Intrinsic momentum transport in up-down asymmetric tokamaks

Recent work has demonstrated that breaking the up-down symmetry of tokamak flux surfaces removes a constraint that limits intrinsic momentum transport, and hence toroidal rotation, to be small. We show, through MHD analysis, that ellipticity is most effective at introducing up-down asymmetry throughout the plasma. We detail an extension to GS2, a local δf gyrokinetic code that self-consistently calculates momentum transport, to permit up-down asymmetric configurations. Tokamaks with tilted elliptical poloidal cross-sections were simulated to determine nonlinear momentum transport. The results, which are consistent with the experiment in magnitude, suggest that a toroidal velocity gradient, (∂u_ζi/∂ρ)/v_thi, of 5% of the temperature gradient, (∂T_i/∂ρ)/T_i, is sustainable. Here v_thi is the ion thermal speed, u_ζi is the ion toroidal mean flow, ρ is the minor radial coordinate normalized to the tokamak minor radius, and T_i is the ion temperature. Though other known core intrinsic momentum transport mechanisms scale poorly to larger machines, these results indicate that up-down asymmetry may be a feasible method to generate the current experimentally measured rotation levels in reactor-sized devices.