Equilibrium properties of spherical torus plasmas in NSTX

Research in NSTX has been conducted to establish spherical torus plasmas to be used for high β, auxiliary heated experiments. This device has a major radius R0 = 0.86 m and a midplane halfwidth of 0.7 m. It has been operated with toroidal magnetic field B0 ⩽ 0.3 T and Ip ⩽ 1.0 MA. The evolution of the plasma equilibrium is analysed between discharges 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. The plasma stored energy reached 92 kJ (βt = 17.8%) with neutral beam heating. A plasma elongation in the range 1.6 ⩽ κ ⩽ 2.0 and a triangularity in the range 0.25 ⩽ δ ⩽ 0.45 have been sustained, with values of κ = 2.6 and δ = 0.6 being reached transiently. The reconstructed magnetic signals are fitted to the corresponding measured values with low errors. 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, characterized by Mirnov oscillations, and a perturbation of the Ip, κ and li evolutions, have been observed. A low q limit was observed and is imposed by a low toroidal mode number kink instability.