Microtearing modes as the source of magnetic fluctuations in the JET pedestal

TL;DR

This study identifies microtearing modes as the primary source of magnetic fluctuations in the JET pedestal, using simulations and experimental data to explain their characteristics, impact on transport, and dependence on collisionality.

Contribution

It provides a comprehensive analysis linking microtearing modes to observed pedestal fluctuations, highlighting the importance of rational surface alignment and nonlinear effects.

Findings

Quantitative agreement between gyrokinetic simulations and experimental fluctuation data.

Microtearing modes produce significant transport levels and saturate by relaxing the electron temperature gradient.

MTM growth rates vary with collisionality, showing complex dependence.

Abstract

We report on a detailed study of magnetic fluctuations in the JET pedestal, employing basic theoretical considerations, gyrokinetic simulations, and experimental fluctuation data, to establish the physical basis for their origin, role, and distinctive characteristics. We demonstrate quantitative agreement between gyrokinetic simulations of microtearing modes (MTMs) and two magnetic frequency bands with corresponding toroidal mode numbers n=4 and 8. Such disparate fluctuation scales, with substantial gaps between toroidal mode numbers, are commonly observed in pedestal fluctuations. Here we provide a clear explanation, namely the alignment of the relevant rational surfaces (and not others) with the peak in the omega star profile, which is localized in the steep gradient region of the pedestal. We demonstrate that a global treatment is required to capture this effect. Nonlinear simulations suggest that the MTM fluctuations produce experimentally-relevant transport levels and saturate by relaxing the background electron temperature gradient, slightly downshifting the fluctuation frequencies from the linear predictions. Scans in collisionality are compared with simple MTM dispersion relations. At the experimental points considered, MTM growth rates can either increase or decrease with collision frequency depending on the parameters thus defying any simple characterization of collisionality dependence.