Wall conditioning of TEXTOR

Abstract

We summarize the experience gained over two years on the wall conditioning of TEXTOR by radiofrequency assisted DC glow discharges (RG-discharges). The importance of the contamination history of the wall for the cleaning rate in H2 RG-discharges is underlined.RG-discharges in hydrogen already lead to some sputter erosion of the wall. This is strongly enhanced when small amounts of other gases, e.g. 1% of methane are added. The resulting material transport between liner and limiter is evaluated quantitatively from direct measurements of the energy and fluxes of the impingling species and measured by means of collection probes. The consequences for the case where the materials of limiter and liner differ are discussed.The plasmachemical deposition of carbon via H2/CH4 RG-discharges has been further examined. A homogeneous carbidic deposit results at elevated wall temperature when ~1% of CH4 is added to the hydrogen. Significant amounts of oxygen are removed in the form of CO even from a well-preconditioned wall. The oxygen concentration in the tokamak discharges which follow the carbidization is lowered by a factor of three whereas the carbon content is increased by roughly the same factor and chlorine as plasma impurity is almost completely suppressed.Except for this, the global behaviour of the confined plasma is almost unaffected. This result is compared to our earlier observations.