High power neutral beam heating on Doublet III

Energy confinement studies with up to 7.5 MW of neutral beam heating power have been performed on the Doublet III tokamak. We have found that during beam injection divertor operation results in up to a factor of two improvement in energy confinement as compared to comparable limiter discharges. In both configurations the confinement improves linearly with plasma current. In addition, there are indications that a new plateau in energy confinement time is reached with injection at high power levels. In all configurations electron transport is the major heat loss channel. Transport analysis indicates that the electron thermal conductivity is typically a factor of 2 to 3 lower in diverted discharges as compared to comparable limiter discharges. However, there are indications that the plasma edge may be important since improved confinement is correlated with the separation between the limiter and the separatrix