Probe measurements of the impurity flux at the plasma edge of the stellarator Wendelstein VII A

Abstract

The progress in stellarator operation in the past 3 years has led to considerable efforts in the planning of future toroidal devices outside the tokamak class. In particular, the access of a currentless regime, in which the plasma was heated by non-ohmic methods, has made it possible to study parameters beyond the range of previous plasma experiments in tokamaks or stellarators [ 1,2]. In the Wendelstein VII A (W VII A) stellarator, the currentless operation was achieved by neutral injection alone after an initial phase with combined ohmic heating and neutral injection. However, an accumulation of impurities in the plasma center resulted in an enhanced radiation loss, which prevented long discharges. A considerable fraction of the impurities appeared to be oxygen from the neutral beams, but heavy impurities such as titanium, molybdenum, or iron/nickel/chromium may contribute as well to the impurity flux. Therefore, it was attempted to collect heavy impurities at the plasma edge of W VII A. The present work contains the first systematic probe study of the impurity flux at the plasma edge of a stellarator.