Gyrokinetic prediction of microtearing turbulence in standard tokamaks

Doerk, H.; Jenko, F.; Görler, T.; Told, D.; Pueschel, M. J.; Hatch, D. R.

doi:10.1063/1.3694663

Cited by 64 publications

(109 citation statements)

References 41 publications

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“…Gene was used primarily in a local mode of operation, employing a real space radial coordinate with Dirichlet boundary conditions, while keeping plasma parameters and gradients constant across the box. This approach was benchmarked against global sim- ulations where possible; a nonlinear global simulation with k y,min = 0.05 produced transport levels differing by only ∼ 10% from the corresponding local simulation, consistent with earlier work suggesting MTM is not strongly affected by global effects [41]. However, nonlinear global simulations could not be extended to lower k y due to numerical issues (likely resolvable with higher resolution simulations that are beyond the scope of this work).…”

Section: Pacs Numbersmentioning

confidence: 89%

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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Section: Pacs Numbersmentioning

confidence: 89%

Microtearing turbulence limiting the JET-ILW pedestal

et al. 2016

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“…However, it has been shown that MTMs [29][30][31] and even the ETG driven electron-gyroradius-scale turbulence 22,32,33 might cause substantial electron heat fluxes which has to be kept in mind if simulation results neglecting these scales and modes are compared with the experiment.…”

Section: Fig 1 Linear Growth Rate (A) And-for Readability-negative mentioning

confidence: 99%

A flux-matched gyrokinetic analysis of DIII-D L-mode turbulence

et al. 2014

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Previous nonlinear gyrokinetic simulations of specific DIII-D L-mode cases have been found to significantly underpredict the ion heat transport and associated density and temperature fluctuation levels by up to almost one of order of magnitude in the outer-core domain, i.e., roughly in the last third of the minor radius. Since then, this so-called shortfall issue has been subject to various speculations on possible reasons and furthermore motivation for a number of dedicated comparisons for L-mode plasmas in comparable machines. However, only a rather limited number of simulations and gyrokinetic codes has been applied to the original scenario, thus calling for further dedicated investigations in order to broaden the scientific basis. The present work contributes along these lines by employing another well-established gyrokinetic code in a numerically and physically comprehensive manner. Contrary to the previous studies, only a mild underprediction is observed at the outer radial positions which can furthermore be overcome by varying the ion temperature gradient within the error bars associated with the experimental measurement. The significance and reliability of these simulations are demonstrated by benchmarks, numerical convergence tests, and furthermore by extensive validation studies. The latter involve cross-phase and cross-power spectra analyses of various fluctuating quantities and confirm a high degree of realism. The code discrepancies come as a surprise since the involved software packages had been benchmarked repeatedly and very successfully in the past. Further collaborative effort in identifying the underlying difference is hence required.

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“…The peak of the growth rate coincides with the experimental value of ν ei for various considered radial positions. The non-monotonic dependence of the MTM growth rate on collisionality is due to the fact that, on one hand both of the driving mechanisms mentioned above require finite collisionality, and therefore stability can be expected as ν ei reduces, but on the other hand, in a strongly collisional regime the strong rate of scattering of the electrons between the field lines prevents the formation of a current layer, hence MTMs are stabilized [14].…”

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confidence: 99%

“…Moreover, it has been shown that the Chirikov criterion [15] for overlapping of the magnetic islands leads to an up-shift of the electron temperature gradient threshold [5,6,13,14].…”

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Microtearing modes in spherical and conventional tokamaks

et al. 2013

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The onset and characteristics of Micro-Tearing Modes (MTM) in the core of spherical (NSTX) and conventional tokamaks (ASDEX-UG and JET) are studied through local linear gyrokinetic simulations with gyro [J. Candy and E. Belli, General Atomics Report GA-A26818 (2011)]. For experimentally relevant core plasma parameters in the NSTX and ASDEX-UG tokamaks, in agreement with previous works, we find MTMs as the dominant linear instability. Also, for JET-like core parameters considered in our study an MTM is found as the most unstable mode. In all these plasmas, finite collisionality is needed for MTMs to become unstable and the electron temperature gradient is found to be the fundamental drive. However, a significant difference is observed in the dependence of linear growth rate of MTMs on electron temperature gradient. While it varies weakly and non-monotonically in JET and ASDEX-UG plasmas, in NSTX it increases with the electron temperature gradient.

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Gyrokinetic prediction of microtearing turbulence in standard tokamaks

Cited by 64 publications

References 41 publications

Microtearing turbulence limiting the JET-ILW pedestal

Microtearing turbulence limiting the JET-ILW pedestal

A flux-matched gyrokinetic analysis of DIII-D L-mode turbulence

Microtearing modes in spherical and conventional tokamaks

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