Induction of poloidal rotation by a radial ponderomotive force of electron cyclotron waves

Abstract

The nonlinear ponderomotive (PM) force due to high-power rf waves is studied as a drive for plasma rotation. It is shown that poloidal rotation may be driven by a radial PM force, in addition to the usual mechanism of direct angular momentum transfer from a poloidal PM force. Here, the effect of a radial PM force producing a radial plasma flow in the presence of viscous damping and neutral collisions is considered. The PM force is produced around an electron cyclotron resonant surface at a specific poloidal location, which naturally creates a poloidally asymmetric steady radial flow, when friction is present. The flow can also arise as a result of poloidal or toroidal PM force components, even in the absence of friction. In toroidal geometry this situation is unstable due to the Stringer spin-up mechanism, for a high enough power of the rf waves. This process is most important near the outer regions of the plasma, where it can then give rise to a high confinement mode (H mode), once a sheared poloidal flow is established. The advantage of this method of driving rotation is that the wave can propagate radially and when it is absorbed at the resonant surface the radial PM force is produced, instead of launching a wave in the poloidal direction. It is shown that this effect may be large enough for electron-cyclotron resonance heating, due to the small width of the resonant surface.