A 2-D Fokker-Planck treatment of the ICRF enhancement of beam-driven currents

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Abstract
A 2-D Fokker-Planck treatment of the interaction between an ICRF wave having k|| = 0 and fast ions arising from neutral-beam injection is undertaken. Fast-ion collisions with the background plasma are treated using a collision operator which takes account of slowing down on both electrons and ions, together with pitch-angle scattering on the thermal ions. Energy diffusion due to electron collisions is neglected. The RF interaction is represented by the usual quasi-linear diffusion operator. The Fokker-Planck equation is solved in the weak RF field limit by treating the effect of the wave interaction as a perturbation of the fast-ion distribution. The current carried by the injected fast ions is found to be increased by the wave interaction in most circumstances. The optimum efficiency (additional current drive per wave power absorbed) of the current enhancement is found to be about twice the optimum efficiency of generation of the straightforward beam-driven current.