Laser Induced Unipolar Arcing

Abstract

High power laser beams interact with targets by a variety of thermal, impulse and electrical effects. The laser heated plasma causes surface ablation by thermal evaporation, ion sputtering, and unipolar arcing. While the first two are purely thermal and mechanical effects, the last one, unipolar arcing, is an electrical plasma-surface interaction process which leads to crater formation, usually called laser-pitting, a process which was often observed but not well understood.

Unipolar arcing occurs when a hot plasma of sufficiently high electron temperature interacts with a conducting wall. Without an external voltage applied, many electrical micro-arcs burn between the surface and the plasma driven by local variations of the sheath potential with the wall acting as both the cathode and anode. Laser induced unipolar arcing represents the most damaging and non-uniform plasma-surface interaction process since the energy available in the plasma concentrates towards the cathode spots. This causes cratering of the materials surface. The ejection of material in the form of small jets from the craters may lead to ripples in the critical density contour. This in turn may contribute to the onset of plasma instabilities, small scale magnetic field generation and laser beam filamentation. The ejection of a plasma jet from the unipolar arc crater also causes highly localized shock waves to propagate into the target, softening it in the process. Thus, local surface erosion by unipolar arcing is much more severe than for uniform energy deposition.