Electromagnetic effects in the stabilization of turbulence by sheared flow

TL;DR

This paper investigates how electromagnetic effects influence the stabilization of plasma microinstabilities by sheared flow, revealing that electromagnetic effects are generally negligible at low beta but become significant at higher beta and stronger shear.

Contribution

It extends previous electrostatic models to include electromagnetic effects, analyzing their impact on plasma microinstability stabilization in sheared flow.

Findings

Electromagnetic effects do not significantly alter growth rates at low beta.

High flow shear and beta can mitigate shear Alfvén wave instabilities.

Unidirectional characteristics require larger flow shear than in electrostatic cases.

Abstract

We have extended our study of the competition between the drive and stabilization of plasma microinstabilities by sheared flow to include electromagnetic effects at low plasma $\beta$ (the ratio of plasma to magnetic pressure). The extended system of characteristic equations is formulated, for a dissipative fluid model developed from the gyrokinetic equation, using a twisting mode representation in sheared slab geometry and focusing on the ion temperature gradient mode. Perpendicular flow shear convects perturbations along the field at the speed we denote as $Mc_s$ (where $c_s$ is the sound speed). $M > 1/ \sqrt{\beta}$ is required to make the system characteristics unidirectional and inhibit eigenmode formation, leaving only transitory perturbations in the system. This typically represents a much larger flow shear than in the electrostatic case, which only needs $M>1$. Numerical investigation of the region $M < 1/\sqrt{\beta}$ shows the driving terms can conflict, as in the electrostatic case, giving low growth rates over a range of parameters. Also, at modest drive strengths and low $\beta$ values typical of experiments, including electromagnetic effects does not significantly alter the growth rates. For stronger flow shear and higher $\beta$, geometry characteristic of the spherical tokamak mitigates the effect of an instability of the shear Alfv\'{e}n wave, driven by the parallel flow shear.