Modelling of carbon transport in fusion devices: evidence of enhanced re-erosion of in-situ re-deposited carbon

The paper presents new Monte-Carlo transport simulations of methane ¹³CH₄ injected through a hole in a testlimiter and exposed to the edge plasma of TEXTOR. The results show that the spatial distribution of ¹³C re-deposited locally on the testlimiter surface can be modelled if the parameter S for the sticking of returning hydrocarbons ¹³CH_y is set to zero or almost zero. This is interpreted as a negligible effective sticking of the returning hydrocarbon radicals due to the instantaneous re-erosion caused by the hydrogen carried with the CH_y radicals (`self re-erosion'). However, the calculated local deposition efficiency of ¹³C, remains too high compared with the observed value. Therefore, in addition an enhanced yield for chemical erosion caused by the background hydrogen for the fresh re-deposits has to be assumed. Similar assumptions can reproduce also the high amount of carbon deposition found on the inner louvers in the MkIIa divertor configuration of JET and on the plasma-shadowed areas of the MkIIGB divertor.