Flux Consumption Optimization and the Achievement of 1MA Discharge on NSTX

The spherical tokamak (ST), because of its slender central column, has very limited volt-second capability relative to a standard aspect ratio tokamak of similar plasma cross-section. Recent experiments on the National Spherical Torus Experiment (NSTX) have begun to quantify and optimize the ohmic current drive efficiency in a MA-class ST device. Sustainable ramp-rates in excess of 5MA/sec during the current rise phase have been achieved on NSTX, while faster ramps generate significant MHD activity. Discharges with IP exceeding 1MA have been achieved in NSTX with nominal parameters: aspect ratio A=1.3-1.4, elongation k=2-2.2, triangularity d=0.4, internal inductance li=0.6, and Ejima coefficient CE =0.35. Flux consumption efficiency results, performance improvements associated with first boronization, and comparisons to neo-classical resistivity are described