Microtearing modes at the top of the pedestal

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.

14 pages, 10 figures (21 unique graphics)

References in corpus (3)

• Kinetic instabilities that limit β in the edge of a tokamak plasma: a picture of an H-mode pedestal
• Micro-tearing modes in the Mega Ampere Spherical Tokamak
• Microtearing Modes in Reversed Field Pinch Plasmas

Cited by in corpus (16)

• Gyrokinetic analysis and simulation of pedestals, to identify the culprits for energy losses using fingerprints
• Microtearing modes as the source of magnetic fluctuations in the JET pedestal
• MHD and Gyro-kinetic Stability of JET Pedestals
• Kinetic microtearing modes and reconnecting modes in strongly magnetised slab plasmas
• Current understanding of the processes underlying the triggering of and energy loss associated with type I ELMs
• Microstability analysis of pellet fuelled discharges in MAST
• Linear gyrokinetic stability of a high $\beta$ non-inductive spherical tokamak
• Three-Dimensional Inhomogeneity of Electron-Temperature-Gradient Turbulence in the Edge of Tokamak Plasmas
• Electromagnetic instabilities and plasma turbulence driven by electron-temperature gradient
• Extended electron tails in electrostatic microinstabilities and the nonadiabatic response of passing electrons
• Nonlinear microtearing modes in MAST and their stochastic layer formation
• The physics of a small-scale tearing mode in collisionless slab plasmas
• New linear stability parameter to describe low-$\beta$ electromagnetic microinstabilities driven by passing electrons in axisymmetric toroidal geometry
• Microstability of $\beta \sim 1$ tokamak equilibria
• Multiscale Modelling for Tokamak Pedestals
• Destabilization mechanism of the collisional microtearing mode in magnetized slab plasmas