Ignition and burning of vacuum arcs on tungsten layers

Dedicated experiments have been carried out in vacuum to study breakdown and arc burning on graphite cathodes covered by a thick (lμm) tungsten layer. The motivation originates from the usage of similar layers as first wall material in fusion devices where a large number of arcs appeared during plasma operation. Besides the standard measurements of current and voltage optical observations of the developing cathode spot and the retrograde motion in a transverse magnetic field on a short time scale have been carried out to determine the arc velocity reliably. Special attention was payed to non-reversible modifications of the layer by arcing. Electron microscopy (SEM) has been applied to specify the geometric structure of the tracks together with the distribution of elements. Main result is the observation that a lμm tungsten layer is permanently perforated by passing of an arc exposing carbon from the underlying graphite to all subsequent erosion processes. The effect depends nonlinearly on the arc current and is used to estimate qualitatively arc currents from remnant tracks found on plasma lacing components. The erosion rate is about one order of magnitude higher than for arcing on bulk W.