Distribution of ion current density in a modified pulse arc process as a function of pulse parameters

Abstract

The ion emission in pulsed and d.c. vacuum arc discharges was investigated in dependence on static and dynamic arc current parameters. Three different power supplies were used with the following pulse currents and durations: 100 A, 3 μs; 500 A, 1 ms; 1000 A, 100 μs. Mainly the angular distribution of ion current density, the total ion current and the maximum velocity of emitted ions were investigated. It was observed that the ion emission occurs preferentially in a direction perpendicular to the cathode plane (0°). The focusing effect increases at higher pulse current values. Therefore the highest ion current density related to arc current was measured at 0° to be 2.6 mA cm −2 A −1 during a 1000 A pulse. This value decreases to 1/10 at 45°. The total ion current was estimated to be 40–50% of the arc current as maximum during the 1000 A pulse. Furthermore the ion velocity was measured with the help of time-related measurements. In general the values are higher at rates of higher current increase. However, there was an angular distribution. At higher angles relative to cathode normal the measured ion velocities are 2–3 times higher than at the 0° position. Collision effects were supposed to be the reason. Furthermore, calculations show the probability that the average ion charge is clearly increased compared with d.c. during current pulses with a high rate of increase. From measurements at 1000 A pulses one can estimate an increase by 1.7. Some investigations were carried out under high vacuum conditions and others at higher pressures with nitrogen. Most of the changes determined become smaller at higher pressures.