Equilibrium properties of ohmically heated plasmas in NSTX

Research in the National Spherical Torus Experiment, NSTX, has been conducted to establish spherical torus plasmas to be used for high-β, auxiliary heated experiments. The device has a major radius R0=0.86 m, a midplane half-width of 0.7 m, and has been operated with toroidal magnetic field B0≤0.3 T and Ip≤1.0 MA. The evolution of the plasma equilibrium is analyzed between shots with an automated version of the EFIT code. Limiter, double-null, and lower single-null diverted configurations have been sustained for several energy confinement times. Plasma stored energy has reached 92 kJ (βt=17.8%) with neutral beam heating. Plasma elongation of 1.6≤κ≤2.0 and triangularity in the range 0.25≤δ≤0.45 have been sustained, with values of κ=2.5 and δ=0.6 being reached transiently. The reconstructed magnetic signals are fit to the corresponding measured values with low error. Aspects of the plasma boundary, pressure, and safety factor profiles are supported by measurements from non-magnetic diagnostics. Plasma densities have reached 0.8 and 1.2 times the Greenwald limit in deuterium and helium plasmas, respectively, with no clear limit encountered. Instabilities including sawteeth and reconnection events (REs), characterized by Mirnov oscillations, and perturbation of the Ip, κ, and li evolution, have been observed. A low q limit was observed and is imposed by a low toroidal mode number kink instability. (author)