Simulation of the Ohkawa-mechanism- dominated current drive of electron cyclotron waves using linear and quasi-linear models

Abstract

Abstract Simulation on the Ohkawa-mechanism-dominated current drive (OKCD) of electron cyclotron (EC) waves is performed using TORAY-GA linear code, and the results are compared with those calculated by CQL3D quasi-linear code. It is found that the radial location of the OKCD profile is almost identical between the linear and the quasi-linear calculations. However, there are significant differences in the calculation of the total driven current I e c and the peak value of the driven current profile j e c p e a k between the two models. The I e c calculated by the CQL3D code is at least 1.4 times larger than the results from the TORAY-GA code. For the calculation of j e c p e a k , the results from CQL3D are at least 1.6 times larger than that calculated by TORAY-GA. With increasing electron temperature, the two models further enlarge the total driven current scaling factor F I and the peak driven current density scaling factor F j . This is mainly because the collision operator in TORAY-GA code adopts a high-speed model and does not retain the first-order Legendre expansion term for momentum conservation of electron self-collision. The quasi-linear effect does not have a significant influence on the total driven current of OKCD when the EC power level does not meet P r f MW m − 3 n e 10 19 m − 3 > 0.5. Therefore, in practical engineering, the TORAY-GA code can be used to calculate OKCD quickly and accurately by multiplying with appropriate scaling factors. The effect of momentum conservation is very important for OKCD and on-axis EC current drive (ECCD), but this effect is not important for off-axis ECCD. The results from this study show that the effects of electron trapping and the collision between resonant passing electrons and trapped electrons are responsible for the decrease in off-axis ECCD efficiency.