Suppression of Ion-Scale Microtearing Modes by Electron-Scale Turbulence via Cross-Scale Nonlinear Interactions in Tokamak Plasmas

Maeyama, Shinya; Watanabe, T.‐H.; Ishizawa, A.

doi:10.1103/physrevlett.119.195002

Cited by 37 publications

(55 citation statements)

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“…Nonlinear energy-transfer analysis. In order to understand the phenomenology described above, the energy transfers shall be studied more closely, e.g., by monitoring the nonlinear mode-to-mode coupling term in the free energy balance equation [19][20][21] In the gyrokinetic formalism, the free energy is a nonlinearly conserved quantity, i.e. N k vanishes when summed over all the wave vector components.…”

Section: Introductionmentioning

confidence: 99%

Electromagnetic turbulence suppression by energetic particle driven modes

et al. 2019

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In recent years, a strong reduction of plasma turbulence in the presence of energetic particles has been reported in a number of magnetic confinement experiments and corresponding gyrokinetic simulations. While highly relevant to performance predictions for burning plasmas, an explanation for this primarily nonlinear effect has remained elusive so far. A thorough analysis finds that linearly marginally stable energetic particle driven modes are excited nonlinearly, depleting the energy content of the turbulence and acting as an additional catalyst for energy transfer to zonal modes (the dominant turbulence saturation channel). Respective signatures are found in a number of simulations for different JET and ASDEX Upgrade discharges with reduced transport levels attributed to energetic ion effects.PACS numbers: 52.65.Tt Introduction.Being an almost ubiquitous phenomenon, turbulence with its highly stochastic and nonlinear character is a subject of active research in various fields. In magnetically confined plasma physics, it is of particular interest since it largely determines the radial heat and particle transport and thus the overall confinement. Any insight on possible reductions of the underlying micro-instabilities which are driven by the steep density and temperature profiles, and/or on modifications of their nonlinear saturation mechanisms can be considered crucial on the way to self-sustained plasma burning and corresponding fusion power plants. A particularly interesting example is the recent experimental and numerical evidence suggesting a link between the presence of fast ions and substantial improvement of energy confinement in predominantly ITG (ion-temperature-gradient) driven turbulence [1][2][3][4][5]. Dedicated theoretical studies have already identified a number of possible energetic ion effects on plasma turbulence like dilution of the main ion species [1], Shafranov shift stabilization [6] and resonance interaction with bulk species micro-instabilities in certain plasma regimes [7]. They furthermore contribute to the total plasma pressure and increase the kinetic-tomagnetic pressure ratio, β, which is a measure for the relevance of electromagnetic fluctuations, known to stabilize ITG modes. Such behaviour could indeed be confirmed in simulations [4,8,9] of JET hybrid discharges [10,11] with substantial fast ion effects that, however, also identified an upper limit for this beneficial fast-ion-pressure effect. If the total plasma pressure exceeds a critical value, kinetic ballooning or Alfvénic ITG modes with smaller toroidal mode numbers and frequencies higher than the ITG modes are destabilized which increase particle/heat fluxes [12]. Similarly, the fast-ion pressure may drive energetic particle (EP) modes if certain thresholds are exceeded. Although a possible relevance of the proximity to the onset of these modes has been noted [4,8], their role was not investigated in more detail. In any case, all of these effects are mainly linear, i.e., alter the growth of the

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Section: Introductionmentioning

confidence: 99%

Electromagnetic turbulence suppression by energetic particle driven modes

et al. 2019

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“…The multiscale DNS provide evidence showing that the presence of short-wavelength turbulence can sometimes modestly enhance long-wavelength, ITG-driven fluxes (Maeyama et al 2015(Maeyama et al , 2017aHoward et al 2016a,b). In contrast, multiscale DNS of microtearing mode (MTM) turbulence show that the presence of short-wavelength ETG modes can suppress the long-wavelength MTM (Maeyama et al 2017b). Most relevant to the results presented in this paper, the multiscale DNS give clear evidence that shows that short-wavelength turbulence can be suppressed in the presence of long-wavelength turbulence as a result of cross-scale interaction (Maeyama et al 2015(Maeyama et al , 2017aHoward et al 2016b).…”

Section: Introductionmentioning

confidence: 65%

“…In contrast, multiscale DNS of microtearing mode (MTM) turbulence show that the presence of short-wavelength ETG modes can suppress the long-wavelength MTM (Maeyama et al. 2017 b ). Most relevant to the results presented in this paper, the multiscale DNS give clear evidence that shows that short-wavelength turbulence can be suppressed in the presence of long-wavelength turbulence as a result of cross-scale interaction (Maeyama et al.…”

Section: Introductionmentioning

confidence: 99%

“…This possibility has sparked considerable interest in multiscale turbulence, see, e.g., Maeyama et al. (2015, 2017 a ); Maeyama, Watanabe & Ishizawa (2017 b ), Howard et al. (2014, 2015, 2016 a , b ), Bonanomi et al.…”

Section: Introductionmentioning

confidence: 99%

“…For the deuterium ions commonly used in magnetic confinement fusion, the small value of (m e /m i ) 1/2 ≈ 1/60 allows for the possibility that two distinct types of turbulence coexist at disparate space-time scales. This possibility has sparked considerable interest in multiscale turbulence, see, e.g., Maeyama et al (2015Maeyama et al ( , 2017a; Maeyama, Watanabe & Ishizawa (2017b), Howard et al (2014Howard et al ( , 2015Howard et al ( , 2016a, Bonanomi et al (2018), Görler & Jenko (2008), Candy et al (2007), , Staebler et al (2016Staebler et al ( , 2017, Creely et al (2019) and Itoh & Itoh (2001).…”

Section: Introductionmentioning

confidence: 99%

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Stabilisation of short-wavelength instabilities by parallel-to-the-field shear in long-wavelength E × B flows

2020

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Magnetised plasma turbulence can have a multiscale character: instabilities driven by mean temperature gradients drive turbulence at the disparate scales of the ion and the electron gyroradii. Simulations of multiscale turbulence, using equations valid in the limit of infinite scale separation, reveal novel cross-scale interaction mechanisms in these plasmas. In the case that both long-wavelength (ion-gyroradius-scale) and short-wavelength (electron-gyroradius-scale) linear instabilities are driven far from marginal stability, we show that the short-wavelength instabilities are suppressed by interactions with long-wavelength turbulence. Two novel effects contributed to the suppression: parallel-to-the-field-line shearing by the long-wavelength ${{\boldsymbol {E}} \times \boldsymbol {B}}$ flows, and the modification of the background density gradient by the piece of the long-wavelength electron adiabatic response with parallel-to-the-field-line variation. In contrast, simulations of multiscale turbulence where instabilities at both scales are driven near marginal stability demonstrate that when the long-wavelength turbulence is sufficiently collisional and zonally dominated the effect of cross-scale interaction can be parameterised solely in terms of the local modifications to the mean density and temperature gradients. We discuss physical arguments that qualitatively explain how a change in equilibrium drive leads to the observed transition in the impact of the cross-scale interactions.

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Multimodal convolutional neural networks for predicting evolution of gyrokinetic simulations

Honda

Narita

Maeyama

et al. 2023

Contributions to Plasma Physics

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Gyrokinetic simulations are required for the quantitative calculation of fluxes due to turbulence, which dominates over other transport mechanisms in tokamaks. However, nonlinear gyrokinetic simulations are computationally expensive. A multimodal convolutional neural network model that reads images and values generated by nonlinear gyrokinetic simulations and predicts electrostatic turbulent heat fluxes was developed to support efficient runs. The model was extended to account for squared electrostatic potential fluctuations, which are proportional to the fluxes in the quasilinear model, as well as images containing fluctuating electron and ion distribution functions and fluctuating electrostatic potentials in wavenumber space. This multimodal model can predict the time and electron and ion turbulent heat fluxes corresponding to the input data. The model trained on the Cyclone base case data successfully predicted times and fluxes not only for its test data, but also for the completely different and unknown JT‐60U case, with high accuracy. The predictive performance of the model depended on the similarity of the linear stability of the case used to train the model to the case being predicted.

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Suppression of Ion-Scale Microtearing Modes by Electron-Scale Turbulence via Cross-Scale Nonlinear Interactions in Tokamak Plasmas

Cited by 37 publications

References 25 publications

Electromagnetic turbulence suppression by energetic particle driven modes

Electromagnetic turbulence suppression by energetic particle driven modes

Stabilisation of short-wavelength instabilities by parallel-to-the-field shear in long-wavelength E × B flows

Multimodal convolutional neural networks for predicting evolution of gyrokinetic simulations

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