On rotation of collisional plasmas in toroidal systems

Abstract

This paper describes the poloidal and toroidal spin-up of an isothermal plasma in toroidal equilibria. The mechanism is the Stringer spin-up mechanism which will be generalized to an arbitrary toroidal equilibrium using the Hamada coordinate system. Viscous damping or magnetic pumping balances the accelerating forces and determines the threshold of spin-up. The accelerating forces arise from the Coriolis forces which couple the radial flow to the poloidal flow velocity. The magnetic pumping rate is computed for conventional stellarators and stellarators of the Helias type. Magnetic pumping depends on the specific structure of B on the magnetic surface and the results exhibit a smaller damping rate in Helias configurations than in conventional stellarators. The paper presents numerical results of viscous damping rates computed for conventional stellarators, the Wendelstein 7-AS and Wendelstein 7-X devices and a tokamak with a toroidal magnetic ripple.