MODIFICATION OF THE CARBON AND BERYLLIUM WALLS IN JET BY EROSION, REDEPOSITION AND DEUTERIUM TRAPPING AFTER THE 1991 DISCHARGE PERIOD

After plasma operation in 1991 samples were cut from C and Be tiles of the JET vessel walls, limiters, and X-point tiles along a poloidal cross section in octant 4W, and the surface layers were analysed by the MeV ion beam methods NRA, PIXE, and RBS to determine the D and metal impurity distributions. The amount of trapped deuterium ranges from 5 x 10(15) to 5 x 10(17) D/cm(2), metal impurities such as Ni, Cr, and Fe are in the range of 10(15) to 3 x 10(16)/cm(2), Cl in the range of 10(16) to 10(18)/Cm-2, K,nd Ca in the range of 10(15) to 10(16)/cm(2) and Ti about 10(15)/cm(2). Except for Cl, all measured concentrations are a factor of up to about ten lower than those observed on JET walls tiles before Be evaporation and Be wall components were used. This observation is in agreement with the reduced impurity concentrations measured in the JET plasma. The D and impurity distributions along the X-point tiles were analysed in some detail. Generally, the deuterium trapping and impurity deposition show minima in areas where erosion dominates and amounts a factor of about ten larger in between.