Micro-tearing modes in the Mega Ampere Spherical Tokamak

TL;DR

This study uses numerical gyrokinetic simulations to analyze micro-tearing modes in spherical tokamaks, revealing their destabilization mechanisms and factors influencing their prevalence, which differ from previous theoretical models.

Contribution

It provides a detailed numerical analysis of micro-tearing modes, highlighting their destabilization by magnetic drifts and electrostatic potential, contrasting with earlier theoretical assumptions.

Findings

Micro-tearing modes are driven by electron temperature gradients.

Magnetic drifts and electrostatic potential destabilize the modes.

Higher plasma beta and smaller radius of curvature increase mode prevalence.

Abstract

Recent gyrokinetic stability calculations have revealed that the spherical tokamak is susceptible to tearing parity instabilities with length scales of a few ion Larmor radii perpendicular to the magnetic field lines. Here we investigate this 'micro-tearing' mode in greater detail to uncover its key characteristics, and compare it with existing theoretical models of the phenomenon. This has been accomplished using a full numerical solution of the linear gyrokinetic-Maxwell equations. Importantly, the instability is found to be driven by the free energy in the electron temperature gradient as described in the literature. However, our calculations suggest it is not substantially affected by either of the destabilising mechanisms proposed in previous theoretical models. Instead the instability is destabilised by interactions with magnetic drifts, and the electrostatic potential. Further calculations reveal that the mode is not significantly destabilised by the flux surface shaping or the large trapped particle fraction present in the spherical tokamak. Its prevalence in spherical tokamak plasmas is primarily due to the higher value of plasma {\beta}, and the enhanced magnetic drifts due to the smaller radius of curvature.