The use of the RG conditioning system to examine some recycling mechanisms occuring in an all-carbon surrounding

Abstract

The plasma facing surfaces in TEXTOR consist of hydrogen (H/D)-rich carbon. The near surface H/D and oxygen concentrations at the start of a tokamak shot depend on the preceding tokamak or conditioning discharges and on the wall temperature. The subsurface layers — which are essentially free of oxygen after carbonization — collect significant amounts (5–10% per collected C atom) of oxygen during the tokamak discharges. This affects the plasma contamination. The amounts of 0 in the plasma and in the wall are similar during H, D and He shots. Short (e.g. 15 min long) RG discharges in He decrease the near surface 0 concentration and reduce the 0 radiation by a factor of ~ 4 in the following tokamak shots. Similarly, RG discharges in He and /or prolonged bake-out at 350–400 ° C reduce the subsurface concentration of H/D and affect its recycling in the next shots. The implications of these and related measurements concerning the recycling mechanisms of D and of O during tokamak operation are discussed. We conclude that, at low wall temperatures (≲ 200 dg C), the D and O inventories in the near surface layer are dominated by implantation and particle induced desorption events which occur during the discharge. At higher temperature (≳ 300 ° C), the thermal release of D 2, between discharges becomes more important. The near surface carbon layer is found to act as a buffer for oxygen of which the primary source is not on the plasma-facing surfaces but elsewhere in the vacuum vessel.