11 - National Spherical Torus eXperiment

Abstract

The National Spherical Torus eXperiment (NSTX) is a medium-sized, low-aspect ratio, spherical tokamak (ST) whose aims are to study 1) the unique physics properties of the ST, 2) complement higher aspect ratio tokamak physics and support ITER, and 3) establish attractive ST operating regimes for future ST-based fusion energy producing devices. Research on NSTX over the past 15 years has been broadly based. Transport and turbulence studies focused on identifying the source of the highly anomalous and dominant electron energy transport, as well as electromagnetic effects on turbulence, in neutral beam and radio frequency heated L- and H-mode plasmas. Macrostability research has identified the importance of kinetic effects in determining high-bT stability, and a multi-parameter-based algorithm was developed capable of predicting plasma disruptions with a near 95% success rate. Energetic particle driven Alfvén Eigenmodes were found to have a significant effect on the fast ion population and resulting beam-driven non-inductive current drive profiles. High heat fluxes were mitigated successfully by operating in a “snowflake” divertor configuration, and test stand studies revealed the surface chemistry necessary to understand lithium wall conditioning, a technique used routinely during the last half of NSTX’s operational lifetime. Finally, solenoid-free operation studies explored non-inductive generation and maintenance of the plasma current; non-inductive operation is a critical component of future ST devices. NSTX has just undergone a major upgrade, which allows for an expanded operating and physics parameter range. The Upgrade also includes the addition of a second, more tangentially directed, neutral beam injector for added power and pressure and current profile control. NSTX-Upgrade operation commenced in 2015.