Fast electron transport during lower-hybrid current drive

In steady state lower-hybrid current drive (LHCD), the toroidal current in tokamaks is sustained by fast electrons which absorb energy and momentum from externally injected waves. The dynamics of the fast electron population is well described by balancing wave induced quasilinear diffusion with collisional slowing down and pitch angle scattering off of fixed Maxwellian field particles. Here we consider a quasilinear-Fokker-Planck formulation which includes the wave induced radial transport of fast electrons, thus generalizing the radially local, velocity space treatments of LHCD. The best current drive efficiencies for LHCD experiments are achieved when the wave spectrum launched into the plasma is narrow and close to the accessibility limit. For central electron temperatures up to a few keV, waves launched near the accessibility limit are very weakly damped and there results a significant 'spectral gap' which must be filled before the waves can Landau damp on electrons. Because of the weak dissipation, the ray trajectories of the waves can make several toroidal transits and suffer numerous radial reflections. It has been shown that by including toroidal effects in the ray dynamics, the poloidal mode numbers of the rays can upshift and thus fill the spectral gap. The fields required to bridge the spectral gap thus have a significant poloidal component, which will contribute to the radial ExB drift of resonant electrons. (author) 6 refs., 2 figs