Gyrokinetic simulations with external resonant magnetic perturbations: Island torque and nonambipolar transport with plasma rotation

摘要

Static external resonant magnetic field perturbations (RMPs) have been added to the gyrokinetic code GYRO [J. Candy and R. E. Waltz, J. Comp. Phys. 186, 545 (2003)]. This allows nonlinear gyrokinetic simulations of the nonambipolar radial current flow j r, and the corresponding j → B → plasma torque (density) R [j r B p / c], induced by magnetic islands that break the toroidal symmetry of a tokamak. This extends the previous GYRO formulation for the transport of toroidal angular momentum (TAM) [R. E. Waltz, G. M. Staebler, J. Candy, and F. L. Hinton, Phys. Plasmas 14, 122507 (2007); errata 16, 079902 (2009)]. The focus is on electrostatic full torus radial slice simulations of externally induced q m / n 6 / 3 islands with widths 5 % of the minor radius or about 20 ion gyroradii. Up to moderately strong E B rotation, the island torque scales with the radial electric field at the resonant surface E r, the island width w, and the intensity I of the high-n micro-turbulence, as E r w I. The radial current inside the island is carried (entirely in the n 3 component) and almost entirely by the ion E B flux, since the electron E B and magnetic flutter particle fluxes are cancelled. The net island torque is null at zero E r rather than at zero toroidal rotation. This means that while the expected magnetic braking of the toroidal plasma rotation occurs at strong co- and counter-current rotation, at null toroidal rotation, there is a small co-directed magnetic acceleration up to the small diamagnetic (ion pressure gradient driven) co-rotation corresponding to the zero E r and null torque. This could be called the residual stress from an externally induced island. At zero E r, the only effect is the expected partial flattening of the electron temperature gradient within the island. Finite-beta GYRO simulations demonstrate almost complete RMP field screening and n 3 mode unlocking at strong E r.