Micro-tearing modes in spherical and conventional tokamaks

TL;DR

This study investigates the behavior of Micro-Tearing Modes in spherical and conventional tokamaks using gyrokinetic simulations, revealing their dominance as instabilities driven by electron temperature gradients and collisionality.

Contribution

It provides a comparative analysis of MTMs in different tokamak configurations, highlighting the dependence of growth rates on electron temperature gradients across devices.

Findings

MTMs are the dominant linear instability in studied tokamaks.

Finite collisionality is essential for MTM instability.

Growth rate dependence on electron temperature gradient varies between devices.

Abstract

The onset and characteristics of Micro-Tearing Modes (MTM) in the core of spherical (NSTX) and conventional tokamaks (ASDEX-UG and JET) are studied through local linear gyrokinetic simulations with gyro [J. Candy and E. Belli, General Atomics Report GA-A26818 (2011)]. For experimentally relevant core plasma parameters in the NSTX and ASDEX-UG tokamaks, in agreement with previous works, we find MTMs as the dominant linear instability. Also, for JET-like core parameters considered in our study an MTM is found as the most unstable mode. In all these plasmas, finite collisionality is needed for MTMs to become unstable and the electron temperature gradient is found to be the fundamental drive. However, a significant difference is observed in the dependence of linear growth rate of MTMs on electron temperature gradient. While it varies weakly and non-monotonically in JET and ASDEX-UG plasmas, in NSTX it increases with the electron temperature gradient.