Nonlinear gyrokinetic simulations of intrinsic rotation in up-down asymmetric tokamaks

Abstract

Experiments and theory show that tokamak plasmas with strong toroidal rotation and rotation shear can suppress turbulent energy transport as well as allow violation of the Troyon [beta] limit. However, using external neutral beams to inject toroidal momentum, as is done in many current experiments, would require a prohibitive amount of energy in larger, reactor-sized devices. The most promising alternative to achieve significant mean plasma flow that scales to large devices is intrinsic rotation, the rotation that is observed in the absence of external momentum injection. Intrinsic rotation is observed in current experiments, but is generated by effects that are formally small in [pi]* =- [pi]i / a, the ratio of the ion gyroradius to the tokamak minor radius. These effects are insufficient in anticipated reactors because [pi]*, will be significantly smaller. Recent theoretical work concludes that up-down asymmetry in the poloidal crosssection of tokamaks can drive intrinsic rotation to lowest order in [pi]*, [1, 2]. In this thesis, we extend GS2, a local [delta] f gyrokinetic code that self-consistently calculates momentum transport, to permit up-down asymmetric configurations. MHD analysis shows that ellipticity is most effective at introducing up-down asymmetry throughout the plasma. Accordingly, tokamaks with tilted elliptical poloidal cross-sections were simulated in GS2 to determine nonlinear momentum transport. The results suggest that the current experimentally measured rotation levels can be generated in reactorsized devices using up-down asymmetry. Surprisingly, linear and nonlinear gyrokinetic simulations also suggest that tilted elliptical flux surfaces may naturally suppress turbulent energy transport. Using cyclone base case parameters [3] (except for an elongation K = 2), a 40% reduction in the linear turbulent growth rate was observed by tilting the flux surface [pi]/4 from vertical. However, this reduction of energy transport was not observed when the background temperature gradient was increased by 50%.