Arcing through a thick B4C layer

The traces of strong erosion found on solid components covered with thick (0.17 mm) B₄C-layers facing the edge of a fusion plasma are identified as relicts of arcing. Spots of light observed on the surface seem to exist during the whole lifetime of the fusion plasma (4...5s) and stay motionless despite the strong external magnetic field (B=2.3 T). The related big craters are single cylindrical holes reaching through the B₄C-layer down to the Cu-substrate. In subsequent discharges the re-ignition of a new arc at the location of an old crater was found to be very probable. The interesting peculiarities of ignition and burning of arcs on B₄C-layers have been confirmed in laboratory experiments (B=0.4 T) revealing that the arc current has remarkably low noise. Theoretical investigations show that a thermal runaway does not exist because of the rise of the conductivity of the semiconducting B₄C with temperature. Long living arcs influence strongly the stability of the imbedding fusion plasma and the final erosion of the substrate material at the bottom of the crater holes reduces the suitability of B₄C as coverage material.