An “Anomalous” Effect of Illumination on the Breakdown in a Long Discharge Tube in Xenon

We studied an electric breakdown in an 80-cm-long discharge tube with the inner diameter of 1.5 cm (the so-called “long discharge tube”) in xenon at 1 Torr. The discharge was initiated by positive pulses of ramp voltage with a rise rate dU/dt on the order of 10^–1–10⁵ kV/s. The breakdown voltage was measured in darkness and upon tube illumination by fluorescent lamps, LEDs, or a diode laser. The effect of illumination depends on the slope of the pulse leading edge. The voltage drops at dU/dt > 100 kV/s, while growing at dU/dt < 100 kV/s. The voltage increases by a factor of six for the voltage slope of 0.1–1 kV/s. The dependence of the observed effect on radiation intensity, wavelength, and position of the illuminated area on the tube surface is studied. The pre-breakdown ionization wave behaves unusually under the described conditions: its speed and intensity of emission at its front grow in the course of wave propagation. Photodesorption of electrons from the tube surface as a result of which the wall near the anode becomes positively charged is assumed to represent the mechanism of the observed phenomena. This causes an increase in the breakdown voltage and accelerated propagation of the ionization wave. Additional experiments confirm the presence of the wall charge in the near-anode region under the discussed conditions.