Spots in Arc Discharges

The cathodes spots are the localized current centers, which appear on the cathode when significant current should be provided but the cathode carmot be heated enough as a whole. The most typical cause of cathode spots is the application of metals with relatively low melting points. The cathode spots can also be caused by low arc currents, which are only able to provide the necessary electron emission when concentrated to a small area. The cathode spots also appear at low gas pressures ( 1 Torr), when metal vapor from the cathode provides atoms to generate positive ions bringing their energy to the cathode to sustain the electron emission. To provide the required evaporation, cmrent is concentrated in spots at pressures 1 Torr and currents 1-10 A, such spots occur even on refractory metals. 

Several problems related to the cathode spot phenomenon are not thus far completely solved. For example, the cmrent-voltage characteristics of vacuum arcs are not decreasing but increasing also there is no complete explanation of the cathode spot motion and splitting. The most intriguing cathode spot paradox is related to the direction of its motion in an 

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