Perturbative Determination of Plasma Microinstabilities in Tokamaks

TL;DR

This paper provides experimental validation for the role of microtearing modes (MTMs) in plasma heat transport in tokamaks, using a novel time-dependent analysis and perturbations to confirm their activity in the pedestal region.

Contribution

It introduces a new time-dependent analysis method to explicitly identify and validate the activity of MTMs in tokamak plasma pedestals.

Findings

MTMs are active in the pedestal region.

Calculated MTM frequencies match observed turbulence.

Perturbations validate the analytical model.

Abstract

Recently, theoretical analysis has identified plasma microinstabilities as the primary mechanism responsible for anomalous heat transport in tokamaks. In particular, the microtearing mode (MTM) has been credited with the production of intense electron heat fluxes, most notably through a thin self-organized boundary layer called the pedestal. Here we exploit a novel, time-dependent analysis to compile explicit experimental evidence that MTMs are active in the pedestal region. The expected frequency of pedestal MTMs, calculated as a function of time from plasma profile measurements, is shown in a dedicated experiment to be in excellent agreement with observed magnetic turbulence fluctuations. Further, fast perturbations of the plasma equilibrium are introduced to decouple the instability drive and resonant location, providing a compelling validation of the analytical model. This analysis offers strong evidence of edge MTMs, validating the existing theoretical work and highlighting the important role of MTMs in regulating electron heat flow in tokamaks.