Results of NSTX heating experiments

The National Spherical Torus Experiment (NSTX) at Princeton University, Princeton, NJ, is designed to assess the potential of the low-aspect-ratio spherical torus concept for magnetic plasma confinement. The plasma has been heated by up to 7 MW of neutral beam injection (NBI) at an injection energy of 100 keV and up to 6 MW of high harmonic fast wave (HHFW) at 30 MHz. NSTX has achieved β_T of 32%. A variety of MHD phenomena have been observed to limit β. NSTX has now begun addressing τ_E scaling, β limits, and current drive issues. During the NBI heating experiments, a broad T_i profile with T_i up to 2 keV, T_i>T_e and a large toroidal rotation were observed. Transport analysis suggests that the impurity ions have diffusivities approaching neoclassical. For L-Mode plasmas, τ_E is up to two times the ITER97L L-Mode scaling and exceeds the ITER98pby2 H-Mode scaling in some cases. Transitions to H-Mode have been observed which result in an approximate doubling of τ_E after the transition in some conditions. During HHFW heating, T_e>T_i and T_e up to 3.5 keV were observed. Current drive has been studied using both coaxial helicity injection with up to 390 kA of toroidal current and HHFW. HHFW has produced H-modes with significant bootstrap current fraction at low I_p, high q, and high β_p.