Theory and modelling of helium enrichment in plasma experiments with pump limiters

Helium enrichment in the exhaust gas stream flowing from a hydrogen-helium plasma is studied using an analytical theory and Monte Carlo simulations. To provide a sensitive experimental test in a tokamak, an unusual configuration, inverted from traditional designs, is proposed for a pump limiter. The principle can be tested in other plasma devices as well. The theory suggests that for typical plasma edge conditions in a confinement device, namely n=1013cm-3 and Tsub(i)=Tsub(e) approx.= 5-30 eV, helium enrichment in the neutral gas exhaust stream can be very high, in the range 5-7, relative to the helium-hydrogen ratio in the plasma. Such high enrichment factors are achieved by exploiting the difference between the ionization rates of hydrogen and helium and the negligible helium charge-exchange rate at these plasma conditions. A limiter arrangement is proposed in which the natural curvature of the toroidal magnetic field is used to isolate, using the plasma itself, the point of plasma neutralization from the location of the gas exhaust. The plasma region then acts to preferentially screen the recycling hydrogen by the processes of ionization and of charge-exchange-induced losses at open boundaries. Theory and analysis suggest that an experiment can provide a sensitive test of models used to describe the plasma edge and of the atomic and surface physics data used in these models. (author)