Poloidal rotation of tokamak plasmas at super poloidal sonic speeds

A tokamak plasma rotating poloidally at speeds in excess of the poloidal sound speed is studied. The study is motivated by the propositions that shear in the rotation could suppress turbulence and improve confinement and that the poloidal sound speed is the critical speed to exceed for flow profiles distributed over the entire minor radius. For such rapidly rotating plasma regimes, the Grad-Shafranov equation is examined, the damping rate of the rotation is calculated, particle orbits are investigated, and a heuristic MHD stability study is done. It is found that for rotation speeds exceeding the poloidal sound speed but less than the poloidal Alfven speed no deleterious effects can be expected from the rapid rotation as regards equilibrium or MHD stability. The damping rate of poloidal rotation is shown to fall off over the peak rate, in all collisionality regimes, as u(p)(-2). This feature makes more efficacious the external driving of such poloidal flow and allows favourable extrapolation to reactor relevant regimes. Finally, it is shown that the fraction of trapped ions is significantly depleted in this regime, suggesting a reduction in the neoclassical heat transport as well.