Measurement of differential wall changes using surface probes in the Alcator-A tokamak

Plasma Impurities resulting from the limiters and walls of tokamaks are a recognized problem for fusion reactors. This paper reports recent measurements of Impurity fluxes and differential changes at a surface probe near the plasma edge of Alcator-A. High purity aluminum samples were prepared and Auger analyzed in an interlocking UHV chamber using a retractable probe. Rutherford backscattertng is used to determine the quantities of implanted species. Time resolved measurements are made by rotating a disc-shaped sample behind an aperature during discharges. Estimates are made for C, N, O, Fe and Mo fluxes at the plasma edge.