PLASMA EDGE PHYSICS WITH SILICONIZATION IN TEXTOR

By using silicon as a material for wall coatings significant improvements in the tokamak performance have been obtained. These include the lowest oxygen level ever achieved in TEXTOR, an improved density limit and an enhanced energy confinement also at high central electron densities. The present paper gives a survey on the plasma properties under siliconized wall conditions with special emphasis on plasma edge physics. The impurity fluxes released from the limiter have been determined as a function of various parameters. An outstanding property of silicon as an impurity in the plasma is its strong influence on plasma edge parameters caused by line radiation. A high radiation level of up to 70% of the input power (1.5 MW) has been achieved, limited by the mechanisms which are responsible for the release of silicon. To describe and understand these processes at the plasma edge Od- and 1d-models are used to simulate the highly nonlinear system. The properties are compared to plasmas in a boronized machine. Silicon and neon are compared with respect to radiation characteristics and transport to the centre. Remarkable differences in the penetration depths and central densities are reported.