Extended electron tails in electrostatic microinstabilities and the nonadiabatic response of passing electrons

TL;DR

This paper investigates ion-gyroradius-scale microinstabilities driven by nonadiabatic passing electrons, revealing extended tails in electrostatic modes and providing a new theoretical framework that enhances understanding of turbulent transport in magnetized plasmas.

Contribution

It introduces a linear gyrokinetic theory including nonadiabatic passing electron responses, uncovering ion-gyroradius-scale modes driven solely by passing electrons, and verifies predictions through numerical simulations.

Findings

Nonadiabatic passing electrons can drive ion-gyroradius-scale modes.

Extended tails in electrostatic modes are linked to passing electron response.

Theory aligns with numerical simulations across collision regimes.

Abstract

Ion-gyroradius-scale microinstabilities typically have a frequency comparable to the ion transit frequency. Due to the small electron-to-ion mass ratio and the large electron transit frequency, it is conventionally assumed that passing electrons respond adiabatically in ion-gyroradius-scale modes. However, in gyrokinetic simulations of ion-gyroradius-scale modes in axisymmetric toroidal magnetic fields, the nonadiabatic response of passing electrons can drive the mode, and generate fluctuations with narrow radial layers, which may have consequences for turbulent transport in a variety of circumstances. In flux tube simulations, in the ballooning representation, these instabilities reveal themselves as modes with extended tails. The small electron-to-ion mass ratio limit of linear gyrokinetics for electrostatic instabilities is presented, in axisymmetric toroidal magnetic geometry, including the nonadiabatic response of passing electrons and associated narrow radial layers. This theory reveals the existence of ion-gyroradius-scale modes driven solely by the nonadiabatic passing electron response, and recovers the usual ion-gyroradius-scale modes driven by the response of ions and trapped electrons, where the nonadiabatic response of passing electrons is small. The collisionless and collisional limits of the theory are considered, demonstrating parallels in structure and physical processes to neoclassical transport theory. By examining initial-value simulations of fastest-growing eigenmodes, the predictions for mass-ratio scaling are tested and verified numerically for a range of collision frequencies. Insights from the small electron-to-ion mass ratio theory may lead to a computationally efficient treatment of extended modes.