Extreme Heat Fluxes in Gyrokinetic Simulations: A New Critical<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>β</mml:mi></mml:math>

Pueschel, M. J.; Terry, P. W.; Jenko, F.; Hatch, D. R.; Nevins, W. M.; Görler, T.; Told, D.

doi:10.1103/physrevlett.110.155005

Cited by 49 publications

(87 citation statements)

References 19 publications

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“…When attempting to heat an experiment beyond such thresholds, a large increase in the heat and particle transport results. One potential such limit is the non-zonal transition (NZT) [1]; while other pressure limits are associated with linear instabilities -of either fluid or kinetic nature -being excited at their respective thresholds, the NZT is a more complex, fundamentally nonlinear phenomenon. In addition to providing an encompassing explanation for the long-standing question why gyrokinetic codes see a sudden onset of extreme transport values well below the ballooning threshold for certain parameters, the concept of the NZT may also be applicable to actual fusion experiments.…”

Section: Introductionmentioning

confidence: 99%

Properties of high-β microturbulence and the non-zonal transition

et al. 2013

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Section: Introductionmentioning

confidence: 99%

Properties of high-β microturbulence and the non-zonal transition

et al. 2013

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“…Some possible explanations of the run-away are proposed by Waltz (2010) and Pueschel et al (2013b). One is a sub-critical instability due to the production of a zonal pressure field.…”

Section: Saturation Problem Of Turbulence At Finite Betamentioning

confidence: 99%

“…10(a) the level of the energy transport for ions and electrons χ i and χ e are not at a physically relevant level for β i = 0.8% and 1%. The saturation problem is called 'run-away' in Waltz (2010) and is called 'non-zonal transition' in Pueschel et al (2013b). The heat diffusion coefficient χ i decreases with plasma beta because of the reduction of the linear growth rate of the ITG mode.…”

Section: Saturation Problem Of Turbulence At Finite Betamentioning

confidence: 99%

“…This implies that the large magnetic perturbation may play a role in the run-away. The stochastic field may prevent the formation of zonal flows (Pueschel et al 2013b;Terry et al 2013). …”

Section: Saturation Problem Of Turbulence At Finite Betamentioning

confidence: 99%

“…This is in contrast with the ITG driven turbulence regulated by zonal flows in low beta torus plasmas. The difficulties are related to the weak zonal flow production, and thus the production process of zonal structures is investigated (Waltz 2010;Pueschel et al 2013b;Terry et al 2013Terry et al , 2014. The identification of the saturation mechanism of microturbulence in regimes where zonal flow generation is weak at finite beta is one of the open issues.…”

Section: Introductionmentioning

confidence: 99%

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Electromagnetic gyrokinetic simulation of turbulence in torus plasmas

et al. 2015

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Gyrokinetic simulations of electromagnetic turbulence in magnetically confined torus plasmas including tokamak and heliotron/stellarator are reviewed. Numerical simulation of turbulence in finite beta plasmas is an important task for predicting the performance of fusion reactors and a great challenge in computational science due to multiple spatio-temporal scales related to electromagnetic ion and electron dynamics. The simulation becomes further challenging in non-axisymmetric plasmas. In finite beta plasmas, magnetic perturbation appears and influences some key mechanisms of turbulent transport, which include linear instability and zonal flow production. Linear analysis shows that the ion-temperature gradient (ITG) instability, which is essentially an electrostatic instability, is unstable at low beta and its growth rate is reduced by magnetic field line bending at finite beta. On the other hand, the kinetic ballooning mode (KBM), which is an electromagnetic instability, is destabilized at high beta. In addition, trapped electron modes (TEMs), electron temperature gradient (ETG) modes, and micro-tearing modes (MTMs) can be destabilized. These instabilities are classified into two categories: ballooning parity and tearing parity modes. These parities are mixed by nonlinear interactions, so that, for instance, the ITG mode excites tearing parity modes. In the nonlinear evolution, the zonal flow shear acts to regulate the ITG driven turbulence at low beta. On the other hand, at finite beta, interplay between the turbulence and zonal flows becomes complicated because the production of zonal flow is influenced by the finite beta effects. When the zonal flows are too weak, turbulence continues to grow beyond a physically relevant level of saturation in finite-beta tokamaks. Nonlinear mode coupling to stable modes can play a role in the saturation of finite beta ITG mode and KBM. Since there is a quadratic conserved quantity, evaluating nonlinear transfer of the conserved quantity from unstable modes to stable modes is useful for understanding the saturation mechanism of turbulence.

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Electromagnetic instabilities and plasma turbulence driven by electron-temperature gradient

et al. 2022

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Electromagnetic (EM) instabilities and turbulence driven by the electron-temperature gradient (ETG) are considered in a local slab model of a tokamak-like plasma. Derived in a low-beta asymptotic limit of gyrokinetics, the model describes perturbations at scales both larger and smaller than the electron inertial length $d_e$ , but below the ion Larmor scale $\rho _i$ , capturing both electrostatic and EM regimes of turbulence. The well-known electrostatic instabilities – slab and curvature-mediated ETG – are recovered, and a new instability is found in the EM regime, called the thermo-Alfvénic instability (TAI). It exists in both a slab version (sTAI, destabilising kinetic Alfvén waves) and a curvature-mediated version (cTAI), which is a cousin of the (electron-scale) kinetic ballooning mode. The cTAI turns out to be dominant at the largest scales covered by the model (greater than $d_e$ but smaller than $\rho _i$ ), its physical mechanism hinging on the fast equalisation of the total temperature along perturbed magnetic field lines (in contrast to kinetic ballooning mode, which is pressure balanced). A turbulent cascade theory is then constructed, with two energy-injection scales: $d_e$ , where the drivers are slab ETG and sTAI, and a larger (parallel system size dependent) scale, where the driver is cTAI. The latter dominates the turbulent transport if the temperature gradient is greater than a certain critical value, which scales inversely with the electron beta. The resulting heat flux scales more steeply with the temperature gradient than that due to electrostatic ETG turbulence, giving rise to stiffer transport. This can be viewed as a physical argument in favour of near-marginal steady-state in electron-transport-controlled plasmas (e.g. the pedestal). While the model is simplistic, the new physics that is revealed by it should be of interest to those attempting to model the effect of EM turbulence in tokamak-relevant configurations with high beta and large ETGs.

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Extreme Heat Fluxes in Gyrokinetic Simulations: A New Criticalβ

Cited by 49 publications

References 19 publications

Properties of high-β microturbulence and the non-zonal transition

Properties of high-β microturbulence and the non-zonal transition

Electromagnetic gyrokinetic simulation of turbulence in torus plasmas

Electromagnetic instabilities and plasma turbulence driven by electron-temperature gradient

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