The National Spherical Torus Experiment: Advancing the Physics of Magnetic Confinement Fusion*

The National Spherical Torus Experiment (NSTX) at the Princeton Plasma Physics Laboratory has provided much of the physics basis for the spherical tokamak (ST) magnetic plasma confinement concept for fusion energy production; concepts which are currently being designed for future fusion pilot plants. NSTX is currently in the midst of an upgrade, and researchers are continuing to advance the physics understanding of ST plasmas to maximize the benefit that will be gained when the upgraded device (NSTX-U) returns to operation and to increase confidence in projections to future devices. STs have certain advantages: their more compact size (than conventional tokamaks) means they can provide higher plasma current more economically. Their low aspect ratio, of major to minor radius, improves stability with favorable average magnetic curvature, enabling high beta (the ratio of plasma pressure to magnetic pressure). There are challenges as well: managing high heat flux, and start-up and sustainment of the plasma without space for an induction coil. The objectives of NSTX-U research are to: (i) to extend particle confinement physics of low aspect ratio, high beta plasmas to the lower particle collisionality levels relevant to future device regimes, (ii) to develop stable, non-inductive scenarios with the self-generated current needed for steady-state operation, and (iii) to develop conventional and innovative power and particle handling techniques to optimize plasma exhaust in high performance scenarios.