PLASMA EDGE PHYSICS WITH SILICONIZATION IN TEXTOR

By using silicon as a material for wall coatings significant improveme nts in the tokamak performance have been obtained. These include the l owest oxygen level ever achieved in TEXTOR, an improved density limit and an enhanced energy confinement also at high central electron densi ties. The present paper gives a survey on the plasma properties under siliconized wall conditions with special emphasis on plasma edge physi cs. The impurity fluxes released from the limiter have been determined as a function of various parameters. An outstanding property of silic on 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 mech anisms which are responsible for the release of silicon. To describe a nd understand these processes at the plasma edge Od- and 1d-models are used to simulate the highly nonlinear system. The properties are comp ared 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 densitie s are reported.