Doublet III operating regimes with improved energy confinement

Over the past three years, extensive energy confinement experiments have been performed on the Doublet III tokamak with up to 8 MW of both hydrogen and deuterium neutral beam heating power. These experiments covered a wide range of plasma parameters and almost a continuum of magnetic configurations from limited to fully diverted. It has been found that the global energy confinement time is linearly proportional to the plasma current and decreases with the application of neutral beam power for all configurations and beam species studied. The majority of our confinement experiments were conducted with discharges limited toward the outside of the vacuum vessel and heated with hydrogen neutral beams. With respect to this standard operating mode, several operating regimes were found with improved values of energy confinement time. Generally, deuterium neutral beam heating is superior to hydrogen beam heating and divertor operation yields superior confinement as compared to limiter operation. Our experiments also indicate that discharges limited toward the inside of the vacuum vessel exhibit somewhat better confinement relative to discharges limited toward the outside. Accordingly, the highest energy confinement values are obtained in high current, well diverted, deuterium beam heated discharges. Deuterium beam heated discharges limited toward the inside yield normalized energy confinement values similar to those from hydrogen beam heated divertor discharges.