Instabilities of prebreakdown currents in vacuum I: late breakdowns

Prebreakdown electron emission between extended metal electrodes is experim entally studied with time resolution of 100 ns. Switching to higher emissio n levels is observed and could be provoked by mechanical shocks. This is fo und to be analogous to spontaneous switching effects. Elementary time const ants of current jumps are 1-3 mu s while the whole switching process lasts up to 40 ms, thus ruling out impact of macroparticles. The breakdown voltag e is found to be correlated with the emission level. Therefore, switching o n electron emission can explain breakdowns with delay times >1 ms. Still lo nger times are found at the deposition of metal vapour plasma originating f rom neighbouring are gaps, in some cases, transient increases of the emissi on occur that last several seconds. Therefore, in vacuum interrupters late breakdowns after current zero can be caused by mechanical shocks without pa rticle impact and also by deposition of residual vapour at the cathode.