Key impact of finite-beta and fast ions in core and edge tokamak regions for the transition to advanced scenarios

García, J.; Challis, C.; Citrin, J.; Doerk, H.; Giruzzi, G.; Görler, T.; Jenko, F.; Maget, P.; Contributors, Jet

doi:10.1088/0029-5515/55/5/053007

Cited by 89 publications

(150 citation statements)

References 28 publications

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“…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 underlying micro-instability.…”

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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“…Some of the key physics that explain such deviation are the increased impact with power of several core plasma turbulence stabilizing mechanisms, such as the stabilization by electromagnetic effects and fast ions pressure gradients or E × B flow shear [8,9]. Additionally, the increased beta tends to stabilize edge ballooning modes and expand the peeling ballooning stability boundary leading to an increase of the pedestal pressure and the onset of a coreedge feedback loop through plasma stiffness.…”

Section: Extrapolation Methodologymentioning

confidence: 99%

“…However, in strong electromagnetic turbulence, i.e. at high electron beta, the effect of E × B flow shear could be overestimated by quasilinear models and actually the electro magnetic stabilization be responsible for most of the reduced turbulence [9]. Recent gyrokinetic simulations for the ITER hybrid scenario show that a strong DT isotope effect can be obtained by the concomitant impact of E × B flow shear, electro magnetic effects and zonal flows [23] leading to oth erwise heat fluxes reductions similar to the one obtained with TGLF for which just the impact of E × B flow shear is the main cause of a strong isotope effect.…”

Section: Isotope and Alpha Heating Effectsmentioning

confidence: 99%

Challenges in the extrapolation from DD to DT plasmas: experimental analysis and theory based predictions for JET-DT

García

Challis

Gallart

et al. 2016

Plasma Phys. Control. Fusion

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A strong modelling program has been started in support of the future JETDT campaign with the aim of guiding experiments in deuterium (D) towards maximizing fusion energy production in Deuterium-Tritium (DT). Some of the key elements have been identified by using several of the most updated and sophisticated models for predicting heat and particle transport, pedestal pressure and heating sources in an integrated modelling framework. For the high beta and low gas operational regime, the density plays a critical role and a trend towards higher fusion power is obtained at lower densities. Additionally, turbulence stabilization by E × B flow shear is shown to generate an isotope effect leading to higher confinement for DT than DD and therefore plasmas with high torque are suitable for maximizing fusion performance. Future JET campaigns will benefit from this modelling activity by defining clear priorities on their scientific program.

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“…For more details on the stabilizing role of the fast ion pressure gradient, the reader is directed to Refs. [22,24,41]. Henceforth, in order to to isolate the effect of fast ions on the local ITG mode, the equilibrium plasma parameters, including the safety factor, magnetic shear, and flux surface shape, will remain fixed.…”

Section: Characterizing the Effect Of Fast Ions On The Linear Itg Modementioning

confidence: 99%

First principles of modelling the stabilization of microturbulence by fast ions

et al. 2018

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The observation that fast ions stabilize ion-temperature-gradient-driven microturbulence has profound implications for future fusion reactors. It is also important in optimizing the performance of present-day devices. In this work, we examine in detail the phenomenology of fast ion stabilization and present a reduced model which describes this effect. This model is derived from the high-energy limit of the gyrokinetic equation and extends the existing "dilution" model to account for nontrivial fast ion kinetics. Our model provides a physically-transparent explanation for the observed stabilization and makes several key qualitative predictions. Firstly, that different classes of fast ions, depending on their radial density or temperature variation, have different stabilizing properties. Secondly, that zonal flows are an important ingredient in this effect precisely because the fast ion zonal response is negligible. Finally, that in the limit of highly-energetic fast ions, their response approaches that of the "dilution" model; in particular, alpha particles are expected to have little, if any, stabilizing effect on plasma turbulence. We support these conclusions through detailed linear and nonlinear gyrokinetic simulations. arXiv:1801.00664v3 [physics.plasm-ph] 5 Jun 2018Effect of fast ions on microturbulence 2 Fast ion stabilization of ITG microturbulenceMicroturbulence fundamentally limits the confinement time in current and future tokamak experiments [2]. The very gradients that are required to achieve high central densities and temperatures also provide a source of free energy. This free energy drives transport of particles, momentum, and energy, usually far in excess of collisional transport. The ion temperature gradient (ITG) mode, for instance, limits the core temperature of tokamaks [3,4,5]. This turbulence occurs on the scale of the thermal ion Larmor radius ρ i .Gyrokinetics is the reduction of the Fokker-Planck kinetic equation that rigorously handles electromagnetic fields whose fluctuations vary on spatial scales similar to ρ i , but on timescales much slower than the gyro-frequency Ω i [6,7,8]. A multitude of computational tools have been developed to solve the nonlinear gyrokinetic equation [9,10,11,12]. These tools take as inputs the equilibrium magnetic field, density, and temperature profiles, and predict the ensuing turbulent fluctuations and the associated transport fluxes. By using experimentally-determined profiles and comparing the computed fluxes to experimentally-inferred fluxes (usually from power-balance calculations), and through more detailed comparisons of turbulence characteristics, the validity of the gyrokinetic approach can be verified [13]. This has been widely demonstrated [14,15,16,17].However, due to the scarcity of computational resources, these matching exercises of necessity entail the use of simulations that neglect various parts of the complex physics of the experimental setup. It was discovered during one of these exercises, in which experiments on the Joint European Tor...

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Key impact of finite-beta and fast ions in core and edge tokamak regions for the transition to advanced scenarios

Cited by 89 publications

References 28 publications

Electromagnetic turbulence suppression by energetic particle driven modes

Electromagnetic turbulence suppression by energetic particle driven modes

Challenges in the extrapolation from DD to DT plasmas: experimental analysis and theory based predictions for JET-DT

First principles of modelling the stabilization of microturbulence by fast ions

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