Multifaceted asymmetric radiation from the edge impurity density limits in tokamaks with poloidal asymmetry and rotation

Abstract

A linear stability analysis of two-dimensional, edge-localized thermal instabilities in tokamaks has been performed using a fluid model which incorporates the effects of large radial gradients and near-sonic rotation speeds. Sufficient stability requirements for fundamental (m=0) and harmonic (m≳0) poloidal modes are established and used to investigate the effects of rotation, poloidal asymmetry in the equilibrium solution, parallel current, and parallel momentum injection on impurity density limits. The higher density limits due to parallel heat conduction and parallel viscosity that are associated with the m≳0 modes, but that are absent for the m=0 mode, account for the onset and stability of MARFEs (multifaceted asymmetric radiation from the edge). Rotation and asymmetry in the equilibrium solution are significant in the determination of impurity density limits. The MARFE threshold density limit can be increased by driving current in the plasma edge counter to the magnetic field.