Observation of Edge Instability Limiting the Pedestal Growth in Tokamak Plasmas

Diallo, A.; Jw, Hughes; Greenwald, M.; LaBombard, B.; Davis, E.M.; Baek, S-G.; Theiler, C.; Snyder, P. B.; Canik, J.; Walk, J.; Golfinopoulos, T.; Terry, J. L.; Churchill, Michael; Hubbard, A.; Porkoláb, M.; Delgado‐Aparicio, L.; Ml, Reinke; White, Anne; Team, Alcator C‐Mod

doi:10.1103/physrevlett.112.115001

Cited by 86 publications

(104 citation statements)

References 22 publications

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“…33, with k h q s $ 0.04, that is, with wavenumbers somewhat lower than for the QCM and WCM discussed above. 155 The mode amplitude scales with electron b (T e and T i are equilibrated) consistent with expectations for the KBM. Immediately after each ELM, this mode disappears.…”

Section: Fluctuations That Regulate Transport In Elmy Dischargesmentioning

confidence: 62%

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20 years of research on the Alcator C-Mod tokamak

et al. 2014

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The object of this review is to summarize the achievements of research on the Alcator C-Mod tokamak [Hutchinson et al., Phys. Plasmas 1, 1511 and Marmar, Fusion Sci. Technol. 51, 261 (2007)] and to place that research in the context of the quest for practical fusion energy. C-Mod is a compact, high-field tokamak, whose unique design and operating parameters have produced a wealth of new and important results since it began operation in 1993, contributing data that extends tests of critical physical models into new parameter ranges and into new regimes. Using only highpower radio frequency (RF) waves for heating and current drive with innovative launching structures, C-Mod operates routinely at reactor level power densities and achieves plasma pressures higher than any other toroidal confinement device. C-Mod spearheaded the development of the vertical-target divertor and has always operated with high-Z metal plasma facing componentsapproaches subsequently adopted for ITER. C-Mod has made ground-breaking discoveries in divertor physics and plasma-material interactions at reactor-like power and particle fluxes and elucidated a) Paper AR1 1, Bull. Am. Phys. Soc. 58, 21 (2013). b) Invited speaker.

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Section: Fluctuations That Regulate Transport In Elmy Dischargesmentioning

confidence: 62%

“…Recent electromagnetic signatures observed between ELMs and described below are possible evidence for KBM pedestal-regulating activity. 155 Separately, ELM precursors have also been documented along with multiple "secondary" filaments following the primary ELM filament ejection. 156 …”

Section: B Elmy H-modementioning

confidence: 99%

20 years of research on the Alcator C-Mod tokamak

et al. 2014

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“…Experimentally, fluctuations have been diagnosed that are consistent with MTM [22,23,25], KBM [22,[24][25][26], and trapped electron modes (TEM) [22], while linear gyrokinetic modeling has identified MTM [19,[27][28][29][30], TEM [30,31], ETG [29,30,32,33], KBM [19,26,27,29,30,34,35], and unidentified drift waves [36]. Due to the large number of possible instabilities, and the complexity of the nonlinear turbulent state (that reflects the underlying instabilities in not so obvious ways), no clear picture has emerged regarding the relative importance of these modes for pedestal transport.…”

Section: Pacs Numbersmentioning

confidence: 83%

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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“…Phenomena presenting a QC signature have been observed at the very edge of the plasma during H-mode [39,40], enhanced D α H-modes [41,42] and I-mode [43]. At the moment, there is no unified explanation for these modes which present rather similar QC spectral signatures.…”

Section: Quasi-coherent Fluctuations At the Very Edge Of The Plasmamentioning

confidence: 99%

Identification of trapped electron modes in frequency fluctuation spectra

Arnichand

Citrin

Hacquin

et al. 2015

Plasma Phys. Control. Fusion

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Abstract.Ion temperature gradient (ITG) and trapped electron modes (TEM) are two important micro-instabilities in the plasma core region of fusion devices (r/a ≤ 0.9). They usually coexist in the same range of spatial scale (around 0.1 < k ⊥ ρ i < 1), which makes their discrimination difficult. To investigate them, one can perform gyrokinetic simulations, transport analysis and phase velocity estimations. In Tore Supra, the identification of trapped electron modes (TEM) is made possible due to measured frequency fluctuation spectra. Indeed, turbulent spectra generally expected to be broad-band can become narrow in case of TEM turbulence, inducing "quasi-coherent" (QC) modes named QC-TEM. Therefore the analysis of frequency fluctuation spectra becomes a possible tool to differentiate TEM from ITG. We have found indications that the TEM can have a QC signature by comparing frequency fluctuation spectra from reflectometry measurements, gyrokinetic simulations and synthetic diagnostic results. Then the scope of the analysis of QC-TEM are discussed and an application is shown, namely transitions between TEM turbulence and MHD fluctuations.

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Observation of Edge Instability Limiting the Pedestal Growth in Tokamak Plasmas

Cited by 86 publications

References 22 publications

20 years of research on the Alcator C-Mod tokamak

20 years of research on the Alcator C-Mod tokamak

Microtearing turbulence limiting the JET-ILW pedestal

Identification of trapped electron modes in frequency fluctuation spectra

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