Measurements of impurity fluxes in the ASDEX-plasma boundary

Impurity fluxes parallel to the magnetic field in the plasma boundary of ASDEX were measured with a collector probe. The probe allows simultaneous measurements of the radial distribution and can be rotated in order to obtain time resolution. The amount of impurities deposited on the graphite collector stripes is subsequently analysed by Rutherford backscattering. The predominant species detected are oxygen and stainless steel components, other impurities such as Ti, Si, Mo, and Au are also found. For typical divertor discharges the average Fe flux near the outer wall is about 1.3 x 1014 Fe-atoms/cm2 x s with an e-folding length of 1.1 cm. The oxygen flux is about one order of magnitude higher. Time-resolved measurements show a maximum deposition rate at the beginning of the discharge. Impurity fluxes for discharges with various parameters including limiter and divertor operation are discussed. (orig.)