Gyrokinetic study of ASDEX Upgrade inter-ELM pedestal profile evolution

Hatch, D. R.; Told, D.; Jenko, F.; Doerk, H.; Dunne, M.; Wolfrum, E.; Viezzer, E.; Pueschel, M. J.

doi:10.1088/0029-5515/55/6/063028

Cited by 58 publications

(94 citation statements)

References 71 publications

(121 reference statements)

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“…ETG simulations are sensitive to not only temperature and density gradients, but also the temperature ratio and Z ef f , all of which are taken directly from the best available experimental estimates. In agreement with earlier work [55,56,50,15], the pedestal ETG turbulence described here is slab-like and isotropic, in contrast with the streamer-dominated core ETG turbulence. Consistent with the large difference in pedestal η e , JET-ILW (92432) produces order of magnitude larger gyroBohm-normalized ETG heat fluxes than JET-C (78697), as shown in Fig.…”

Section: Etg Turbulence: Comparison Of Jet-ilw and Jet-csupporting

confidence: 92%

Direct gyrokinetic comparison of pedestal transport in JET with carbon and ITER-like walls

Hatch

Kotschenreuther²,

Mahajan³

et al. 2019

Nucl. Fusion

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104

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This paper compares the gyrokinetic instabilities and transport in two representative JET pedestals, one (pulse 78697) from the JET configuration with a carbon wall (C) and another (pulse 92432) from after the installation of JET's ITERlike Wall (ILW). The discharges were selected for a comparison of JET-ILW and JET-C discharges with good confinement at high current (3 MA, corresponding also to low ρ * ) and retain the distinguishing features of JET-C and JET-ILW, notably, decreased pedestal top temperature for JET-ILW. A comparison of the profiles and heating power reveals a stark qualitative difference between the discharges: the JET-ILW pulse (92432) requires twice the heating power, at a gas rate of 1.9×10 22 e/s, to sustain roughly half the temperature gradient of the JET-C pulse (78697), operated at zero gas rate. This points to heat transport as a central component of the dynamics limiting the JET-ILW pedestal and reinforces the following emerging JET-ILW pedestal transport paradigm, which is proposed for further examination by both theory and experiment. ILW conditions modify the density pedestal in ways that decrease the normalized pedestal density gradient a/L n , often via an outward shift of the density pedestal. This is attributable to some combination of direct metal wall effects and the need for increased fueling to mitigate tungsten contamination. The modification to the density profile increases η = L n /L T , thereby producing more robust ion temperature gradient (ITG) and electron temperature gradient driven instability. The decreased pedestal gradients for JET-ILW (92432) also result in a strongly reduced E × B shear rate, further enhancing the ion scale turbulence. Collectively, these effects limit the pedestal temperature and demand more heating power to achieve good pedestal performance. Our simulations, consistent with basic theoretical arguments, find higher ITG turbulence, stronger stiffness, and higher pedestal transport in the ILW plasma at lower ρ * .

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Section: Etg Turbulence: Comparison Of Jet-ilw and Jet-csupporting

confidence: 92%

Direct gyrokinetic comparison of pedestal transport in JET with carbon and ITER-like walls

Hatch

Kotschenreuther²,

Mahajan³

et al. 2019

Nucl. Fusion

Self Cite

104

View full text Add to dashboard Cite

show abstract

“…Plasma turbulence often retains signatures of the underlying linear eigenmodes in a strongly turbulent state, as found both in theory and simulation [1][2][3][4][5][6] as well as observations [7][8][9][10][11]. When this is the case, one can predict important features of the turbulence using only the much more-accessible linear information (a notable example is the various quasilinear techniques that have been quite successful in modeling turbulent transport [12][13][14][15][16]).…”

Section: Introductionmentioning

confidence: 86%

Linear signatures in nonlinear gyrokinetics: interpreting turbulence with pseudospectra

et al. 2016

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A notable feature of plasma turbulence is its propensity to retain features of the underlying linear eigenmodes in a strongly turbulent state-a property that can be exploited to predict various aspects of the turbulence using only linear information. In this context, this work examines gradient-driven gyrokinetic plasma turbulence through three lenses-linear eigenvalue spectra, pseudospectra, and singular value decomposition (SVD). We study a reduced gyrokinetic model whose linear eigenvalue spectra include ion temperature gradient driven modes, stable drift waves, and kinetic modes representing Landau damping. The goal is to characterize in which ways, if any, these familiar ingredients are manifest in the nonlinear turbulent state. This pursuit is aided by the use of pseudospectra, which provide a more nuanced view of the linear operator by characterizing its response to perturbations. We introduce a new technique whereby the nonlinearly evolved phase space structures extracted with SVD are linked to the linear operator using concepts motivated by pseudospectra. Using this technique, we identify nonlinear structures that have connections to not only the most unstable eigenmode but also subdominant modes that are nonlinearly excited. The general picture that emerges is a system in which signatures of the linear physics persist in the turbulence, albeit in ways that cannot be fully explained by the linear eigenvalue approach; a nonmodal treatment is necessary to understand key features of the turbulence.

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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: 92%

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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Gyrokinetic study of ASDEX Upgrade inter-ELM pedestal profile evolution

Cited by 58 publications

References 71 publications

Direct gyrokinetic comparison of pedestal transport in JET with carbon and ITER-like walls

Direct gyrokinetic comparison of pedestal transport in JET with carbon and ITER-like walls

Linear signatures in nonlinear gyrokinetics: interpreting turbulence with pseudospectra

Microtearing turbulence limiting the JET-ILW pedestal

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