Generation of Non-Inductive H-Mode Plasmas with 30 MHz Fast Wave Heating in NSTX-U

A Fusion Nuclear Science Facility based on a spherical tokamak must generate the plasma current (I$_{\mathrm{p}})$ with little or no central solenoid field. The NSTX-U non-inductive (NI) plasma research program is addressing this goal by developing NI start-up, ramp-up and sustainment scenarios separately. 4 MW of 30 MHz fast wave power is predicted to ramp I$_{\mathrm{p}}$ to 400 kA, a level sufficient to avoid significant shine-through of 90 keV ions from neutral beam injection. In 2010, experiments in NSTX demonstrated that 1.4 MW of 30 MHz high-harmonic fast wave (HHFW) power could generate an I$_{\mathrm{p}}=$ 300 kA H-mode discharge with a NI I$_{\mathrm{p}}$ fraction, f$_{\mathrm{NI}}$, around 0.7 at the maximum axial toroidal field (B$_{\mathrm{T}}$(0)) in NSTX of 0.55 T. NSTX-U is a major upgrade of NSTX that will eventually allow the generation of plasmas with B$_{\mathrm{T}}$(0) up to 1 T. Full wave simulations of 30 MHz HHFW heating in NSTX-U predict reduced FW power loss in the plasma edge as B$_{\mathrm{T}}$(0) is increased. HHFW experiments this year aim to couple 3 -- 4 MW of 30 MHz HHFW power into an I$_{\mathrm{p}}=$ 250 -- 350 kA plasma with B$_{\mathrm{T}}$(0) up to 0.75 T to generate a f$_{\mathrm{NI}}=$ 1 H-mode plasma. These experiments should benefit from the improved fast wave coupling predicted at higher B$_{\mathrm{T}}$(0) in NSTX-U.