High β, long pulse, bootstrap sustained scenarios on the National Spherical Torus Experiment (NSTX)

Long pulse, high β scenarios have been established on the National Spherical Torus Experiment (NSTX) [M. Ono et al., Nucl. Fusion 40, 557 (2000)]. βt(≡2μ0〈P〉/Bt02)∼35% has been achieved during transient discharges. The machine improvements that lead to these results, including error field reduction and high temperature bakeout of plasma facing components are described. The highest βt plasmas have high triangularity (δ=0.8) and elongation (κ=2.0), at low aspect ratio A≡R/a=1.4. The strong shaping permits large values of normalized current, IN(≡Ip/(aBt0))≊6 while maintaining moderate values of q95=4. Long pulse discharges up to 1 s in duration have been achieved with substantial bootstrap current. The total noninductive current drive can be as high as 60%, comprised of 50% bootstrap current and ∼10% neutral beam current drive. The confinement enhancement factor H89P is in excess of 2.7. βN * H89P≳15 has been maintained for 8 * τE∼1.6 * τCR, where τCR is the relaxation time of the first radial moment of the toroidal current density. The ion temperature for these plasmas is significantly higher than that predicted by neoclassical theory.