Neutral beam heating in W VII-A Stellarator

A high density current-free deuterium plasma with β(o)<=0,5%, nsub(e)(o)>=1014 cm-3, Tsub(e), Tsub(i)>350 eV can be maintained in W VII A stellarator by injection of 600 kW of H0 at 27 kV. The plasma energy content rises linearly with power, and in first experiments with 1.1 MW of neutral power 3.5 kJ have been achieved. The plasma shows no major instabilities but the energy balance is dominated by radiation losses, which are attributed to an increase of impurity ions in the plasma center, predominantly oxygen. The observed heating efficiency in particular for the plasma ions, seems to be higher than predicted by the beam deposition code. Possible mechanisms for this are discussed. It appears that radial electric fields as low as kTsub(e)/a improve the computed efficiency as well as the ion heating considerably. Despite the uncertainty about actual input power, it can be shown that the plasma confinement improves during injection