ECCD for Advanced Tokamak Operations Fisch-Boozer versus Ohkawa Methods

Abstract. Current can be driven using electron cyclotron waves (ECW) by optimizing either the Fisch-Boozer mechanism (ECCD) or the Ohkawa mechanism (OKCD). In ECCD, perpendicular heating due to ECW creates an asymmetric resistivity. In OKCD, current is generated by ECW-induced asymmetric electron trapping. OKCD is a good candidate for off-axis CD where the ECCD effectiveness is reduced due to trapped electrons. The two mechanisms are described using the kinetic, bounce-averaged, Fokker-Planck code DKE with a quasilinear ECW operator. Currents and CD efficiencies for the two methods are calculated and compared in different regions of an advanced tokamak plasma. Numerical results confirm the experimental observations that ECCD is best for central CD but becomes ineffective beyond a certain radial distance from the plasma center. On the low-field side (LFS) of this outboard region, OKCD can very effectively generate localized currents. As it is optimized on the LFS, OKCD requires lower wave frequencies than ECCD- an advantage when considering ECW sources