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atmospheric pressure plasma displays

Atmospheric presure plasma displays: 
slightly "cleaned" up repost.


There isn't really much to it:

	There was, for a while, some interest on the USA-TELSA mailing list in
how plasma globes are built. They generally consist of a sphere of glass
through which there is placed a single electrode (mine is a 1/4" copper pipe
with a steel ball bearing soldered on top). Some means of pumping out and
sealing off the globe is also needed. Many people including me have made them 
by pumping out a glass jar, sphere or flask to a vacuum level of ~1mm Hg or 
1/750 of an atmosphere. Some Use other gases than air- like neon (expensive) 
or argon but at similar pressures (1/100-1/10000 atm). This works pretty well 
if you've got a vacuum pump and are not afraid of an implosion. In this form, 
the "plasma globe" is rather similar to Tesla's own carbon button lamp, 
although he generally ran much more current into them - often with the effect 
of heating the discharge terminal to incandescence due to energetic ion 
bombardment.

	The electrode is driven with "high" frequency current at voltage 
levels of 10kV or so. By "high" I mean people typically use a TV flyback 
oscillator driven by transistors - like in Ianni's book. DC will not work 
since the discharge would quickly extinguish itself due to space charge 
build-up. I have not tried 60Hz, but I assume it's too close to DC to 
offeranything bu weak discharges. 1Mhz would be interesting- somebody 
willing to fry( High SWR) their Ham transmiiter should try this!

	Then someone[ can't recall the name] said he had heard that commerical
plasma globes were made to work at or near normal atmospheric pressure. Well,
with air alone this obviously can't be done with the voltage levels available -
10KV or so from a flyback oscillator (like in a TV set). But this seemed
possible to me if  you keep the same partial pressure of air or other gas
(1/1000 atm) and fill the rest of the space with helium gas. This is a trick
used by gas laser people for 30 years to raise the working pressure of
electriccal discharge gas lasers (See SCi AM amatuer scientist from 1970-1972
on N2 gas laser).

	So, I tried it. It works quite well. You can easilly obtain 5" arcs
inside a glass globe with only 5kV or so using atmospheric pressure helium and
1/100-1/10000 parial pressure of air or other "dopant" gas. It is the dopant
gas which is generally doing most of the glowing. As a natural extension of
this I believe it was Bill Beatty (sorry if I got the spelling wrong) who
suggested making plasma "displays" from plastic bags filled with helium. I
tried using a rubber balloon instead, and found pretty good results if the
balloon was well purged with helium to get out most of the air. Again 5" arcs
were obtained inside the balloon with under 10kV. Intersting semi-staionary
forked structures similar to those seen with high power vacuum tube telsa coils
can be seen with much lower voltage levels. I see many different types of 
dischages depending on voltage and pressure.


That's where the experiment stands with me right now. I still want to get a
nice big plastic bag as Bill suggested, but the purging could be much more
difficult.

Ed Harris