Theory and Modelling of Helium Enrichment in Plasma Experiments with Pump Limiters

Abstract

Helium enrichment in the exhaust gaB stream flowing from a hydrogenhelium 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 11mlter. 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-10 13 cm-3 and T. « T • 5-30eV, helium enrichment in the x e neutral gas exhaust 3tream can be very high, in the range 5 to 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 limlter arrangement la proposed in which the natural curvature of the toroidal magnetic field is used to 1BOlate, 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. The theory and analysis suggests that an experiment can provide a sensitive test of modules used to describe the plasma edge and of atomic and surface physics data used in these models.

Helium removal from tokamaks

“…Therefore, if EH* and EH denote the pumping probabilities for neutral helium and hydrogenic particles, respectively, one may have, on the hasis of different atomic physics and largely depending on geometrical and plasma conditions, ratios E H ~/ E H larger or smaller than 1. This effect has for example led to the proposal of an "inverted pumplimiter configuration" for preferential helium removal (Prinja and Conn, 1985). However, things turn out to be more complicated: * Upon ionization, in general, hydrogenic particles are much more energetic than helium (e.g.…”