Laboratory investigations of arcing on W-coated graphite components

Results of laboratory experiments of arcing on graphite tiles coated by a W-layer are reported. The samples have been taken from coated tiles manufactured for ASDEX Upgrade (AUG). The motion of the arcs was observed by high-speed cameras. Additionally, sample plates have been exposed to collect macro-particles emitted by the arc. The eroded surfaces of the cathodes were investigated after experiment to characterize surface changes, tracks, and re-deposited particles. On the cathode strongly radiating immobile spots are observed by the cameras acting as sources of numerous macro-particles. At the surface large holes (diameter 17 μm) are found that perforate the W-layer and extend into the graphite bulk. Subsequent arcs tend to locate at the pre-existing holes. Hence, locally the W-coating is quickly and effectively broken, the W erosion is enhanced as compared to bulk W, and carbon is locally liberated despite the existence of an undamaged W-coating outside the arcing region.