Efficient generation of noninductive, off-axis, Ohkawa current, driven by electron Bernstein waves in high β, spherical torus plasmas

Abstract

Off-axis rf-driven current can play a critical role in sustaining high β, spherical torus (ST) plasmas without a central solenoid. Numerical modeling of electron Bernstein wave current drive (EBWCD) for a β∼40% ST plasma predicts efficient, off-axis, Ohkawa EBWCD. Current can be efficiently driven at r/a>0.5 where the large trapped electron fraction precludes conventional Fisch-Boozer current drive and provides favorable conditions for Ohkawa EBWCD. Calculated normalized current drive efficiency increases with r/a and is a factor of 2 higher at r/a=0.7 than has been obtained with electron cyclotron current drive near the axis of large aspect ratio tokamaks.