Gap length dependence of two-dimensional electron and copper vapour density distribution over vacuum plasma

Shack-Hartmann type laser wavefront sensors were used for simultaneous visualisation of two-dimensional electron and copper vapour density distributions over vacuum plasmas in a breakdown phase and sustainable arc phase. Our experimental results for a 3-mm gap between Cu electrodes showed that the electrons in the vacuum breakdown stage were mainly supplied from the ionisation process of the copper vapour evaporating from the anode. In addition, the electron and copper vapour densities in an intense arc mode for the 3-mm gap were comparable to those for the 1.2-mm gap in our previous experiments. The validity of the observation results was verified by simultaneously recorded voltage and current waveforms demonstrating an inversely proportional decrease in arc column conductance with increasing gap length