Dynamics of cathode spot plasma parameters in spark and arc stages of vacuum discharge

Modern notations do not foresee principle differences in cathode spot mechanisms in the breakdown stage as compared with steady burning arc. There is no reason to have doubts in this statement. Really, cathode spots generate similar craters regardless of discharge burning time under the condition of a cold cathode. Cathode spot plasma has rather stable parameters while gradual changes in plasma parameters can be convincingly explained in terms of cathode heating effects. The only difference is that the beginning stage gives a chance to trace a separate cycle of cathode spot operation and confidence that the initial moment of a cycle is a proper one. Unlike stable long-lasting arc, the first spot cycle is not hazy with prehistory; and spot behaviour looks different as a result. Having utilized a first spot cycle for diagnostics, the authors have revealed dynamics of atomic and ionic spectral lines in single spot radiation with high temporal resolution. Higher charge state ions start to emit light first followed by lower charge state ions and neutrals being latest. All the spectral lines start to be highly broadened. These data was successfully used to revise conclusions drawn earlier concerning experiments on resonant laser diagnostics of cathode spot plasma.