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 (IW) 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 IW revealed the presence of extra waves preceding the regular pre-breakdown IW. 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.