Effects of trapping electrons on synergy of lower-hybrid wave and electron cyclotron wave

Abstract

Steady state operation is essential for Tokamak-based fusion reactor, in which the plasma current has to be fully sustained and controlled by non-inductive methods. Lower-hybrid current drive is the most effective radio-frequency current drive method, which, however, has the drawback that the driven current profile is difficult to control. Electron cyclotron current drive has the ability to deposit power and drive current in a highly localized and robustly controllable way, while the efficiency of electron cyclotron current drive is known to be significantly lower than that of lower-hybrid current drive. Due to those complementary features, the combinative usage of lower-hybrid wave and electron cyclotron wave has been proposed. The current driven by simultaneously using the waves might be significantly larger than the sum of the currents driven by the waves individually in the same plasma conditions, which is the so-called synergy effect. While the lower-hybrid current drive and the electron cyclotron current drive are both affected by the trapping effect, which implies that the synergy effect between lower-hybrid current drive and the electron cyclotron current drive may also closely related to the trapping effect. In this paper, the effects of trapping on the synergy of lower-hybrid current drive and the electron cyclotron current drive are investigated by solving the bounce-averaged quasi-linear equation with different trapping angles. The diffusions induced by the lower-hybrid wave and the electron cyclotron wave are considered simultaneously. The resulting steady-state electron distribution function as a balance between the collisions and the wave-induced diffusions is obtained numerically by the CQL3D code, which is then integrated to calculate the driven plasma current. The velocity-space fluxes are analyzed for understanding the mechanism and the physics of the synergy process. It is found that the currents driven by the waves decrease as trapping angle increases. The synergy factors also decrease as trapping angle increases, which means that the current drive processes in the synergy case are more sensitive to the trapping effect than in the single wave case. The current driven by electron cyclotron wave drops rapidly with the increase of trapping angle, while the existence of lower-hybrid wave is helpful in decelerating the dropping. The lower-hybrid wave reduces the dependency of the electron cyclotron current drive on the trapping effect. The decouple effect turns stronger as the resonance region of the lower-hybrid wave becomes wider. Increasing the power of the electron cyclotron wave leads to more accelerated electrons and more electrons with relatively high parallel velocities, which results in stronger synergy effect and less dependence on trapping.