The NSTX-U Research Program: closing the gaps towards a Fusion Pilot Plant

There is significant interest both within and outside the U.S. in producing net electricity in a Fusion Pilot Plant based on the Spherical Tokamak concept. With 12 MW of neutral beam heating, 6 MW of High Harmonic Fast Wave heating, and the capability of 2 MA, 1 T, 5 sec discharges, the National Spherical Tokamak Experiment-Upgrade (NSTX-U) is uniquely positioned to target several science and technology gaps in physics regimes close to those expected in a compact, steady-state FPP. In particular, NSTX-U will target the most significant gaps: core confinement improvement, heat flux mitigation, and non-inductive operation in a core-edge optimized plasma with majority self-driven current. NSTX-U capabilities will allow for assessing confinement improvements seen in NSTX and other STs in physics regimes much closer, than ever before, to those anticipated in ST-based FPPs. Both conventional and transformative heat flux mitigation methods, such as liquid lithium plasma facing components, will be developed and tested in-situ in NSTX-U at incident heat fluxes of ~100 MW/m², which will inform plans to upgrade to fully heated, high-Z wall and full liquid lithium divertor capability. Non-inductive operating scenarios will be developed through a unique high-β_N, high-κ route.