Progress in EBW Current Drive Research Towards Enabling Sustained High Beta, Solenoid-Free Operation on NSTX

Off-axis rf-driven current can play a critical role in sustaining high beta spherical torus (ST) plasmas without a central solenoid. Numerical modeling of electron Bernstein wave current drive (EBWCD) for a beta ∼ 40% ST plasma predicts efficient, EBWCD at r/a > 0.5 where the large trapped electron fraction provides favorable conditions for Ohkawa EBWCD. Normalized EBWCD efficiency increases with r/a and is a factor of two higher at r/a = 0.7 than has been obtained with ECCD near the axis of large aspect ratio Tokamaks. Calculations predict 3-4 MW of 28 GHz RF power would maintain the ∼ 100 kA off-axis current needed to stabilize a beta ∼ 40% plasma. Modeling shows that efficient coupling to EBWs can be readily accomplished using near-circularly polarized electromagnetic waves via an ``O-X-B" oblique launch to the confining magnetic field. Initial measurements of EBW emission support pursuing this approach.