Global anomalous transport of ICRH- and NBI-heated fast ions

Wilkie, George; Pusztai, István; Abel, Ian; Dorland, W.; Fülöp, Tünde

doi:10.1088/1361-6587/aa5902

Cited by 8 publications

(17 citation statements)

References 18 publications

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“…Therefore, the distribution exhibits a ‘bump-on-tail’ similar to what was previously observed (Wilkie et al. 2016, 2017). In order for this inversion to be observable, this maximum must be well separated from the helium ash population.…”

Section: Benchmarking and Discussionsupporting

confidence: 85%

“…Previous studies (Wilkie et al 2017) found that the D rv , D vr and D vv coefficients play a sub-dominant role compared to radial diffusion (D rr ) at high energy, so this will be the † Here, nα refers to the density of helium that is not Maxwellian ash so that the total helium density is nHe = n ash + nα only transport term kept from Eq. (1.1).…”

Section: Model Distribution Functionmentioning

confidence: 99%

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Analytic slowing-down distributions as modified by turbulent transport

Wilkie

2018

J. Plasma Phys.

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The effect of electrostatic microturbulence on fast particles rapidly decreases at high energy, but can be significant at moderate energy. Previous studies found that, in addition to changes in the energetic particle density, this results in nontrivial changes to the equilibrium velocity distribution. These effects have implications for plasma heating and the stability of Alfvén eigenmodes, but make multiscale simulations much more difficult without further approximations. Here, several related analytic model distribution functions are derived from first principles with reasonable approximations. A single dimensionless parameter characterizes the relative strength of turbulence relative to collisions, and this parameter appears as an exponent in the model distribution functions. Even the most simple of these models reproduces key features of the numerical phase-space transport solution and provides a useful a priori heuristic for determining how strong the effect of turbulence is on the redistribution of energetic particles in toroidal plasmas.

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Section: Benchmarking and Discussionsupporting

confidence: 85%

Section: Model Distribution Functionmentioning

confidence: 99%

Analytic slowing-down distributions as modified by turbulent transport

Wilkie

2018

J. Plasma Phys.

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“…The PHS is unfolded into a NES, which contains information on the fast ion distribution. The deconvolution is performed using two codes, both based on the maximum entropy ansatz: the MAXED code [23] and the Deconvolution method with Adaptive Kernel (DAK) [24]. In Fig.…”

Section: Energetic Tails In Pulse Height Spectra and Neutron Emissionmentioning

confidence: 99%

Detection of neutral beam deuterons acceleration by 2nd harmonic radio frequency heating in ASDEX Upgrade via neutron spectrometry

et al. 2019

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Second harmonic absorption of radio frequency in the Ion Cyclotron Range of Frequency (ICRF) by fast D is expected in the case of H minority heating at the fundamental cyclotron frequency, provided the ion Larmor radius is of the order of the RF-wavelength, as it is for Neutral Beam Injection (NBI)-D ions. In tokamaks a strong increase of neutron production is observed when such an ICRF heating is applied on top of NBI, often referred to as "synergy effect".On ASDEX Upgrade a scintillator-based Compact Neutron Spectrometer (CNS) is available outside the torus hall, with perpendicular view through the plasma equatorial plane, measuring Pulse Height Spectra (PHS), which allow to infer the ion distribution functions, in particular the suprathermal tail.The measured PHS exhibit clear energetic tails when ICRF is applied in the presence of NBI, proving there's an overproportional tail in the fast-ion distribution at the high energy end. The PHS in the CNS line-of-sight are modelled coupling kinetic ‡ Corresponding author: git@ipp.mpg.de Detection of neutral beam deuterons acceleration by 2nd harmonic radio frequency heating in ASDEX Upg solvers with wave codes. Unfolding the PHS into neutron emission spectra allows to quantify the maximum fast ion energy with significant occurrence, showing a nonnegligible population around 500 keV, significantly higher than the NBI energy. A sensitivity study assesses the maximum fast ion energy compatible with the measured PHS, confirming the presence of 500 keV D ions.

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“…The different types of fast ions are characterized by whether their strong radial variation is one of particle density ("NBI-like") or energy density ("ICRH-like"). It has previously been shown that these different classes of fast ions respond differently to turbulence when passive [54,27]. It is worth examining to what extent their effect on turbulence differs compared to their respective dilution effects.…”

Section: Differentiating Classes Of Fast Ionsmentioning

confidence: 99%

“…This is a generalization of the dilution model presented in Ref. [27], which did not include the kinetic effect of fast ions approximated by R 0f . It is also analogous to the τ parametrization used in Ref.…”

Section: Effective Parameter Modelmentioning

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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Global anomalous transport of ICRH- and NBI-heated fast ions

Cited by 8 publications

References 18 publications

Analytic slowing-down distributions as modified by turbulent transport

Analytic slowing-down distributions as modified by turbulent transport

Detection of neutral beam deuterons acceleration by 2nd harmonic radio frequency heating in ASDEX Upgrade via neutron spectrometry

First principles of modelling the stabilization of microturbulence by fast ions

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