Heating and Current Drive Options for a Spherical Tokamak Advanced Reactor

A spherical tokamak advanced reactor (STAR) project at PPPL has settled on design with an aspect ratio A = 2, major radius R = 4 m, toroidal magnetic field B_T = 5.2 T, and plasma elongation κ = 2.3. Here we consider two leading candidates for heating and current drive (CD) systems: neutral beam injection (NBI) and electron cyclotron (EC) heating. The plasma equilibrium relies on 100% non-inductive CD, a vast majority of which is bootstrap current, but externally driven current is needed near the axis. An array of EC gyrotrons can match the desired current more precisely, but with beam ion energy correctly tailored for penetration to the core, NBI can also provide the needed CD. Additionally, the plasma core must maintain a certain temperature profile to be useful, and this is determined by a balance between a heating source and thermal transport. The NBI and EC solutions are assessed with TRANSP to determine the evolution in time of the heating vs. transport balance and whether the necessary high temperatures can be maintained in steady state. Preliminary assessments indicate the energy confinement time implied by the prescribed temperature profile is close to low aspect ratio scalings. A more detailed predictive assessment of turbulent transport is ongoing.