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 e xperiments. Contrary to the usual Deuterium experiments, a significant redu ction of the power load to the divertor is observed in Helium discharges wh ile 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 di vertor into the bulk plasma with the consequent power loss. At this stage o f Helium detachment, most of the outer divertor plasma remains at an electr on temperature of similar to 5-10 eV, for which Helium recombination does n ot occur.