Kinetic instabilities that limit {\beta} in the edge of a tokamak plasma: a picture of an H-mode pedestal

TL;DR

This paper investigates the plasma behavior at the edge of a tokamak during ELM cycles, revealing how kinetic instabilities like KBMs and MTMs influence pedestal formation and stability.

Contribution

It provides a new physical picture of pedestal formation and arrest based on gyrokinetic analysis of microinstabilities during ELM cycles.

Findings

Identification of kinetic ballooning modes in the pedestal

Detection of microtearing modes near the pedestal top

Growth rate spectra support a new physical understanding of pedestal dynamics

Abstract

Plasma equilibria reconstructed from the Mega-Amp Spherical Tokamak (MAST) have sufficient resolution to capture plasma evolution during the short period between edge-localized modes (ELMs). Immediately after the ELM steep gradients in pressure, P, and density, ne, form pedestals close to the separatrix, and they then expand into the core. Local gyrokinetic analysis over the ELM cycle reveals the dominant microinstabilities at perpendicular wavelengths of the order of the ion Larmor radius. These are kinetic ballooning modes (KBMs) in the pedestal and microtearing modes (MTMs) in the core close to the pedestal top. The evolving growth rate spectra, supported by gyrokinetic analysis using artificial local equilibrium scans, suggest a new physical picture for the formation and arrest of this pedestal.