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

Abstract

The effect of irradiation with visible spectrum light on breakdown in discharge tubes 75–80 cm long and 1.5 cm in inner diameter in rare gases at a pressure of ∼1 Torr was studied. A ramp voltage of variable slope in the range of ∼10–1–105 kV s−1 was applied to the tube anode. The tube was illuminated by radiation from fluorescent lamps operating in a continuous mode, as well as by LEDs or a laser diode operating in a pulsed mode. The breakdown voltage and the pre-breakdown ionization wave (IW) velocity were measured. Illumination led to a change in the breakdown potential. The sign of this change depended on the anode voltage rise rate dU/dt. At dU/dt > 102–103 kV s−1, the breakdown voltage decreased. A similar effect was observed earlier and was explained by the appearance of electrons in the discharge gap under the light action, as a result of which the breakdown delay time decreased. This, in turn, caused a decrease in the breakdown voltage. At dU/dt < 101–102 kV s−1, on the contrary, the breakdown potential increased; at dU/dt ∼ 0.1 kV s−1, this increase could reach 5–6 times. The dependence of the observed effect on the radiation intensity, its wavelength, and the illuminated area position on the tube surface is studied. The pre-breakdown IW behaved in an unusual way under these conditions: its velocity and the signal amplitude recorded by the capacitive probe increased when moving from the high-voltage anode to the cathode. It is assumed that the observed features are caused by the desorption of weakly bound electrons from the tube wall surface under the action of irradiation. These electrons create a current that charges the wall near the anode. Since the first stage of discharge ignition is the initial breakdown between the anode and the tube wall, the anode potential for such a breakdown should increase, which means an increase in the breakdown voltage. Additional experiments with the initiation of a preliminary IW by a pulse applied to the cathode, confirmed the existence of a charge on the wall near the anode.