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.

The combined drive current of the lower hybrid wave (LHW) and the high harmonic fast wave (HHFW) is studied for the first time, based on the use of low and higher β_e operational parameters in EAST. Broad and significant synergistic effects are found in the simulation, the local synergy factor reaching a factor of 4.3 in the off-axis region. The current drive (CD) efficiency is greatly improved, and the current profile is modified as a result of the synergy between the two types of waves . the LHW interacted with the resonant electrons in low parallel velocity region and pushes them into the adjacent resonance region of the high phase velocity wave (HHFW), the number of fast electrons resonant with the HHFW is increased dramatically, and the driven current is enhanced. Therefore the synergy effect strongly depends on the positional relational between the velocity resonance regions of the two waves. Moreover, the effects of the parallel refractive index and the wave power on the synergy effect are examined. Some problems well known in the single LHW CD or the HHFW CD may be overcome by the combined CD.

A localized and efficient current drive method in the outer-half region of the tokamak with a large inverse aspect ratio is proposed via the Ohkawa mechanism of electron cyclotron (EC) waves. Further off-axis Ohkawa current drive (OKCD) via EC waves was investigated in high electron beta β e HL-2M-like tokamaks with a large inverse aspect ratio, and in EASTlike tokamaks with a low inverse aspect ratio. OKCD can be driven efficiently, and the driven current profile is spatially localized in the radial region, ranging from 0.62 to 0.85, where the large fraction of trapped electrons provides an excellent advantage for OKCD. Furthermore, the current drive efficiency increases with an increase in minor radius, and then drops when the minor radius beyond a certain value. The effect of trapped electrons greatly enhances the current driving capability of the OKCD mechanism. The highest current drive efficiency can reach 0.183 by adjusting the steering mirror to change the toroidal and poloidal incident angle, and the total driven current by OKCD can reach 20-32 kA MW −1 in HL-2M-like tokamaks. The current drive is less efficient for the EAST-like scenario due to the lower inverse aspect ratio. The results show that OKCD may be a valuable alternative current drive method in large inverse aspect ratio tokamaks, and the potential capabilities of OKCD can be used to suppress some important magnetohydrodynamics instabilities in the far off-axis region.

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This paper reports numerical study of the 2/1 NTM stabilized by the Ohkawa-mechanism-dominated current drive (OKCD) of EC waves, and the results are compared with that of the traditional Fisch-Boozer mechanism dominated electron cyclotron current drive (ECCD). The peak value, radial position and radial width 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. Comparing with the results of ECCD to stabilize the 2/1 NTM, when the choice 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 electron trapping effect is large and important. This paper shows 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.