Theory of synergy between electron cyclotron and lower hybrid waves

Abstract

A theoretical study of the improvement of the electron cyclotron current drive (ECCD) efficiency in regimes in which most of the current is driven by lower hybrid (LH) waves is presented. A perturbation technique is employed to solve the adjoint equation and derive the response function including both collisional and LH effects in the limit where the former dominate. An alternative treatment of the problem, involving a numerical solution of the Langevin equations, is proposed to gain insight into the current drive mechanism and to confirm the obtained results. The existence of a cross-effect between the two waves is demonstrated and the conditions for the synergy (i.e., significant enhancement of the ECCD efficiency in the presence of LH power) are identified.