Operation of the National Spherical Torus Experiment

Summary form only given. The National Spherical Torus Experiment (NSTX) is a proof-of-principle experiment designed to study the physics of low aspect ratio toroidal plasmas. NSTX can be operated with a wide range of shapes to produce plasmas with R/a=0.85/0.68 m/spl sim/1.25, I/sub p/=1 MA, B/sub T//spl les/0.6 T, /spl kappa//spl les/2.2, /spl delta//spl les/0.5, and, eventually, with heating powers of up to 11 MW (6 MW high harmonic fast waves, 5 MW, 80 keV, D/sup 0/ neutral beam injection). Inductive breakdown is assisted by electron cyclotron resonance heating preionization. Co-axial helicity injection (CHI) can provide the opportunity for V-sec savings during startup. A mission of the NSTX research program is to study the global confinement and local transport properties of ST (Spherical Tori) plasmas. The large field line pitch on the outboard side, and small pitch on the inboard side, increases the time the field line spends in the good curvature regime, which should benefit both the transport and micro- and macro-stability properties of the NSTX plasmas. This increase in good curvature, coupled with a large shearing rate due to the low toroidal field, may reduce dramatically the electrostatic and electromagnetic microinstabilities that cause transport. Furthermore, with the presence of close fitting conducting plates to suppress external kink modes, and with optimized current and pressure profiles, plasma betas of up to 40% with 70% bootstrap fractions have been calculated to be stable.