To expand a little more on the pressure-voltage issue with glow discharge.
Since the "glow" we see is from excited gas molecules relaxing, and since the ionization potentials of the residual gases is a bit less than 20 eV, it really doesnt take a lot of voltage to light up the plasma, just that the mean free path (mfp) is long enough for the electron to acquire ionization energy. But as the gas density gets lower, the mean free path lengthens. The stand off voltage at first lowers, then increases as the system passes through the Paschen minimum for that electrode geometry.
The glow technically is always there, until the mfp is soo long that the probability of electrons colliding with a gas molecule is too small for a detectable number of events in the observing period.
The geometry of fusor potentials concentrates the ions in the central "star" where the gas density is considerably higher than the surroundings, thus we see the glow.
If we think of the star as approximately like a spherical cluster of charge, then the potential of the outer charges varies as 1/r. IF we assume a certain charge density is needed to be able to see the glow, then as the absolute gas pressure drops, the star gets smaller and the potential needed to maintain it, increases. Hence the voltage to create the glow increases as the pressure drops. Dave Cooper
Created on Saturday, June 02, 2001 3:25 AM EDT by David Cooper