Numerical study of m = 2/n = 1 neoclassical tearing mode stabilized by the Ohkawa-mechanism-dominated current drive of electron cyclotron waves

Abstract

This paper reports a numerical study of the 2/1 neoclassical tearing mode (NTM) stabilized by the Ohkawa-mechanism-dominated current drive (OKCD) of electron cyclotron (EC) waves, and the results are compared with those of the traditional Fisch–Boozer mechanism dominated EC current drive (ECCD). The peak values, radial positions and radial widths of the driven current profiles by EC waves are passed to the modified Rutherford equation to study the effect of OKCD/ECCD on the 2/1 NTM. Well-localized current density profiles and large driven current can be achieved for 2/1 NTM stabilization in a low-aspect-ratio tokamak (R/a ∼ 2.7) by using OKCD. The optimal minimum EC powers are calculated for both OKCD and ECCD to fully stabilize the 2/1 NTM. We compare our results with those of ECCD to stabilize the 2/1 NTM, when the choices of magnetic field strength and gyrotron frequency are such that off-axis deposition on the high-field side is not practical to generate localized current effectively, so that the electron trapping effect is large and important. In this paper, we show that it is better to use lower gyrotron frequencies optimized for the Ohkawa mechanism to obtain a higher current drive efficiency for 2/1 NTM stabilization.