Electrical breakdown resulting in the ignition of a low-pressure low-current glow discharge is investigated in long (length much larger than the diameter) tubes. New features characterizing the breakdown are found. Breakdown begins with synchronous sharp drop of the anode voltage and the peak in the anode current, which is not accompanied by the current at the grounded cathode. This proves the existence of the first (initial) breakdown occurring between the highvoltage electrode and the nearby section of the tube wall. Simultaneously, an ionization wave starts from the anode. The cathode current initiates noticeably later, at the moment when the ionization wave reaches the cathode. The distribution of the breakdown statistic delay time is governed by the Laue law. This study has revealed a profound effect on the breakdown of illumination of the tubes by visible-spectrum light. Illumination diminishes the average breakdown delay time; for the breakdown mode when breakdown occurs at the pulse leading edge this leads to a decrease in the average breakdown voltage. The long-wavelength threshold of the effect is 520 nm. Electron photodesorption from the wall surface is supposed to be the mechanism of the effect. Quantum efficiency for this process is 0.6×10 −9 . Unlike in most previous studies, all the measurements were carried out with unshielded tubes; screening of the tube by a grounded shield has a strong influence on the breakdown characteristics.

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The influence of the pulses of visible-spectrum light on the breakdown in a discharge tube was studied experimentally. The tube of 80 cm length and 1.5 cm inner diameter contained neon at a pressure of 0.6 Torr. High-voltage rectangular or ramp pulses were applied to the anode. The light pulses of 0.1–100 µs duration were produced by the light-emitting diode or laser diode at a wavelength of 460 and 407 nm, respectively. A small area of the tube wall near the anode was illuminated at different moments after or before the voltage pulse onset. It has been found that the light pulse exerts the strongest influence on the breakdown. In the case of rectangular pulse, it drastically decreases the statistical delay time of the breakdown. As a result, all the breakdowns, which did not happen before the light pulse, occur during several microseconds after its onset. The light pulse influences the breakdown delay time even if it terminates before the voltage pulse onset. This can be explained by the finite lifetime of the secondary electrons produced by the light. In the case of the ramp voltage pulse, an essential decrease in the breakdown voltage scatter is observed under illumination. The effect is especially pronounced if the light pulse is applied at the moment when the anode voltage is equal to or somewhat lower than the minimum breakdown voltage. For later moments, the breakdown voltages are strictly bounded by both sides. The average breakdown voltage decreases noticeably. As for the mechanism responsible for the observed effect, electron photodesorption from the tube wall is suggested. This process presumably occurs in dielectric barrier discharge at atmospheric pressure where it can influence the breakdown voltage and provide synchronous breakdown of micro discharges, thus causing the formation of multiple filaments.

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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.