Comparative study of Fisch-Boozer and Ohkawa current drive mechanisms for electron cyclotron waves

Abstract

Localized current drive by electron cyclotron (EC) waves is of significant importance in the outer half region of tokamak plasmas. Using the coupled GENRAY/CQL3D suite codes, a systematic comparative study between Ohkawa current drive (OKCD) and electron cyclotron current drive (ECCD) is performed. The results show that OKCD has more advantages than ECCD for far off-axis localized current drive in tokamaks with large inverse aspect ratios, while ECCD is more efficient than OKCD in tokamaks with low inverse aspect ratios. The results of local current driving on the q = 2 rational surface show that both OKCD and ECCD are effective for control of the m = 2/n = 1 tearing mode or neoclassical tearing mode (NTM). It seems that efficient Ohkawa current can be driven in a specific far off-axis radial position as long as the local inverse aspect ratio is large enough. The effect of collisionality imposes a significant impact on OKCD and results in reduction of the net current driven by unit EC power. The dimensionless current drive efficiency of OKCD increases with increasing electron beta βe in a medium range. The results further confirm that OKCD can be a valuable alternative localized current drive method to replace ECCD in large inverse aspect ratio tokamaks or in the radial position where the local inverse aspect ratio is large enough.