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

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 ω * 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.

Review: Turbulence dynamics during the pedestal evolution between edge localized modes in magnetic fusion devices

“…where k y ρ s = ρsq ρtora n) profile localized in the pedestal [85]. The authors of [84] showed excellent agreement with observed spectra. The important assumption is that the modes follow MTM dispersion relation and only the nnumber such that there exits an alignment with the rational surface appear to correspond to the observed frequencies.…”

“…As stated above, ITER plans to operate near the L-H power threshold, which will lead presumably to long inter-ELM phases during which pedestal modes will saturate. Following Hatch et al [84] hypothesis with MTM being the key player, it seems like ω * e will be small as ω * e scales with the inverse of I p . Assuming low n-modes (∼10), one would expect sub-kHz modes to be present in ITER in the plasma frame.…”

“…This is a challenging question to tackle without an hypothesis. That being said, recent work by Hatch et al [84] provide some insights. In JET, for instance, substantial gaps between toroidal modes were reported (similar to AUG and DIII-D).…”

“…In JET, for instance, substantial gaps between toroidal modes were reported (similar to AUG and DIII-D). Using GENE, Hatch et al [84] provided an hypothesis related to the alignment of the relevant rational surfaces with the peak of ω * e = k y ρ s c s…”