Threshold density for toroidal rotation reversal bifurcation induced by symmetry breaking

Abstract

Threshold density for intrinsic toroidal rotation reversal is studied using a newly proposed model [Li et al., Nucl. Fusion 61, 104002 (2021)] based on the neoclassical toroidal plasma viscosity (NTV) effect induced by symmetry breaking due to the existence of internal kink mode. In this model, rotation reversal happens when the NTV steady state flow jumps between “electron root” and “ion root,” which depends on plasma collisionality regimes. To make it convenient for cross machine comparisons or extrapolation to the future device, a scaling of threshold density for rotation reversal is proposed based on the physics of transition between the 1/ν and superbanana plateau regimes, i.e., the normalized collisionality over the particle toroidal precessional frequency rather than over the particle bounce frequency that is frequently used in neoclassical theory. The theoretical scaling of the threshold density on electron temperature and magnetic field strength is well verified by the NTVTOK modeling. Other effects, such as boundary condition, temperature gradient, and temperature ratio, which cannot be included in the theoretical scaling, are also investigated by using the NTVTOK modeling.