Investigation of toroidal and poloidal asymmetries of the edge density profiles in the CASTOR tokamak using a thermal lithium beam

Abstract

The plasma edge of the CASTOR tokamak ( R = 0.4 m, a = 0.1 m) has been investigated using a thermal lithium beam. The radial profiles of the electron density have been derived from the radiation profiles of the excited lithium atoms at λ = 671nm. Probe measurements have shown that the outward particle flux due to the turbulent density and potential fluctuations via an associated E × B drift, which could be responsible for the observed anomalous diffusion at the plasma edge in ohmic discharges, is poloidal strongly asymmetric. These results have been confirmed by measurements of the edge density profiles on top and at the bottom of the torus. A 45° toroidally movable sector limiter gives the possibility of varying the distance to the limiter continuously. The influence of the sector limiter position on the edge density profiles has been investigated and compared to measurements with a radially movable sector limiter far from the lithium beam (180° ). The additional heating with RF ( f = 1.25 GHz) produces in the CASTOR tokamak a lower hybrid current drive effect, which enhances the particle confinement time. This behaviour has been indicated by steeper edge density profiles during LHCD.