Heavy impurity collection at the plasma edge of the stellarator W VII A

The presence of impurities at the plasma edge of the Wendelstein VII-A stellarator was studied by means of carbon probes that were exposed to up to 200 plasma discharges in helium. The probes were subsequently analysed with 1 MeV 4He+ Rutherford Backscattering. The average impurity deposition for Ti, Mo and wall components (Fe, Cr, Ni) was 2-4 x 1012 atoms/cm2, 6 x 1010 atoms/cm2 and 1 x 1011 atoms/cm2 per discharge, respectively. With the exception of Ti this impurity deposition is more than one order of magnitude smaller than the corresponding results from comparable tokamaks. (orig.)