Geodesic extended modes in low magnetic shear tokamaks and stellarators

TL;DR

This paper develops a new gyrokinetic theory for geodesic extended modes in low magnetic shear tokamaks and stellarators, revealing a coupled electron-ion microinstability validated by simulations.

Contribution

It introduces a multiscale gyrokinetic theory for geodesic extended modes, highlighting their coupled electron-ion physics at low magnetic shear.

Findings

Validation against gyrokinetic simulations confirms the theory.

The new mode's parameter dependence is systematically studied.

The mode couples non-adiabatic electron and ion responses.

Abstract

Theories of ion-scale microinstabilities in tokamaks and stellarators typically assume that the passing electrons respond adiabatically due to their fast propagation speed. However, when the magnetic shear becomes sufficiently small, ion-scale modes can extend far along the magnetic field and the non-adiabatic response of passing electrons becomes important. We derive a theory of extended modes at low magnetic shear through a multiscale expansion of the gyrokinetic equation. The theory elucidates the physics of the geodesic extended mode, a new type of microinstability. The new mode couples the non-adiabatic physics of both electrons and ions, unlike extended modes at magnetic shear of order unity. The theory is validated against gyrokinetic simulations and the parameter dependences of the new mode are studied.