the the refrigerant lines to remove the compressor without recovering the Freon is against the law. Therefore, consider having a HVAC service company purge the Freon for you - it is even possible they will do this free of charge (as long as you deliver and pick up the appliance) since the recovered Freon is worth something.
It is critical that there always be adequate lubricating oil in the system. There is no telling how much was actually in the compressor when you cut it away from the rest of the appliance. An HVAC service company may be able to help. Some of the proper oil can be SLOWLY added via the suction port (some compressors will be damaged attempting to compress an incompressible fluid if it is added too quickly). If too much oil is in the compressor, it will spurt out the pressure port in excessive quantities.
During operation, check the amount of oil in the container from time to time (by weight if necessary). There will always be a small amount of oil expelled out the pressure port of the pump. However, if the loss becomes too great, you will have to add some oil (very slowly to the input) to maintain adequate lubrication.
WARNING: As noted above, catching the expelled oil isn't just to prevent that mess. The significance of the health and fire hazards cannot be over emphasized.
In any case, to prevent oil from back-streaming into the vacuum system, provide a filter in-line with the compressor suction port.
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!
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).
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.
(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.
Here are some of the types in common use:
I have a bunch of Bourdon tube gauges. They are useful for low vacuum work but unless you listen to the local weather report, you may think your pump isn't operating as well on those days where the barometric pressure is low (because at perfect vacuum, the pointer will only move an amount equal to the local pressure)! You can get around this, sort of. :) Neither of the following is particularly recommended but might work in a pinch:
For greater sensitivity, it may be possible to use a gauge with a smaller range but its needle will be pegged off scale with no or low vacuum and this may harm the mechanism. If you have a Bourdon type sphygmomanometer (a.k.a., blood pressure gauge - not the mercury type!) that you are willing to try, that could give a usable reading down to less than 1 Torr but might not survive too high a pressure differential (1 atm would end up being over 2.5X its typical range of 300 mm of Hg).
This is essentially identical in concept to a mercury barometer.
By using a valve at the closed end instead of a hard seal, pressure differences can be measured.
A pair of these is actually adequate for the gas lasers being discussed:
See the section: Home-built Closed-End Manometer Gauge for details.
Unlike the others, this is not automatic - it must be tilted and righted to read the pressure. This action captures a precise quantity of the rarified atmosphere which can be balanced against a measured column of mercury.
Its readings are independent of the type(s) gas in the system which is advantageous where gas fill is constantly changing.
Thermocouple (TC) gauges operate at the lower end of the range in which we are interested. The most common ones provide useful readings between 1 Torr and 1 milliTorr (1 micron) though some go as high as 50 Torr and as low as 0.1 micron (but not with the same sensor). However, the "best sensitivity" range may be somewhat less as each end is a bit squished. The TC gauge is among the most widely available, especially on the surplus market or eBay. However, keep in mind that a good deal on a controller isn't that great if you have to pay full price for the compatible TC gauge tube (sensor). It seems that when vacuum systems are dismantled, the tubes are often neglected and remain with the plumbing. TC gauges with sensors tend to go for about $50 to $100 on eBay, somewhat more from used vacuum equipment companies.
It is possible to build a TC controller for next to nothing with parts from a reasonably well stocked junk drawer but the sensors are still costly (by scrounger standards) and the effort - despite the simplicity of the circuitry - may not justify the money saved. See the next section for my experience doing this. The article: Building a Thermocouple Vacuum Gauge also includes information on a home-made version which can be constructed inexpensively. It also mentions the Pirani and thermistor gauges, which operate on similar principles.
However, a thermistor gauge can be built even less expensively than a thermocouple gauge since the sensor can also be a common thermistor if you have a reference vacuum gauge for calibration. See Thermistor Vacuum Gauge. This is from The Vacuum Technology Page (ECE Department, University of Alberta) which has other related information.
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.
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! :)
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.
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):
Set the Meter Select switch to the "Vacuum" position (with the system remaining pumped down to below 1 micron):
(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.
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. :)
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.
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!
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.
(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.
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.
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.
The usual choice is a silicone based compound appropriately called 'vacuum grease'. While common lithium axle grease or Vasiline may work at modest levels of vacuum, there is no telling what volatile compounds these release to poison your laser.
When I was involved in vacuum work, the favorite was a compound called 'Red Glyptal' (Red Glyptal Insulating Varnish is made by General Cement. It is available in small quantites from electronics distributors like Allied: # 796-3670, GC # 10-9002 for a two ounce bottle, and by the quart or gallon from chemical/scientific supply houses). This is something like a thick red enamel paint and makes an excellent seal to most types of materials (as well as for use in impregnating and excluding moisture from motor and transformer windings). Epoxy can also be used for permanent connections.
For threaded fittings that may need to be disassembled, white Teflon plumbing tape should work medium vacuums - down to well below 1 Torr. Vacuum grease may even have a high enough viscosity to prevent it from being sucked out of the threads in this case.
TorrSeal is another ultra high vacuum compatible cement. It does not outgas and is for all practical purposes a nonconductive metal when hard - and that is very hard. No common solvents will touch it so you better be really really sure that you want the parts connected if you use TorrSeal as they won't come apart - ever!
(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.
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:
A Bordon tube type can be used in a pinch but accuracy will be poor at the low end of its range.
Alternatively, a McLeod gauge can be used in place of this manometer. A McLeod gauge can be more precise but will be more expensive and/or more difficult to fabricate (and more of a pain to use!)
WARNING: High pressure gas cylinders MUST be fitted with proper regulators to supply low pressure gas!!! You cannot hook a 2,000 psi gas cylinder directly to your laser! Following this should be a flow restricting orifice (or metering valve with a small maximum size opening) followed by a relief valve (set for a few psi at most - just over 1 atm) to protect your glasswork and other low pressure tubing from 'accidents' that might have unfortunate consequences!
If a diffusion pump is added (between the mechanical/roughing pump and the dryer), a thermocouple and/or ion gauge will also be needed.
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.
Some, like Pfeiffer, have complete manuals for their equipment which may be downloaded for free (useful for that pump you found in Grandma's attic).
There are also many vacuum technology related companies at The Bell Jar - Suppliers Page and another one linked from The Vacuum Technology Page(ECE Department, University of Alberta).
A quick Internet search or business directory should turn up numerous other possibilities as well as sources for used equipment (see below).
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! :)
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.
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!
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.
(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.
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.
A recommended book on this topic is:
The new book prices are stratospheric ($64 and $47 respectively) but these are the sorts of books that may show up on the clearance tables of bookstores like Borders. :)
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:
S-L glass is not suitable for high temperature apparatus (e.g., the bore of a high power laser) but should be fine for most other uses.
B-S glass can probably be used everywhere but the difficulties in working make this unattactive. However, if someone else is doing the glass-work...
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.
All you need is a small triangular file (new or in excellent condition, not rusty and clogged with something disgusting) and perhaps some spit. :-)
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.
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.
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!
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!
For large diameter (e.g., greater than 1/4") or thin walled tubing, cap one end and attach a rubber hose to the other to use as a blow-pipe so you can apply a little positive pressure (by *gently* blowing into the hose) to keep the glass from collapsing - don't blow too hard or you will blow a bubble or rupture the tube entirely!
Note that the key to this is to get a seal all the way around when the pieces are first brought together. Otherwise, any gaps will tend to open up and there will be no way to apply positive pressure to keep the whole affair from collapsing.
Note that the key to this is to get a seal all the way around when the pieces are first brought together. Otherwise, any gaps will tend to open up and there will be no way to apply positive pressure to keep the whole affair from collapsing.
I actually prefer to cheat: Instead of joing a straight piece to the side of another, where possible, I start with a small glass T connection, cut off its ends if necessary, and joint it to the main tube using the butt joint. Depending on the specific diameters and types of glass, this may be a better approach.
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.
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:
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.
(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.
Neon sign electrodes may need to be 'activated' by heating prior to use. See the sections below for information on ways of doing this.
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. :)
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:
Having said that, with care, it might be worth trying. See the section: Using a Microwave Oven to Evaluate and Revive HeNe Laser Tubes. However, whether such treatments help, hurt, and the extent to which they last, seem to depend on many factors which aren't readily obvious.
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.
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.
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:
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.
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!
First, the major players (complete names are not included since I am unable to contact them for permission to make this public):
Donald is now a high power lawyer. :-)
Douglas - is/was involved in Washington, D.C. and at one time held the title of "Deputy Assistant to Someone or Something".
Fred now develops, markets, and manufactures special purpose linear ICs.
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.
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.
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...