LOW-FREQUENCY CURRENT DRIVE AND HELICITY INJECTION

For ω≪Ωi, where Ωi is the ion cyclotron frequency, circularly polarized waves can drive current far exceeding the current resulting from linearly polarized waves. Further, the efficiency can be independent of plasma density. In some cases, this circular polarization may be interpreted in terms of helicity injection. For tokamak applications, where the wavenumber in the toroidal direction is a real quantity, wave helicity is injected only with finite Ez waves, where z is the direction of the static magnetic field. The Alfvén waves are possible current drive candidates but, in the cylindrical model considered, the compressional wave is weakly damped because Ez = 0, while the shear Alfvén wave is totally absorbed at the surface because of finite Ez. A mixture of the two modes is shown to drive an oscillatory surface current even though the efficiency is high and independent of density. A more promising current drive candidate is a fast wave that propagates to the plasma interior and is damped by the minority cyclotron resonance. Near the minority mode conversion region, the fast wave is left-handed circularly polarized and it has a small but finite Ez component at high electron temperatures. The current drive efficiency, although not as high as that of the Alfvén wave, is still good and independent of density, making it attractive for fusion reactors. © 1990 American Institute of Physics.