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 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.

Perturbative Determination of Plasma Microinstabilities in Tokamaks

“…Since ω e * is inversely related to L Te , a peak in the ω e * (ψ n ) profile corresponds to a peak in the MTM instability drive from ∇T e (ψ n ). Therefore MTMs are most likely to occur when a rational q surface aligns with the peak of the ω e * (ψ n ) profile 25 . This formulation has been used to explain steady-state frequency bands observed in magnetic fluctuation data on the JET tokamak, which were identified as MTMs through comparisons with gyrokinetic simulations 25 , and it forms the theoretical foundation of the dynamical experimental analysis presented here.…”

“…Notably, recent theoretical work suggests that the MTM 13,14 , a small-scale resistive magnetohydrodynamic (MHD) mode not yet included in leading predictive models 15 , might play a critical role in limiting electron thermal transport through the pedestal 16,17 . The presence of pedestal MTMs has been suggested through analysis of so-called "transport fingerprints" 18 and through comparisons of measured magnetic fluctuations with sensitive theory-based (gyrokinetic) simulations [16][17][18][19][20][21][22][23][24][25][26] . However, a conclusive experimental identification of these modes has not yet been presented and is needed to validate the theoretical results.…”

Reduced models for ETG transport in the pedestal

“…Notably, the disparity between heat diffusivity and particle diffusivity (the latter being much smaller than the former) identified by edge modeling, points toward a vigorous electron heat transport mechanism that needs to be accounted for. Two instabilities are likely at play: ETG and microtearing modes (MTM) [11,12,13]. Since ETG fluctuations exist at scales that are typically inaccessible to diagnostics, we must rely on theory and simulation to infer their activity.…”

“…[14,22], which identified toroidal ETG modes destabilized at large radial wavenumbers). This results in turbulence (1) that is isotropic in comparison with the streamer-dominated core ETG [23,24,25]; (2) exhibits high-k z structure, which demands extreme resolution in the parallel direction; and (3) has contributions from a high number (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) of unstable eigenmodes at each wavenumber. Properties (2) and (3) challenge some of the standard approaches to reduced quasilinear modeling, making the present work challenging and timely.…”

Turbulent broadening of electron heat-flux width in electromagnetic gyrokinetic simulations of a helical scrape-off layer model

“…Previous Gkeyll results include electrostatic studies of the SOL of the National Spherical Torus Experiment (NSTX) 19 , the Texas Helimak 38 , and the Large Plasma Device (LAPD) 39 . While we have been focused on demonstrat-ing the capability to simulate the SOL region, Gkeyll will eventually be used to simulate the pedestal region as well, where it is known that kinetic ballooning and peeling-ballooning modes play an important role in setting the height and width of pedestals 40,41 , and microtearing modes are also important in some regimes 42,43 . Thus, it is important to demonstrate a robust capability to handle electromagnetic fluctuations in an edge gyrokinetic code.…”