Modeling study of synergistic effects of neutral beam injection hot ions on helicon wave current drive in HL-3 tokamak

Helicon waves with very high ion cyclotron harmonics have been proposed for active off-axis current drive in tokamak with high performance operations. Under the application of ion cyclotron resonant heating or the neutral beam injection, hot ions with very high energy exist. For helicon waves with a wave frequency of 476 MHz and n_∥ = 3.0, as utilized in DIII-D and KSTAR tokamaks, theoretical analysis based on the fast wave dispersion relation indicates that the ion damping coefficient can be comparable to electron damping coefficient once the hot ions are injected. The synergistic effects of hot ions produced by neutral beam injection on helicon wave current drive in the HL-3 tokamak are investigated numerically using GENRAY/CQL3D simulations. The results indicate that electron damping remains the dominant mechanism owing to the low concentration of hot ions, and robust off-axis current drive can generally be achieved in the device. Considering the hot ions with an average energy 〈E_avg〉 ∼ 80 keV produced by neutral beam injection, positive synergistic effects between the hot ions and the helicon wave are observed, where super hot ions with energy exceeding 100 keV appear, as evidenced by the formation of a pronounced plateau in the hot ion velocity distribution function. Finally, the introduction of minority hot ions is proved to have a limited effect on the total current drive in the device.