EROSION OF LONG-TERM WALL SAMPLES IN JET

The erosion of carbon and the deposition of carbon and impurities have been determined for graphite samples exposed at the wall of JET during the six months of operation in 1985. Graphite erosion was determined using SIMS analysis of the depth change of implanted **1**3C marker layers in the samples. Impurity deposition was measured using SIMS and Rutherford backscattering. The samples showed high erosion ranging from 0. 2 to 0. 6 mu m at various locations, with significant poloidal variation in both erosion and deposition. The deposition of substantial quantities of metal impurities and carbon was related to a probe accident and a deliberate heavy carbonization procedure during the operation period. The highest observed erosion exceeds the expected sputter erosion due to charge exchange neutrals by more than an order of magnitude, unless neutral fluxes are enhanced at the inner wall due to ions arriving along open field lines during current ramp-down.