Modeling of plasma-wall interaction in tokamak experiments with high-Z materials*

Since high-Z material W will be used for ITER divertor, the understanding of related plasma-wall interaction processes is indispensable and only can be achieved in a combined effort of experiment and modelling. The 3D Monte Carlo code ERO taking into account a material mixing surface model has been used to simulate W erosion and re-deposition on DIII-D with toroidally continuous W rings embedded in the divertor and EAST with an upper full W divertor. Modeling shows that the transport of C impurities not only dominates the W sputtering but also determines the overall erosion and deposition balance in the mixed materials surface. With a self-consistent calculation of C impurity distribution, W gross erosion rates measured by WI spectroscopy can be well reproduced by the modeling. The ExB drift and lower electron temperature at the radial outboard side lead to a net deposition zone where W and C are accumulated. In the net erosion zone closer to the outer strike point, the W coverage on C is very low and saturated independent of exposure time, agreeing with the measurements by collector probes. Strong sheath effects on material erosion rates have been observed using external biasing samples, which have been simulated by the 2D PIC code SPICE2 and the ERO code. Both the PIC simulation and the measured Dα emission reveal that with increasing biasing voltage the ion flux decreases at the biased area while increases at the adjacent downstream tile, although the biased sample potential is far below the plasma potential. The reason indicated by modeling is the strong gradient of the electric field in the sheath, which results in different magnitude of the polarization drift above the biased and non-biased surface. More than an order of magnitude reduction in erosion with slight positive voltage biasing in the experiments is due to the reduced incident energy and ion flux.