Boronization of ASDEX

Boronization is the plasmachemical deposition of thin boron/carbon or pure boron films onto the inner wall of a tokamak device. Since mid-April 1989 the ASDEX vessel has been boronized seven times in glow discharges, first with a gas mixture of 10% B2H6+10% CH4+80 % He (A), once with 10% B2H6+90% H2 (B) and once with 10% B2H6+90% He (C). All boronization runs were done at room temperature vessel walls. Local surface analysis of the deposited layers yielded thicknesses of up to 2000 A. Boronization provided considerably improved plasma conditions. In ohmic discharges, reductions of CO by a factor of more than five in the divertor chamber and of two for H2O were observed, in relation to a reduction of oxygen by a factor of more than five as measured by spectroscopy, but the reduction of hydrocarbons was only marginal. Iron disappeared from the spectra and even copper, the target plate material, was significantly reduced. As a consequence, Zeff is < 1.5 for anti ne>3 x 1019 m-3. The density limit was improved by 25% (boronization A) and more (boronization B and C). The oxygen-diminishing effect of boron/carbon and pure boron layers was found to be longer lasting than the suppression of metal impurities. Deteriorated wall conditions occurred only after hundreds of discharges, including numerous additional heating shots with ICRH, NI, LH and combined scenarios, but pre-boronization conditions were never reached. The main emphasis of this paper is to describe the impact of boronization on aspects of plasma operation. (orig.)