Structure of Micro-instabilities in Tokamak Plasmas: Stiff Transport or Plasma Eruptions?

TL;DR

This paper investigates micro-instabilities in tokamak plasmas, revealing how plasma profiles influence mode structure and stability, and proposing a model for plasma eruptions like ELMs based on flow shear effects.

Contribution

It introduces a 2D eigenmode model showing the sensitivity of micro-instabilities to plasma profiles and identifies conditions leading to transitions resembling plasma eruptions.

Findings

Maximally unstable modes balloon on the outboard side.

Most profiles favor modes ballooning near the top or bottom.

A critical flow shear triggers a transition to more unstable ballooning modes.

Abstract

Solutions to a model 2D eigenmode equation describing micro-instabilities in tokamak plasmas are presented that demonstrate a sensitivity of the mode structure and stability to plasma profiles. In narrow regions of parameter space, with special plasma profiles, a maximally unstable mode is found that balloons on the outboard side of the tokamak. This corresponds to the conventional picture of a ballooning mode. However, for most profiles this mode cannot exist and instead a more stable mode is found that balloons closer to the top or bottom of the plasma. Good quantitative agreement with a 1D ballooning analysis is found provided the constraints associated with higher order profile effects, often neglected, are taken into account. A sudden transition from this general mode to the more unstable ballooning mode can occur for a critical flow shear, providing a candidate model for why some experiments observe small plasma eruptions (Edge Localised Modes, or ELMs) in place of large Type I ELMs.