Fast electrostatic microinstability evaluation in arbitrary toroidal magnetic geometry using a variational approach

TL;DR

This paper introduces a semi-analytical dispersion relation for microinstabilities in toroidal fusion devices, accurately capturing ITG and TEM modes across various geometries with reduced computational cost.

Contribution

It develops a variational, semi-analytical model for electrostatic microinstabilities in arbitrary toroidal geometry, incorporating resonances and non-local effects, validated against gyrokinetic simulations.

Findings

Accurately models ITG and TEM instabilities in diverse geometries.

Padé approximation reduces computational costs with high accuracy.

Good agreement with high-fidelity gyrokinetic simulations.

Abstract

Small-scale turbulence originating from microinstabilities limits the energy confinement time in magnetic confinement fusion. Here we develop a semi-analytical dispersion relation based on lowest-order solutions to the gyrokinetic equations in an asymptotic expansion in the ratio of transit (bounce) frequency to the mode frequency for ions (electrons), capable of describing two common instabilities: the ion temperature gradient (ITG) mode and trapped-electron mode (TEM), in the electrostatic limit. The dispersion relation, which is valid in arbitrary toroidal geometry, takes into account resonances with the magnetic ion and bounce-averaged electron drifts, incorporates non-local effects along the magnetic field line, is valid for arbitrary sign of the growth rate and magnetic curvature, and is shown to satisfy a variational property. Several common approximation models are introduced for both the magnetic drift and finite Larmor radius (FLR) damping, with the Pad\'e approximation for FLR effect in particular resulting in remarkable agreement with the baseline dispersion relation model at significantly reduced costs. The baseline model is verified by comparing solutions of the dispersion relation model to high-fidelity linear gyrokinetic simulations, where the exact eigenfunction of the electrostatic potential from simulations is used as a trial function, showing good quantitative agreement for ITGs and TEMs in (shaped) tokamaks as well as low-magnetic-shear stellarators.