Deionisation of an interrupted vacuum arc

Measurements have been made of time-dependent saturation ion currents flowing to a probe and to the discharge electrodes following the forced extinction of high-current (1¿5 kA) vacuum arcs of short duration (¿1 ms) on copper electrodes. The results have been utilised to derive ion-flux distributions irrespective of ion-charge number, assuming collisionless plasma expansion from the cathode spots, and significant differences have been found compared with previous measurements on steady-state discharges. The results on interrupted and steady-state discharges are reconcilable if a source of low-energy ions exists, which is constrained in the cathode-spot region in the steady state and which is released at the zero current condition when the constraining forces vanish.