MATERIAL TRANSPORT BY EROSION AND REDEPOSITION ON SURFACE PROBES IN THE SCRAPE-OFF LAYER OF JET

The material transport by erosion and redeposition at the plasma facing wall areas in high temperature plasma experiments has been studied using limiter-like carbon probes with well defined surface deposits and depth markers. The probes have been exposed in the scrape-off layer (SOL) of the Joint European Torus (JET) during single discharges. For the evaluation of these experiments a computer program, ERO, has been developed. The calculated erosion-deposition rates for carbon as a function of the distance to the last closed-flux surface (LCFS) agree well with the experimental results. For a single He-4 JET discharge, erosion yields of 530 angstrom for the silicon deposit and 80 angstrom for vanadium have been measured near the LCFS. A large amount of redeposited silicon (about 17% of the sputtered atoms) has been found on the probe surface in co-deposition with carbon an area not favoured by the proposed model. This observation can be explained by an additional force on the impurity ions (e.g. the existence of a local electric field), which may cause the deposition.