Cathode spot transition from one cathode to another.

The process of transition of the cathode spot of the vacuum arc from one cathode to another, located at a distance of 5-7 mm from the first, is investigated. It is found that the current transition occurs in portions (Fig.1), this suggests that the cathode spot consists of individual cells and the time of occurrence and disappearance of the cell is less than 10 ns. Immediately before the transition of the cell to the second cathode, a positive polarity current is observed at the second cathode in the form of short bursts of 10-50 ns duration and an amplitude of 0.1 - 1 A (Fig. 2). On the surface of the cathode after such current surges remain microcraters of the correct form, shown in Fig. 3. Under the assumption that the appearance of these microcraters is caused by an ion current from the plasma (the oscillogram of this current in Fig. 2), it is possible to estimate the density of this current as 10⁷ A/cm² and the power density of 10⁸ - 10⁹ W/cm². This is enough to heat the cathode surface to a boiling point in the order of nanoseconds. It can be assumed that this phenomenon plays a significant role in the formation of new cells of the cathode spot.

Fig. 1. Oscillograms of the current at the first and second cathodes in the transition of the cathode spot of the arc on a tungsten with a current of 10 A. the Upper beam: the first cathode 3.3 A/poi. Lower beam: the second cathode 5 A/poi. Time 200 ns/poi.

Fig. 2. Current waveforms on the first and second cathodes immediately before the transition of the arc cathode spot on tungsten with a current of 5 A. Upper beam: the first cathode of 1.5 A/poi. Lower beam: the second cathode of 0.5 A/poi. (inverted signal). Time 100 ns/poi.

Fig. 3. The surface of the second cathode after a single exposure to the current shown in figure 2. There was no transition of cathode spot cells to the second cathode.