Studies in inverse magnetron discharges of vacuum interrupters — Part 3 — Anomalies

This is the third part of a series of investigations into inverse magnetron discharges in vacuum interrupters. In the previous papers the effect of variations in the magnetic and electric fields were studied. A number of anomalies in the discharge waveforms were seen, including what appeared to be step changes in the characteristics. Different anomalies occurred for single and double magnetron discharges. In this paper we examine the anomalous effects more deeply. We examined only the Single Inverse Magnetron (SIM) discharge, and not the Double Inverse Magnetron type. specifically the Pressure-Voltage relationship for a fixed magnetic field, where we see that the relationship reverses at a particular voltage, and also the Arc Stability-Magnetic Field relationship for a fixed voltage, where the arc characteristic changes from smooth to noisy, and there is a distinct change in the waveform shape. We determined that the reversal of the Pressure- Voltage relationship is real and occurs over a small range of voltage. However we have not been able to explain this. For the Arc Stability-Magnetic Field relationship, we determined that this is also a real effect, and believe that it comprises of two distinct discharges, a smooth discharge that occurs for all values of magnetic field and a noisy discharge that occurs and increases once the magnetic field reaches a critical value. We believe that this may be due to local outgassing of the electrodes commencing which causes a secondary discharge involving these evolved gasses.