Synergy effect of the Ohkawa current drive of electron cyclotron waves and the lower hybrid current drive: a new mechanism

A new synergy mechanism between the Ohkawa current drive (OKCD) of electron cyclotron (EC) waves and the lower hybrid current drive (LHCD) is discovered and discussed. The methodology to achieve this synergy effect is also introduced. Improvement of OKCD efficiency can be achieved by up to a factor of ~2.5 in the far off-axis radial region (ρ > 0.6) of tokamak plasma. By making EC wave heating of electrons have a parallel velocity in the co-Ip direction and the lower hybrid wave heating of electrons have a parallel velocity in the counter-Ip direction, the mechanism of this new synergy effect comes from the results of the electron trapping and detrapping processes. The OKCD makes the low-speed, barely-passing electrons be trapped (trapping process), the LHCD pulls some of the high-speed barely-trapped electrons out of the trapped region in velocity space (detrapping process) and accelerates the detrapped electrons to a higher speed.