Microtearing modes at the top of the pedestal

TL;DR

This study investigates microtearing modes at the top of the pedestal in MAST, revealing a collisionless drive mechanism influenced by magnetic drifts, which differs from core MTMs and may be common in tokamak edges.

Contribution

It identifies a new collisionless drive mechanism for edge microtearing modes, contrasting with existing theories and highlighting their potential ubiquity in tokamak pedestals.

Findings

Edge MTMs are driven by a collisionless trapped particle mechanism.

Growth rates do not peak at finite collision frequency for edge MTMs.

High magnetic shear and trapped particle fraction enhance edge MTMs.

Abstract

Microtearing modes (MTMs) are unstable in the shallow gradient region just inside the top of the pedestal in the spherical tokamak experiment MAST, and may play an important role in the pedestal evolution. The linear properties of these instabilities are compared with MTMs deeper inside the core, and further detailed investigations expose the basic drive mechanism. MTMs near the MAST pedestal top are not well described by existing theories. In particular the growth rate of the dominant edge MTM does not peak at a finite collision frequency, as frequently reported for MTMs further into the core. Our study suggests that the edge MTM is driven by a collisionless trapped particle mechanism that is sensitive to magnetic drifts. This drive is enhanced in the outer region of MAST by a high magnetic shear and a high trapped particle fraction. Observations of similar modes on conventional aspect ratio devices suggests this drive mechanism may be somewhat ubiquitous towards the edge of current day and future hot tokamaks.