Studies on plasma edge transport and layer deposition by local injection of reactive gases during TEXTOR-94 discharges

To study local particle transport, silane was injected through a hole in a test limiter surface during discharges in TEXTOR-94. The gas injection lead 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. The layer was not only formed 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 ∼ 25° relative to the magnetic field lines Calculations with the ERO-TEXTOR code recover the general shape of the Si-deposition pattern. To get accordance between the calculations and the experimental results a strong electric field parallel to B and a sticking coefficient much smaller than 1 has to be considered to reproduce the low silicon deposition efficiency, whereas the tilting of the symmetry line can be explained by the existence of a radial electric field.