Gyrokinetic studies of microinstabilities in the reversed field pinch

Carmody, D.; Pueschel, M. J.; Terry, P. W.

doi:10.1063/1.4803509

Cited by 35 publications

(38 citation statements)

References 29 publications

(59 reference statements)

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“…In particular, the stability of ion-temperature-gradient (ITG), trapped-electron mode (TEM), and microtearing modes have been investigated analytically and numerically. [14][15][16][17][18][19] Comparisons of experimental measurements with comprehensive gyrokinetic models that use experimental equilibria for several of these improved-confinement cases have confirmed the presence of microtearing modes, 20 TEM, and ITG turbulence. 21 It should also be noted that many of the observed features presented in this paper are similar to features seen in the space turbulence.…”

Section: Introductionmentioning

confidence: 80%

Evidence for drift waves in the turbulence of reversed field pinch plasmas

et al. 2017

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A detailed characterization of the high-frequency range of the fluctuation spectrum in reversed field pinch plasmas is presented, revealing a variety of new features distinct from global tearing modes and the cascade that they are thought to drive. The anisotropic broadband spectrum of the fluctuating electric field is measured. The power in the fluctuating kinetic energy ð1=2Þm i n iṼ 2 EÂB 0 , previously measured to be smaller than the magnetic energy in the tearing-mode-unstable frequency range, becomes greater than and diverges from the magnetic energy above 60-80 kHz. The lack of equipartition at high frequencies coincides with the measured signatures of the independent fluctuation activity broadly consistent with the drift-wave fluctuations. Statistical coherence measurements reveal the mode activity that is compressive with a large amplitude in the vicinity of strong density gradients and with a phase speed comparable to the electron drift speed. There is a distinct highfrequency correlation between the fluctuations of density and the parallel magnetic field. Elevated coherences associated with this fluctuation feature return more quickly after a sawtooth event than the corresponding coherences associated with tearing activity. Published by AIP Publishing.

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Section: Introductionmentioning

confidence: 80%

Evidence for drift waves in the turbulence of reversed field pinch plasmas

et al. 2017

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“…Parameter scans determine that MTM growth rates increase strongly with electron temperature gradients, are weakly suppressed by density and ion temperature gradients, and are largely independent of collisionality [28,37,38]. The MTM exhibits moderate cancellation in E || (Ê || 0.3 being typical), although the electrostatic component of the mode is not crucial to its existence; the MTM growth rate smoothly decreases to ∼ 55% of its nominal value when the amplitude of the electrostatic potential is artificially reduced towards φ = 0.…”

Section: Pacs Numbersmentioning

confidence: 99%

Microtearing turbulence limiting the JET-ILW pedestal

et al. 2016

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Gyrokinetic simulations identify microtearing modes (MTM) to be the dominant microinstabilities in the JET-ILW (ITER like wall) pedestal. Nonlinear simulations show that MTM-driven turbulence produces the bulk of the transport in the steep gradient region, demonstrating that MTM may be the principal mechanism limiting JET-ILW pedestal evolution. The combination of MTM, electron temperature gradient (ETG), and neoclassical transport reproduces experimental power balance across most of the pedestal. Kinetic ballooning modes are significant only in the local limit and only at low β, far below the experimental operating point. PACS numbers:The tokamak H-mode [1] is defined by a narrow insulating region-the pedestal-at the plasma edge, where turbulence is suppressed and sharp pressure gradients develop. The pedestal is at the center of the most pressing issues facing fusion energy. ITER, for example, must reach a sufficient temperature at the pedestal's inner boundary in order to achieve its fusion power targets [2]. This work reports the results of, perhaps, the very first, first-principles simulations of the H-mode pedestal dynamics that reproduce experimentally observed transport levels. In addition to providing unprecedented insight into the dynamics of the existing H-mode pedestals, such simulations are likely to advance our capabilities towards predictive modeling of future burning plasma devices.This study targets the JET-ILW (ITER-like wall) pedestal [4][5][6], which approaches ITER conditions in two important ways: 1) as the largest tokamak in operation, it most-closely approximates plasma parameters that are dependent on machine size (like ρ * , the ratio of the gyroradius to minor radius), 2) to approximate ITER conditions even better, JET has recently installed an ITERlike wall (ILW) (composed of a tungsten divertor and beryllium chamber). This modification decreases the global performance of discharges by 20-30%, attributable largely to changes in pedestal structure. In addition to the performance loss, certain observed key properties of the ILW pedestal are inconsistent with predictions of the leading pedestal model (EPED [7,8]).In this work, we elucidate possible mechanisms limiting profile evolution in the JET-ILW discharges. We demonstrate, through simulations using the gyrokinetic code Gene [9,10], that the microtearing mode (MTM) [11][12][13][14] is the dominant instability in the pedestal. Interestingly, the simulations do not find the kinetic ballooning mode (KBM), a basic component of the EPED model, except locally in a narrow region near the separatrix. Most importantly, we determine via nonlinear gyrokinetic simulations that a combination of MTM and electron temperature gradient (ETG) [9,[15][16][17][18] driven turbulence plus the neoclassical flux, produces transport levels closely matching experimental power balance across most of the pedestal, demonstrating the capacity of these mechanisms to limit JET-ILW pedestal evolution.The JET-ILW Pedestal-JET pulse 82585 (described in Ref. [4]) is pa...

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“…Although often co-existing with, or sub-dominant to ITG or TEM, such cases have now been predicted for ASDEXUpgrade [26][27][28], DIII-D [29], and JET [30]. Microtearing is also predicted to occur in improved confinement regimes of reversed field pinches (RFPs) [31,32], as well as in the pedestal region of MAST [33,34], NSTX [35], JET [36] and even for model ITER profiles [37]. The prevalence of MT instability in such a variety of toroidal confinement devices highlights the need for nonlinear simulations to determine the nonlinearly saturated transport amplitude and fluctuation characteristics for comparison with experiments.…”

Section: Introductionmentioning

confidence: 99%

Progress in simulating turbulent electron thermal transport in NSTX

et al. 2013

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Abstract. Nonlinear simulations based on multiple NSTX discharge scenarios have progressed to help differentiate unique instability mechanisms and to validate with experimental turbulence and transport data. First nonlinear gyrokinetic simulations of microtearing (MT) turbulence in a high-beta NSTX H-mode discharge predict experimental levels of electron thermal transport that are dominated by magnetic flutter and increase with collisionality, roughly consistent with energy confinement times in dimensionless collisionality scaling experiments. Electron temperature gradient (ETG) simulations predict significant electron thermal transport in some low and high beta discharges when ion scales are suppressed by E B shear. Although the predicted transport in H-modes is insensitive to variation in collisionality (inconsistent with confinement scaling), it is sensitive to variations in other parameters, particularly density gradient stabilization. In reversed shear (RS) Lmode discharges that exhibit electron internal transport barriers, ETG transport has also been shown to be suppressed nonlinearly by strong negative magnetic shear, s<<0. In many high beta plasmas, instabilities which exhibit a stiff beta dependence characteristic of kinetic ballooning modes (KBM) are sometimes found in the core region. However, they do not have a distinct finite beta threshold, instead transitioning gradually to a trapped electron mode (TEM) as beta is reduced to zero. Nonlinear simulations of this "hybrid" TEM/KBM predict significant transport in all channels, with substantial contributions from compressional magnetic perturbations. As multiple instabilities are often unstable simultaneously in the same plasma discharge, even on the same flux surface, unique parametric dependencies are discussed which may be useful for distinguishing the different mechanisms experimentally.

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Gyrokinetic studies of microinstabilities in the reversed field pinch

Cited by 35 publications

References 29 publications

Evidence for drift waves in the turbulence of reversed field pinch plasmas

Evidence for drift waves in the turbulence of reversed field pinch plasmas

Microtearing turbulence limiting the JET-ILW pedestal

Progress in simulating turbulent electron thermal transport in NSTX

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