Non-linear erosion effects in plasma experiments

The erosion and the deposition behaviour of various materials (Be, C, Si, Mo, and W) exposed to a plasma, containing impurities, have been investigated by means of computer simulation with the program TRIDYN and analytical calculations. A strong dependence on the impurity concentration in the plasma, the exposure time and the kind of material has been found. Regions can be defined in this parameter field where erosion of the target material or deposition of the plasma impurities (such as carbon) dominates. Further, a transition from an erosion phase to a deposition phase (and also reversed) during the exposure was observed under certain conditions. These highly dynamical changes occur in a time scale of about some seconds and, thus, are of the same order of the discharge times in today's fusion experiments. The developed analytical model describes well the observed behaviour and can be used for the calculation of the target erosion in fusion experiments if diffusion and segregation effects do not play a role.