Light emission of cathode spot plasma in spark and arc stages of vacuum discharge

Modern notations don´t foresee principle differences in cathode spot mechanisms after a few tens of nanoseconds after discharge ignition. Really, cathode spots generate similar craters regardless of discharge burning time under the condition of cold cathode. The cathode spot plasma has almost stable parameters in the microsecond time scale while gradual changes in the plasma parameters can convincingly be explained with cathode heating effects. However, invariability of spot mechanisms is not the feature of initial period of spot burning since the moment of gap breakdown. We recently observed a surprising behaviour of cathode spot plasma characteristics within the first microsecond of discharge burning. Though ion energies per charge ratios are the same for all charge states at the long-burning arc discharge, there is a dependency of the ratios on charge states in the starting phase. The similar transition is seen in spot light radiation. Higher ion charge state lines start to shine first, being followed by lower charge state lines. Finally, atomic lines appeared after the delay as long as half microsecond. The latter fact correlates with exactly the same delay of resonant absorption by spot plasma at atomic lines.