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

Hatch, D. R.; Kotschenreuther, M.; Mahajan, S. M.; Pueschel, M. J.; Michoski, C.; Merlo, G.; Hassan, E.; Field, A. R.; Frassinetti, L.; Giroud, C.; Hillesheim, J. C.; Maggi, C. F.; Perez von Thun, C.; Roach, C. M.; Saarelma, S.; Jarema, D.; Jenko, F.; Contributors, JET

doi:10.1088/1741-4326/abd21a

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

Journal Article · Sun Feb 07 23:00:00 EST 2021 · Nuclear Fusion

DOI:https://doi.org/10.1088/1741-4326/abd21a· OSTI ID:1849563

^[1]; Kotschenreuther, M.; Mahajan, S. M.; ; Michoski, C.; ; ; ; ; Giroud, C.; Hillesheim, J. C.; ; ; ; Saarelma, S.; Jarema, D.; Jenko, F.; Contributors, JET

OSTI

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 numbersn= 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 theω_*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 a simple MTM dispersion relation. 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.

Research Organization:: Univ. of Texas, Austin, TX (United States)

Sponsoring Organization:: USDOE Office of Science (SC)

DOE Contract Number:: FG02-04ER54742; SC0018429

OSTI ID:: 1849563

Journal Information:: Nuclear Fusion, Journal Name: Nuclear Fusion Journal Issue: 3 Vol. 61; ISSN 0029-5515

Publisher:: IOP Science

Country of Publication:: United States

Language:: English

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