Letteratura scientifica selezionata sul tema "Gyrofluid models"

The verification of a new saturation rule applied to the quasi-linear fluid model EDWM (extended drift wave model) and the calibration of several other features are presented. As one of the computationally fastest first-principle-based core transport models, EDWM can include an arbitrary number of ions and charge states. This feature is especially important for experimental devices with plasma-facing components made of heavy elements, such as the upcoming ITER device. As a quasi-linear model, EDWM solves a linear dispersion relation to obtain the instabilities driving the turbulence and combines the linear description with an estimation of the saturation level of the electrostatic potential to determine the fluxes. A new saturation rule at the characteristic length combined with a spectral filter for the poloidal wavenumber dependency is developed. The shape of the filter has been fitted against the poloidal wavenumber dependency of the electrostatic potential from non-linear gyrokinetic simulations. Additionally, EDWM's collision frequency and safety factor dependencies, as well as the electron heat flux level, have been calibrated against gyrokinetic and gyrofluid results. Finally, the saturation level has been normalized against non-linear gyrokinetic simulations and later validated against experimental measured fluxes from 12 discharges at JET.

Reduced models are derived for a strongly magnetized collisionless plasma at scales which are large relative to the electron thermal gyroradius and in two asymptotic regimes. One corresponds to cold ions and the other to far sub-ion scales. By including the electron pressure dynamics, these models improve the Hall reduced magnetohydrodynamics (MHD) and the kinetic Alfvén wave model of Boldyrev et al. (2013 Astrophys. J., vol. 777, 2013, p. 41), respectively. We show that the two models can be obtained either within the gyrofluid formalism of Brizard (Phys. Fluids, vol. 4, 1992, pp. 1213–1228) or as suitable weakly nonlinear limits of the finite Larmor radius (FLR)–Landau fluid model of Sulem and Passot (J. Plasma Phys., vol 81, 2015, 325810103) which extends anisotropic Hall MHD by retaining low-frequency kinetic effects. It is noticeable that, at the far sub-ion scales, the simplifications originating from the gyroaveraging operators in the gyrofluid formalism and leading to subdominant ion velocity and temperature fluctuations, correspond, at the level of the FLR–Landau fluid, to cancellation between hydrodynamic contributions and ion finite Larmor radius corrections. Energy conservation properties of the models are discussed and an explicit example of a closure relation leading to a model with a Hamiltonian structure is provided.

An electromagnetic reduced gyrofluid model for collisionless plasmas, accounting for electron inertia, finite ion Larmor radius effects and Landau-fluid closures for the electron fluid is derived by means of an asymptotic expansion from a parent gyrofluid model. In the absence of terms accounting for Landau damping, the model is shown to possess a non-canonical Hamiltonian structure. The corresponding Casimir invariants are derived and use is made thereof, in order to obtain a set of normal field variables, in terms of which the Poisson bracket and the model equations take a remarkably simple form. The inclusion of perpendicular temperature fluctuations generalizes previous Hamiltonian reduced fluid models and, in particular, the presence of ion perpendicular gyrofluid temperature fluctuations reflects into the presence of two new Lagrangian invariants governing the ion dynamics. The model is applied, in the cold-ion limit, to investigate numerically a magnetic reconnection problem. The Landau damping terms are shown to reduce, by decreasing the electron temperature fluctuations, the linear reconnection rate and to delay the nonlinear island growth. The saturated island width, on the other hand, is independent of Landau damping. The fraction of magnetic energy converted into perpendicular kinetic energy also appears to be unaffected by the Landau damping terms, which, on the other hand, dissipate parallel kinetic energy as well as free energy due to density and electron temperature fluctuations.

A two-field Hamiltonian gyrofluid model for kinetic Alfvén waves retaining ion finite Larmor radius corrections, parallel magnetic field fluctuations and electron inertia, is used to study turbulent cascades from the magnetohydrodynamic (MHD) to the sub-ion scales. Special attention is paid to the case of imbalance between waves propagating along or opposite to the ambient magnetic field. For weak turbulence in the absence of electron inertia, kinetic equations for the spectral density of the conserved quantities (total energy and generalized cross-helicity) are obtained. They provide a global description, matching between the regimes of reduced MHD at large scales and electron reduced MHD at small scales, previously considered in the literature. In the limit of ultra-local interactions, Leith-type nonlinear diffusion equations in the Fourier space are derived and heuristically extended to the strong turbulence regime by modifying the transfer time appropriately. Relations with existing phenomenological models for imbalanced MHD and balanced sub-ion turbulence are discussed. It turns out that in the presence of dispersive effects, the dynamics is sensitive on the way turbulence is maintained in a steady state. Furthermore, the total energy spectrum at sub-ion scales becomes steeper as the generalized cross-helicity flux is increased.

Abstract Tokamak operation at negative triangularity has been shown to offer high energy confinement without the typical disadvantages of edge pedestals [Marinoni et al 2021 Nucl. Fusion 61 116010]. In this paper, we examine impurity transport in DIII-D diverted negative triangularity experiments. Analysis of charge exchange recombination spectroscopy reveals flat or hollow carbon density profiles in the core, and impurity confinement times consistently shorter than energy confinement times. Bayesian inferences of impurity transport coefficients based on laser blow-off injections and forward modeling via the Aurora package [Sciortino et al 2021 Plasma Phys. Control. Fusion 63 112001] show core cross-field diffusion to be higher in L-mode than in H-mode. Impurity profile shapes remain flat or hollow in all cases. Inferred radial profiles of diffusion and convection are compared to neoclassical, quasilinear gyrofluid, and nonlinear gyrokinetic simulations. Heat transport is observed to be better captured by reduced turbulence models with respect to particle transport. State-of-the-art gyrokinetic modeling compares favorably with measurements across multiple transport channels. Overall, these results suggest that diverted negative triangularity discharges may offer a path to a highly-radiative L-mode scenario with high core performance.

La reconnexion magnétique est une modification de la topologie du champ magnétique, responsable de la libération explosive d'énergie magnétique dans les plasmas astrophysiques, comme dans le cas des orages magnétosphériques et des éjections de masse coronale, ainsi que dans les plasmas de laboratoire, comme dans le cas des crashs en dents de scie dans les tokamaks. Dans les plasmas sans collisions comme, par exemple, la magnétosphère et le vent solaire, l'inertie des électrons devient particulièrement pertinente pour provoquer la reconnexion dans les régions de courant localisé intense, appelées feuilles de courant. Dans ces environnements non collisionnels, la température peut souvent être anisotrope et les effets à l'échelle électronique sur le processus de reconnexion peuvent devenir non négligeables.Dans cette thèse, la stabilité des feuilles de courant bidimensionnelles dans des plasmas sans collisions avec un fort champ guide est analysée sur la base de modèles gyrofluides assumant des ions froids. Ces modèles peuvent prendre en compte une anisotropie de température d'équilibre, et un βe fini. Ce dernier est un paramètre correspondant au rapport entre la pression cinétique électronique d'équilibre et la pression magnétique.Nous dérivons et analysons une relation de dispersion pour le taux de croissance des modes tearing sans collisions tenant compte de l'anisotropie de la température d'équilibre des électrons. Les prédictions analytiques sont testées par des simulations numériques, montrant un très bon accord quantitatif.Dans le cas isotrope, en tenant compte des effets βe finis, nous observons une stabilisation du taux de croissance du mode tearing lorsque les effets du rayon de Larmor fini des électrons deviennent pertinents. Dans la phase non linéaire, des phases de ralentissement et des phases d'accélération sont observées, de manière similaire à ce qui se produit en présence d'effets de rayon de Larmor fini ionique.Nous étudions également les conditions de stabilité marginale des feuilles de courant secondaires, pour le développement de plasmoïdes, dans des plasmas sans collisions. Dans le régime isotrope βe → 0, nous analysons la géométrie qui caractérise le feuillet de courant, et identifions les conditions pour lesquelles elle devient instable à l'instabilité plasmoïde. Notre étude montre que des plasmoïdes peuvent être obtenus, dans ce contexte, à partir de feuille de courants aillant un rapport d'aspect beaucoup plus petit que dans le régime collisionnel. De plus, nous étudions la formation de plasmoïdes en comparant les simulations gyrofluides et gyrocinétiques.Ceci a permis de montrer que l'effet de βe favorise l'instabilité plasmoïde. Enfin, nous étudions l'impact de la fermeture appliquée sur les moments, effectuée lors de la dérivation du modèle gyrofluide, sur la distribution et la conversion de l'énergie lors de la reconnexion
Magnetic reconnection is a change of topology of the magnetic field, responsible for explosive release of magnetic energy in astrophysical plasmas, as in the case of magnetospheric substorms and coronal mass ejections, as well as in laboratory plasmas, which is the case of sawtooth crashes in tokamaks. In collisionless plasmas as, for instance, the magnetosphere and the solar wind, electron inertia becomes particularly relevant to drive reconnection at regions of intense localized current, denoted as current sheets. In these non-collisional environments, the temperature can often be anisotropic and effects at the electron scale on the reconnection process can become non-negligible.In this thesis, the stability of two-dimensional current sheets, with respect to reconnecting perturbations, in collisionless plasmas with a strong guide field is analysed on the basis of gyrofluid models assuming cold ions. These models can take into account an equilibrium temperature anisotropy,and a finite βe, a parameter corresponding to the ratio between equilibrium electron kinetic pressure and magnetic pressure.We derive and analyze a dispersion relation for the growth rate of collisionless tearing modes accounting for equilibrium electron temperature anisotropy. The analytical predictions are tested against numerical simulations, showing a very good quantitative agreement.In the isotropic case, accounting for finite βe effects, we observe a stabilization of the tearing growth rate when electron finite Larmor radius effects become relevant. In the nonlinear phase, stall phases and faster than exponential phases are observed, similarly to what occurs in the presence of ion finite Larmor radius effects.We also investigate the marginal stability conditions of secondary current sheets, for the development of plasmoids, in collisionless plasmas. In the isotropic βe → 0 regime, we analyze the geometry that characterizes the reconnecting current sheet, and identify the conditions for which it is plasmoid unstable. Our study shows that plasmoids can be obtained, in this context, from current sheets with an aspect ratio much smaller than in the collisional regime. Furthermore, we investigate the plasmoid formation comparing gyrofluid and gyrokinetic simulations.This made it possible to show that the effect of finite βe, promotes the plasmoid instability. Finally, we study the impact of the closure applied on the moments, performed during the derivation of the gyrofluid model, on the distribution and conversion of energy during reconnection
La riconnessione magnetica è un cambiamento nella topologia delcampo magnetico, responsabile del rilascio esplosivo di energia magnetica nei plasmiastrofisici, come nelle tempeste magnetosferiche e nelle espulsioni di massa coronale,nonché nei plasmi di laboratorio, come nel caso delle oscillazioni a dente di sega neitokamak. Nei plasmi non-collisionali come, ad esempio, la magnetosfera e il vento solare,l’inerzia elettronica diventa particolarmente efficace nel causare la riconnessionein regioni di corrente intensa e localizzata, detti strati di corrente. In tali plasmi noncollisionali,la temperatura può essere spesso anisotropa e gli effetti su scala elettronicasul processo di riconnessione possono diventare non trascurabili.In questa tesi, viene analizzata la stabilità di strati di corrente bidimensionali inplasmi non-collisionali con un forte campo guida, sulla base di modelli girofluidi cheassumono ioni freddi. Questi modelli possono tenere conto di un’anisotropia di temperaturadi equilibrio e di un βe finito. Quest’ultimo è un parametro corrispondente alrapporto tra la pressione cinetica elettronica di equilibrio e la pressione magnetica.Deriviamo e analizziamo una relazione di dispersione per il tasso di crescita dei moditearing non-collisionali tenendo conto dell’anisotropia della temperatura di equilibriodegli elettroni. Le previsioni analitiche sono verificate mediante simulazioni numeriche,che mostrano un ottimo accordo quantitativo. Nel caso isotropico, tenendoconto degli effetti di βe finito, si osserva una stabilizzazione del tasso di crescita delmodo tearing quando diventano rilevanti gli effetti del raggio finito di Larmor deglielettroni. Nella fase non lineare si osservano fasi di decelerazione e fasi di accelerazione,simili a quanto avviene in presenza di effetti del raggio di Larmor finito ionico.Studiamo anche le condizioni di stabilità marginale degli strati di corrente secondaria,per lo sviluppo di plasmoidi, in plasmi senza collisioni. Nel regime isotropicocon βe → 0, analizziamo la geometria che caratterizza lo strato di corrente e identifichiamole condizioni in cui esso diventa instabile a causa di un’instabilità che generaplasmoidi. Il nostro studio mostra che i plasmoidi possono essere ottenuti, in questocontesto, da strati di corrente aventi un rapporto d’aspetto molto più piccolo rispettoal regime collisionale. Inoltre, studiamo la formazione di plasmoidi confrontando simulazionigirofluidi e girocinetiche. Ciò ha permesso di dimostrare che l’effetto di βe promuove l’instabilità che genera plasmoidi. Infine, si studia l’impatto della chiusuraapplicata ai momenti, eseguita durante la derivazione del modello girofluido, sulla distribuzionee conversione dell’energia durante la riconnessione