Here it is at last... The definitive Plasma Globe!
The objective of this project is simple: To produce the
best plasma globe possible given the resources available. This means using the
largest glass globe obtainable, and equipping it with the best vacuum system /
electrode assembly / power supply I can obtain and/or make. This globe is meant
to be used as a decorative piece for eventual exhibition, and should therefore
be made presentable (nice base, no hanging wires, etc), safe (all H.V. parts
enclosed, and shatter-proof glass). It should also last long and be able to
operate for extended periods of time, which can only be achieved through a very
well sealed vacuum system and a well designed power supply.
Here is a picture
of the flask. It is made by Schott, a German glass manufacturer, and is made
from Duran®
borosilicate glass (Special Thanks to U. Zimmerman for pointing that out to
me), which is highly resistant to chemical
attack and drastic changes in temperature. The flask is the round bottom, long
neck type and has a capacity of 6000mL. It cost me $100 with the 5cm dia cork. The
flask outer diameter is approximately 30cm (12"), and its walls are some
2mm thick (its HEAVY!)
I am considering removing the writing on it... Initial tests with cheaper glassware and Hydrofluoric acid (HF) proved unsatisfactory (a frosted spot remains where the label used to be). Anyone have any better ideas of what to use?
Here
is a picture of the center electrode being assembled. The electrode itself is a
26cm tall glass column with a 4cm diameter glass ball in the end. The tube is
than filled with 25grams of aluminum filings, a high voltage insulated wire is
inserted through the end and the tube is sealed with epoxy. Overall cost here (aluminum
filings, glass for blowing the tube, and high voltage wire) was approximately 20
dollars.
Assembled electrode inside flask: A hole was drilled into the cork that was approximately 80% the size of the tube, making for a very tight fit. The tube was than smeared with epoxy and inserted through the hole. Also notice the H.V. wire sticking out of the bottom.
Here you see the
parts used for the vacuum plumbing. From left to right: Pressure rated (to
200PSI) nitrile tubing, for connecting the pump and gas tank to the lines, copper
tubing for connecting the valves to the flask, tube clamps, tube adaptor, valve,
extension, "T" junction, vacuum gauge, gauge adaptor, and so on... In
the bottom there is another valve, the cork for the flask, and some liquid Teflon
thread sealant (a must for any vacuum work), rated at 8000PSI.
And
here is the assembled vacuum plumbing. There are two sections to it: The bottom
section which includes a valve and a tube adaptor, and is meant to be attached
to the gas tank and the top section which incorporates a vacuum gauge to the valve / tube
adaptor assembly and is meant to be connected to the vacuum pump. The advantages
of having the system split in two are obvious: The flask can be pumped down and
backfilled with any gas simultaneously, and the pressure can be varied and monitored at all
times. Hence all 5 factors (voltage, frequency, gas, pressure and power) can be
adjusted in real time while the system operates, and when optimal conditions
have been reached it can be sealed and should remain that way. This plumbing system cost
me $50
After months of searching around for an oil-sealed rotary vacuum pump,
and finding that the cheapest model available cost $500 (no second hand units
were found), I finally got a $15 deal on a small refrigeration unit pump (given
its size it probably comes from an air conditioner or a very small
refrigerator). It is *FAR* from ideal, but hey, it works! :). As a plus, this
unit is unusually quiet (it can not be heard even when working at full power)
and rather fast (pumps 6 litres down to its lowest pressure in under 3 minutes).
As a minus, it can barely reach 690mmHg (27"), and it gets rather hot
in trying to do so! The pump weights 4,8kG and measures 20X10X15 (lXWXH).
The
flask is baked and pumped down to the pump's limit. Several other (higher) pressures were
tried, but with air this proved to have the best results. I believe even lower
(>700mmHg) pressures would be desirable with air, but have no way of testing yet.
The vacuum system in the flask is flawless and so far has held its vacuum for 2 days
without any changes in the vaccuometer.
Below are 4 still frames of a 30 second video I took of the second test of this globe. As new and better test videos come, this and the pictures will be substituted (what you are seeing now is already version 5.0 of this page). For now the specs are: Air at 690mmHg, 50kV high frequency power source (de-powered Tesla Coil). The video shows arcs tracking my hand along the glass with the lights on, and than off. Click any of the 4 pictures below to download the 1.18mB .mpg video. Note: Tesla Coils are far from ideal power sources for plasma globes: The arcs are very hot and tend to move around and rise very quickly, making a rather frantic display. I personally prefer the slower moving arcs from a lower frequency power source.
Still under construction. Check back by the end of the
week for another update.
If you have any comments, questions, or suggestions, don't hesitate to e-mail me!
People have visited this page since 09/04/00
Last updated 02/05/01
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