At a given voltage, discharges in noble gases are many times as long as those in air. For my experiments I'm using welding grade argon because it's the cheapest noble gas. As it's still too expensive to flood my cellar with argon, I'm using a 37mm thick, 70cm long glass tube.
To the ends of the tube I attached pieces of printed circuit board with brass tubes passing through. The brass tubes serve both as gas inlets and electrodes and are soldered to the pcb's which in turn are glued to the glass tubes with epoxy resin.
Here's the experimental setup. The Plasma Tube hangs from a handy construnction
of drain pipes with a height adjustable cross beam.
argon is slightly heavier than air, so I attached the Ar bottle to the bottom
tof the tube and the replaced air leaves at the top end. I didn't have a vacuum
pump when I first performed this experiment so this discharge works at atmospheric
pressure - and it works pretty well!
To power the discharge I'm using my Flyback power
supply.
The top electrode is not grounded, because with a grounded top electrode the
discharge becomes spark-like and don't look as good.
Close-up of the bottom electrode of the Plasma Tube. I wrapped the brass pipe and the PVC hose with a piece of aluminium foil which suppresses discharges back into the hose.
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The discharge photographed at different exposure times. The middle one represents
best what the arc looks like live. Note that the Ar atmosphere has to be pretty
pure and some argon has to flow through the tube before the discharges reach
their full length - where "full length" is limited by the length of
the glass tube. I wonder how long the arcs would get in a bigger tube.
Hmmm...maybe filling a room with argon would be a good idea actually - and most
likely cheaper than building a Tesla Coil...