High-density, low-q discharges with D-shaped and circular cross-sections in Doublet III

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Abstract
The maximum plasma current in Doublet III is found to be limited by disruptions when the limiter safety factor is approximately two. However, because of the strong toroidal and shaping-field effects on the rotational transform at the outer plasma edge associated with a vertical elongation of 1.5 D-shape, the safety factor estimated from simple geometric considerations for D-shaped plasmas corresponds to values as low as 1.5. These discharges operate stably with considerably higher plasma current than most reactor design studies assume. These low-q discharges show excellent plasma performance: very flat spatial electron temperature profiles, high-density operation with eR/BT up to 7.8, and good energy confinement producing a volume-average β of up to 1% with Ohmic heating only. This operational regime appears to be applicable to future high-β tokamaks with D-shaped cross-sections.