Cathode fall-dominated Ar discharge: Transient and steady-state experiments

Abstract

The time-dependent transition of an Ar discharge from a low-current diffuse or Townsend discharge to a moderately high-current diffuse or abnormal discharge is investigated. We measure the current, voltage, and optical emission transients after a high-voltage pulse is superimposed on the direct current voltage of the low-current discharge. Emission transients are obtained near the cathode, at the end of the steady-state cathode fall, and near the anode. Spatial scans of absolute optical emission at the 750 and 811 nm lines are taken during the development of the cathode fall. These data show the initial growth of current and emission while the electric field is essentially uniform; the early minimum in emission near the anode marking the collapse of the electric field; the motion toward the cathode of the peak of emission; the drop in the discharge voltage, current, and emission during the approach to the steady-state; and the spatial distribution of emission from the steady-state discharge. Measurements are for pressure times electrode separations from 0.5 to 2 Torr cm and steady-state current densities of 500–100 μA/cm2, respectively. The initial rate of rise of the discharge voltage is roughly 100 V/μs and the times required to reach steady state are about 10 μs.