Breakdown voltage in long tubes: the effect of surface charge

Abstract

The study focuses on ignition processes in long discharge tubes (the length of which is large compared to the diameter) in rare gases Ne, Ar, and their mixture at a low pressure (~ 1 Torr). Gas breakdown was caused by ramp voltage pulses of positive or negative polarity applied to the active electrode. The breakdown voltage was determined by the voltage drop at breakdown. The emission of the ionization wave preceding the breakdown was explored. The discharge tubes were exposed to two types of external influences. The first was illumination of the tube cathode with visible spectrum light, while the second was the constant or pulsed bias of the cathode potential by a value lower than that of maintaining discharge. In both cases the breakdown voltage increased up to doubling under some conditions. The observation of the ionization wave revealed the presence of extra waves preceding the regular pre-breakdown ionization wave. The extra wave velocity and emission intensity differed from those of the regular waves. Their main feature is that they do not overcome the entire inter-electrode gap, but weaken and disappear in between. It is assumed that the extra waves deposit the wall surface charge, which in turn affects the breakdown voltage. The increased breakdown voltage value remains for tens of minutes, which could indicate the surface charge lifetime of the same order. This was confirmed by direct wall- potential measurements using an electrostatic voltmeter.

Hindering breakdown in a long discharge tube by visible spectrum light illumination

“…(1) Non-monotonicity of the dependences obtained under illumination, specifically, the presence of a minimum at dU/ dt ∼ 10 2 -10 3 kV s -1 . Similar curves were observed in [13,14]. (2) An increase in U b at higher dU/ dt, especially pronounced for a non-illuminated tube.…”

“…Additional experiments were carried out, which seems to confirm the above interpretation. The applied voltage pulses were of the positive polarity, while in [14] a similar non-monotonic U b on dU/ dt dependence was observed in Ne-Ar mixture for negative polarity pulses. The minimum of the U b (dU/ dt) curve was at 3 × 10 2 -3 × 10 3 kV s -1 .…”

“…At dU/ dt ∼ 10 kV s -1 and below, illumination leads to the breakdown voltage increase, while at dU/ dt ∼ 100 kV s -1 and above it leads to the breakdown voltage decrease. A decrease in the breakdown voltage in the illuminated discharge tubes as noted in the introduction, was observed in most works done on this subject [14,[22][23][24][25][26].…”

“…The left branch of the curves for the illuminated discharge was obtained earlier in [13] (figure 1) and [14]. In the former, it was assumed that the high breakdown voltage is explained by the charging of the tube wall near the anode due to the finite glass conductivity.…”

“…It seems most reasonable to attribute this effect to the photodesorption of weakly bound electrons from the dielectric wall surface [14,[23][24][25]. The desorbed electrons appearing in the volume decrease the breakdown delay time and according to (1) reduce the breakdown voltage.…”