Helium behaviour in expanded boundary divertor discharges

Helium was injected into expanded boundary divertor discharges in Doublet III to evaluate the helium exhaust and enrichment capabilities of the divertor. Helium was found to enter the main body of the plasma readily, achieving a concentration similar to that without the divertor. At a helium concentration in the main plasma of 5% of e, helium pressure of 5 × 10−5 torr was observed in the pumping duct with no enrichment of helium relative to atomic hydrogen, while at higher densities helium pressure decreased and significant de-enrichment was observed. Simplified calculations of the relative penetration depths of neutral hydrogen and helium are consistent with these results.