Effect of Operating Pressure on Plasma Formation in the Transient Hollow Cathode Discharge

Abstract

The discharge development of a triggered pulsed hollow cathode discharge is investigated with respect to two regimes of operating pressure: the high pressure regime at ∼ 10−2 mbar and the low pressure regime at ∼ 10−5 mbar. The hollow cathode discharge studied here is performed with the UMFX device powered by either eight 2700 pF door‐knob capacitors connected in parallel and discharged at 40 kV or a single 1.85 μF capacitor discharged at 20 kV. The characteristics of the discharge is monitored by using magnetic probe for dI/dt measurement; PIN diodes and XRD for time‐resolved X‐ray signals; and pinhole camera for time‐integrated X‐ray imaging. At the high operating pressure regime, X‐rays are observed to be emitted from the tip of the anode only. At the low operating pressure regime, however, the heating of the plasma by the main capacitor discharge is significantly enhanced. In the case of low energy discharge (with 22 nF capacitor bank), a plume‐like plasma emitting in the soft X‐ray region is observed to be formed attached to the tip of the anode. The pre‐breakdown electron beam, which is a well‐known phenomenon in this type of discharge, is also observed to be significantly enhanced. For the case of discharges performed with the 1.85 μF capacitor, dense plasma spots are observed at low operating pressure of 10−4 mbar or lower. These dense plasma spots are not observed for discharges operated at pressures higher than 10−4 mbar