Deposition and erosion in local shadow regions of TEXTOR-94

Carbon erosion and deposition were investigated on the surface of a flat target covered with an a-C:H film and exposed for 197 s in the SOL of TEXTOR-94. The target was declined by 20 degrees with respect to the toroidal direction and partly protected by an aluminum (3 mm) plate which created an 8 mm wide local shadow. Thickness changes were measured by colorimetry after each plasma discharge. Carbon is eroded from surface areas near the plasma edge (LCFS +1 cm) and transported into the local shadow regions. Accumulation rates up to approximate to 7nm/s were found. The erosion in the local shadow regions (about - 0.1 nm/s) is due to charge exchange neutrals. The observations are confirmed by ion beam analyses and by preliminary calculations with the B2-EIRENE and ERO-TEXTOR code.