Theory of kinetic Alfvén wave helicity injection and current drive

Kinetic shear Alfvén wave (KSAW) helicity injection motivated by the goal of simultaneous implementation of radio frequency current and flow [Phys. Rev. Lett. 67, 1535 (1991)] drive is analyzed. The quasilinear helicity flux results in a net helicity increase only via transport through the boundaries. This, in turn, requires a compressional component at the plasma edge boundary and electron dissipation at the Alfvén resonance, as well as throughout the region, where helicity transport occurs. A comparison is made to direct KSAW current drive, which can add constructively with helicity injection by tailoring of the sign of poloidal and parallel wave numbers. The KSAW helicity flux is related to the α effect due to MHD (magnetohydrodynamic) resistive kink and shear-Alfvén-driven helicity fluxes. The helicity flux need not have a zero flux surface average. Finally, the efficiency (IpR/Pabs) of helicity injection by the KSAW is found to be significantly smaller than that by viscoresistive shear Alfvén waves, and scales as (IpR/Pabs)∼n0−3/2B03, where n0 is the density and B0 is the toroidal magnetic field. Application to edge current drive for ELM control is discussed.