Impurity Flux Studies in the Limiter Shadow of the TEXTOR Tokamak

The temporal and radial dependence of the flux of impurity particles in the plasma scrape of layer in the TEXTOR tokamak is measured during a series of reproducible discharges by using passive collection probe techniques. Oxygen, titanium, chromium, iron and nickel impurities are detected. The flux of heavy impurities is observed to be reproducable. A typical peak value of the iron flux during a discharge is 2 · 1015 at./cm2 · s. No saturation effects of the measured amount of impurities on the probe surface is observed for surface densities of up to 5 · 1015 at./cm2, indicating that in this range losses due to possible release mechanisms of collected impurities are negligible. The e-folding length of the radial decay of the impurities detected on the probe surface is observed to vary with the discharge time. The value of the e-folding length is determined to be 20-30 mm at the flat top part of the discharge and about 13 mm at the end, for distances between 10 and 24 mm from the plasma edge.