Theoretical investigation on r.f. current drive in reversed-field pinch plasma

In a previous paper, a fast magnetosonic (FM) wave (ωDC < ω < ωLH, where ωDC and ωLH are the deuterium cyclotron and the lower hybrid angular frequencies, respectively) was reported to be a useful candidate for the current drive, especially in a high-beta plasma such as a reversed-field pinch (RFP) plasma, on account of the good accessiblity to the high-density region with a wide spectrum and strong damping. The main damping mechanism is due to transit magnetic pumping, which has a relatively high current efficiency. This paper describes a theoretical investigation of the propagation and damping in RFP plasmas of various waves and the current profile control by FM waves. The results suggest the possibility of generating a steady-state, dynamo-free stable RFP configuration by arbitrarily selecting a power spectrum. Also, a lower hybrid wave might be useful for the current drive in the outer regions to reverse the toroidal field. By a single-ray fast-wave partial current drive, where the wave-driven current is combined with an ohmic current, the field configuration approaches Taylorʹs minimum energy state and a higher shear state, indicating the enhancement of the plasma stability and confinement. Further, FM and also LH waves might be useful for providing a sufficient poloidal current in the outer region, which reverses the toroidal field and would eliminate or reduce the need for dynamo action, and then could improve the energy confinement time.