On Synergy effect of Ohkawa Current Drive of Electron Cyclotron Waves and Lower Hybrid Current Drive: A New Mechanism

Abstract: A new synergy mechanism between Ohkawa current drive (OKCD) of electron cyclotron (EC) waves and lower hybrid current drive (LHCD) is discovered and discussed. And the methodology to achieve this synergy effect is also introduced. Improvement of OKCD efficiency can be achieved up to a factor of ~ 2.5 in far off-axis radial region (\r{ho} > 0.6) of tokamak plasmas. Making EC wave heating the electrons of co-Ip direction and LH wave heating the electrons of counter-Ip direction, the mechanism of this new synergy effect comes from the results of electron trapping and detrapping processes. The OKCD makes the low speed barely passing electrons to 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.