Investigation of transport processes close to limiter surfaces in TEXTOR-94

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Abstract
Silane gas has been injected through a hole in the surface of test limiters during ohmically heated tokamak discharges in TEXTOR-94. The gas injection led to the deposition of a thin layer around the injection hole which was close to the area of the highest heat load. The layer was eroded in discharges without silane injection and spectroscopic measurements prove that the erosion of limiter material is reduced in areas where the layer is deposited. Surface analyses reveal that the layer was formed not only by the injected silicon but also by a large amount of carbon which must originate from other plasma-facing components. The preferential direction (symmetry line) of the deposition pattern is tilted by relative to the magnetic field lines. Calculations with the Monte Carlo code ERO-TEXTOR were used to interpret the data. The shape of the Si-deposition pattern can be recovered quantitatively by including an drift in a radial electric field originating from the gradient in the electron temperature profile above the limiter surface. The deposition pattern allows the `imaging' of the complex plasma flow close to the surface. To obtain agreement between calculated and experimentally observed deposition efficiencies a sticking coefficient much smaller than 1 and the influence of an electric field parallel to B have to be considered.

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