Influence of Plasma Composition on Divertor Detachment

The phenomena that lead to the observed power and particle flux detachment have been studied with the code B2-Eirene for DIII-D Helium and Deuterium experiments. Contrary to the usual Deuterium experiments, a significant reduction of the power load to the divertor is observed in Helium discharges while the ion flux remains high. Modelling indicates that this is due to the longer ionisation mean free path of Helium, which can penetrate from the divertor into the bulk plasma with the consequent power loss. At this stage of Helium detachment, most of the outer divertor plasma remains at an electron temperature of ∼5-10 eV, for which Helium recombination does not occur.