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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