Time resolved metal impurity concentrations in the dite tokamak using RBS analysis

Abstract

A carbon disc collector has been inserted into the boundary layer of a tokamak plasma to a position radially inside the limiter. The disc is rotated about an axis parallel to the toroidal field B T during a discharge so that a time resolved measure of the flux of impurities to the collector is obtained. The concentration of impurities on the surface is subsequently analysed using Rutherford backscattering of 3.5 MeV 14N + ions. It has been observed that the flux of metal impurities to the probe rises rapidly at the beginning of all discharges and that in discharges with few low Z impurities the flux remains high throughout the discharge. The flux can be reduced, however, by injecting a low Z impurity, e.g. neon and can be reduced even further by injection of hydrogen gas during a discharge.