Temporal and constriction behavior of low-pressure, cathode-dominated argon discharges

Measurements of quasi-steady-state voltage-current characteristics, low-current voltage and current oscillations, and constriction formation are reported for cathode-dominated discharges in low-pressure Ar. Pulsed a discharges (similar to 5 ms long at 1-3 Hz) reduce heating and improve the accuracy of differential voltage measurements. Values of pressure p times the separation d of the parallel-plane electrodes are 0.12 to 2 Torr cm corresponding to discharge breakdown voltages of 2 kV to 230 V. Discharge currents range from 2 mu A to 40 mA. The quasi-steady-state discharge voltages decrease from their breakdown values in direct proportion to the current, i.e., the negative-differential-voltage-to-current ratio is constant. At currents below the onset of self-sustained sustained oscillations and at the higher pressures, the frequency and damping of transient oscillations are in agreement with a perturbation model based on changes in ion-induced electron emission at the cathode caused by space-charge electric fields. At the higher currents and low pressures transient voltage and current waveforms show the growth of low-frequency oscillations accompanying the onset of constrictions. Rotation of the constricted discharge is observed at these pressures. At the higher pressures the constrictions are stable during the pulse and emission scans parallel to the electrodes provide quantitative data on constricted discharge areas and emissivity per unit current as the discharge increases in size with increasing current. Comparisons are made of these experiments with recent models where possible.