Search

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.

Experiments designed to heat electrons by Landau damping of waves at approximately twice the lower hybrid frequency have been carried out on Doublet IIA. This objective is in contrast to other lower hybrid experiments which are designed to heat ions using frequencies corresponding to the lower hybrid resonance frequency. Up to 500 kW of rf power was applied to discharges with approximately 100 kW ohmic input using parallel wavelengths chosen to optimize the spatial distribution of the power deposition based on linear or quasi-linear Landau damping. Coupling of the power to both electrons and ions was observed, but there was no indication of effective bulk heating of either species. The desired slow wave propagated into the plasma and efficient coupling of the wave energy to the plasma occured, but this energy was poorly confined. Two possible models of the absorption and loss mechanisms remain unresolved: 1) The power is coupled to energetic electrons by Landau damping and is lost via anomolous electron transport before it can thermalize. 2) The power is coupled to energetic ions and is lost by direct particle losses before it can thermalize.