Recent Progress in ST Research - MAST and NSTX -

Abstract

Research on spherical tokamak (ST) plasmas is progressing rapidly. Recent results from the mega-ampere class ST devices, MAST and NSTX, are herein reviewed. A toroidal beta of up to 35% and a normalized beta of over 6 were achieved on NSTX. The highest betas are obtained with low internal inductance and a flat pressure profile, exceeding the no-wall stability limit by up to20%. Diamagnetic plasmas with a noninductive current fraction of 60% and a self-driven current fraction of 40% have been obtained. Ion thermal diffusivity is consistent with neoclassical transport, where as electron thermal diffusivity is anomalous. A strong toroidal flow (20-30% of Alfven velocity) is observed. H-mode is routinely observed in both MAST and NSTX. Energy confinement is consistent with the ITER 98y2 ELMy H-mode scaling, but the H-mode transition threshold power is about 50% higher than the ITER scaling. The divertor heat load is highly asymmetric, the outboard side being higher than the inboard side, typically by an order of magnitude. Up to 0.5 MA of plasma current has been obtained using the merging-compression technique, and further increase was demonstrated by ramping up the vertical field on MAST. Up to 0.4 MA of toroidal current has been obtained by coaxial helicity injection on NSTX. Noninductive current drive techniques using the neutral beam, high harmonic fast wave, and electron Bernstein wave are being developed.