Erosion/re-deposition modeling in an ITER divertor-like high-density, low-temperature plasma beam

Transport of hydrocarbon impurities in a high-density (>10 ²⁰m^-3), low-temperature (<2eV) plasma beam was studied with the ERO code. The high ion density and low temperature cause strong Coulomb collisionality between plasma ions and impurity ions. The collisionality is so strong that ions typically do not complete their Larmor orbits. The high collisionality causes impurity entrainment: impurity ions quickly acquire a velocity close to the plasma flow velocity. This causes a relatively high surface impact energy: the calculated mean impact energy of CH_x was 8.1eV in a plasma with T_e=0.7eV. Simulation results were compared to an a-C: H erosion experiment in the linear plasma generator Pilot-PSI. The large uncertainties in literature values for the sticking probability of hydrocarbon radicals are shown to cause a serious uncertainty in the calculated re-deposition pattern. In contrast, the radial electric field component perpendicular to the axial magnetic field lines did not have a major effect on the redeposition profile.