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

Diallo, A.; Laggner, F. M.

doi:10.1088/1361-6587/abbf85

Cited by 16 publications

(21 citation statements)

References 88 publications

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“…The success of the perturbative approach applied here also implies a possibility for the expansion of dynamic turbulence identification to other unexplained tokamak regimes. Similar instability markers have been reported on a wide variety of machines and scenarios 6,32 , but the underlying physics remains largely undetermined. The analysis presented here offers a new mechanism to uncover explanations for these observations, potentially enabling a comprehensive perturbative study of experimental transport signatures in tokamak devices.…”

Section: Discussion and Outlooksupporting

confidence: 65%

“…Remarkably, this profile-based calculation exactly matches the mode chirping behavior seen in fast magnetic fluctuation measurements, as shown in figure 2(c). Through this theoretically-motivated analysis, we thus explain the distinctive up-chirping behavior observed in magnetic spectrograms 6,25,[30][31][32] as follows: The recovery of density and temperature gradients after an ELM 32 introduces periodic growth into the ω e * profile described by equation 1. MTMs, being locked at a particular rational q surface, will simultaneously experience a local increase in ∇T e and ω e * .…”

Section: Time-dependent Mtm Identificationmentioning

confidence: 99%

“…First, strong velocity shear caused by variation in the radial electric field suppresses transport in the pedestal by tearing apart turbulent eddies, allowing for the formation of steep temperature and density gradients that would otherwise be eliminated by diffusion 5 . Second, various metastable microinstabilities induce transport across the pedestal despite the high levels of turbulent shear, controlling the evolution of the pedestal structure 6 . If left unmitigated, non-linear interactions between these microinstabilities periodically spark global explosive events 7,8 called edgelocalized modes (ELMs) 9 which can melt and erode the machine wall 10 .…”

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confidence: 99%

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Perturbative Determination of Plasma Microinstabilities in Tokamaks

Nelson,

Laggner,

Diallo

et al. 2021

Preprint

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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.

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Section: Discussion and Outlooksupporting

confidence: 65%

Section: Time-dependent Mtm Identificationmentioning

confidence: 99%

mentioning

confidence: 99%

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Perturbative Determination of Plasma Microinstabilities in Tokamaks

Nelson,

Laggner,

Diallo

et al. 2021

Preprint

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“…Recently, innovative experimental diagnostics of internal magnetic fluctuations [17] have further established MTM as a common pedestal fluctuation in DIII-D as discussed in [18,19]. Related gyrokinetic modeling will be reported in [20].…”

Section: Introductionmentioning

confidence: 95%

Identifying the microtearing modes in the pedestal of DIII-D H-modes using gyrokinetic simulations

Hassan¹,

Hatch²,

Halfmoon³

et al. 2021

Nucl. Fusion

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Recent evidence points toward the microtearing mode (MTM) as an important fluctuation in the H-mode pedestal for anomalous electron heat transport. A study of the instabilities in the pedestal region carried out using gyrokinetic simulations to model an ELMy H-mode DIII-D discharge (USN configuration, 1.4 MA plasma current, and 3 MW heating power) is presented. The simulations produce MTMs, identified by predominantly electromagnetic heat flux, small particle flux, and a substantial degree of tearing parity. The magnetic spectrogram from Mirnov coils exhibits three distinct frequency bands---two narrow bands at lower frequency ($\sim$35-55 kHz and $\sim$70-105 kHz) and a broader band at higher frequency ($\sim$300-500 kHz). Global linear GENE simulations produce MTMs that are centered at the peak of the $\omega_*$ profile and correspond closely with the bands in the spectrogram. The three distinctive frequency bands can be understood from the basic physical mechanisms underlying the instabilities. For example (i) instability of certain toroidal mode numbers (n) is controlled by the alignment of their rational surfaces with the peak in the $\omega^*$ profile, and (ii) MTM instabilities in the lower n bands are the conventional collisional slab MTM, whereas the higher n band depends on curvature drive. While many features of the modes can be captured with the local approximation, a global treatment is necessary to quantitatively reproduce the detailed band gaps of the low-n fluctuations. Notably, the transport signatures of the MTM are consistent with careful edge modeling by SOLPS.

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“…ELMs set a maximum limit on the normalized pressure gradient (α) and average toroidal current density ( j tor ) in the pedestal region, but they do not by themselves fully constrain the achievable pedestal pressure or the resulting plasma performance [4]. Instead, the recovery of the pedestal structure between ELMs is additionally influenced by so-called microinstabilities, which regulate local profile gradients by inducing additional transport across the ETB [5]. In conjunction with the global peeling-ballooning (PB) constraint, models describing sub-critical pedestal instabilities can be used to predict the pedestal height and width, demonstrating the critical role that these microinstabilities play in determining plasma performance [6,7].…”

Section: Introductionmentioning

confidence: 99%

Time-dependent experimental identification of inter-ELM microtearing modes in the tokamak edge on DIII-D

et al. 2021

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In a series of discharges on the DIII-D tokamak, fast vertical plasma jogs are used to induce current perturbations in the steep gradient region of the H-mode edge. These current perturbations directly influence the edge q profile, decoupling the resonant location and instability drive of pedestal-localized microtearing modes (MTMs). By exploiting this effect, we develop and apply a new experimental technique to track the dynamical frequency evolution of MTMs in the pedestal region, providing a compelling validation of the MTM model. The frequency of potential MTMs is calculated as the Doppler-shifted electron diamagnetic frequency at rational q = m/n surfaces, showing remarkable agreement with chirped frequency behavior of n = 3, 4 and 5 modes detected with fast magnetics. Data is collected throughout multiple ELM cycles in order to build robust statistics describing the time-dependent frequency evolution of MTMs, which can be explained by examining the recovery of pedestal gradients after an ELM event. MTMs have a dominant transport contribution in the electron thermal channel, so the presented results indicate that reduced models of pedestal transport must be electromagnetic in nature and constructed with accurate calculations of MTM stability; inclusion of this physics is essential for accurate predictions of the electron temperature pedestal profile. Supporting measurements of mode saturation, propagation direction and transport fingerprints are made to support the dynamic frequency determination, unambiguously and experimentally identifying MTMs in the pedestal region of DIII-D.

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

Cited by 16 publications

References 88 publications

Perturbative Determination of Plasma Microinstabilities in Tokamaks

Perturbative Determination of Plasma Microinstabilities in Tokamaks

Identifying the microtearing modes in the pedestal of DIII-D H-modes using gyrokinetic simulations

Time-dependent experimental identification of inter-ELM microtearing modes in the tokamak edge on DIII-D

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