Spontaneous extinction of the arc by reducing the current and threshold currents.

The only study of the arc discharge threshold current values was carried out by Prof. Kesaev in the late 60s, we do not know the later measurements, while the threshold current value is very important for understanding the cathode processes. We have undertaken a study of the phenomenon of spontaneous extinction of the arc discharge with a gradual decrease in the discharge current on the clean surface of the cathode with a high time resolution (Fig. 9). New data on the threshold arc discharge current for three materials with significantly different thermophysical properties: cadmium, copper and tungsten are obtained. The data obtained by us on the threshold currents of the arc discharge are close to the data of Kesaev, however, the threshold currents were slightly lower, and for cadmium is significantly lower (about three times) than that of Kesaev. In our opinion, this is due to the higher temporal resolution of the method we use. Based on our observations, we can conclude that the current flow in the cathode spot cell is a current pulse of approximately rectangular shape (Fig. 10). The rise and fall time of the current is 1 - 2 ns for fusible material (cadmium) and less than 0.8 ns for refractory cathode material (tungsten).

Fig. 9. An example of a current waveform for a copper cathode. Current - 1.5 A/poi.; time - 20 ns/poi.

Fig. 10. Example of an oscillogram of current and light for a tungsten cathode.