13 research outputs found
Free-energy model for fluid helium at high density
We present a semi-analytical free-energy model aimed at characterizing the
thermodynamic properties of dense fluid helium, from the low-density atomic
phase to the high-density fully ionized regime. The model is based on a
free-energy minimization method and includes various different contributions
representative of the correlations between atomic and ionic species and
electrons. This model allows the computation of the thermodynamic properties of
dense helium over an extended range of density and temperature and leads to the
computation of the phase diagram of dense fluid helium, with its various
temperature and pressure ionization contours. One of the predictions of the
model is that pressure ionization occurs abruptly at \rho \simgr 10 g
cm, {\it i.e.} P\simgr 20 Mbar, from atomic helium He to fully ionized
helium He, or at least to a strongly ionized state, without He
stage, except at high enough temperature for temperature ionization to become
dominant. These predictions and this phase diagram provide a guide for future
dynamical experiments or numerical first-principle calculations aimed at
studying the properties of helium at very high density, in particular its
metallization. Indeed, the characterization of the helium phase diagram bears
important consequences for the thermodynamic, magnetic and transport properties
of cool and dense astrophysical objects, among which the solar and the numerous
recently discovered extrasolar giant planets.Comment: Accepted for publication in Phys. Rev.
Relativistic magnetic reconnection in collisionless ion-electron plasmas explored with particle-in-cell simulations
Magnetic reconnection is a leading mechanism for magnetic energy conversion
and high-energy non-thermal particle production in a variety of high-energy
astrophysical objects, including ones with relativistic ion-electron plasmas
(e.g., microquasars or AGNs) - a regime where first principle studies are
scarce. We present 2D particle-in-cell (PIC) simulations of low
ion-electron plasmas under relativistic conditions, i.e., with inflow magnetic
energy exceeding the plasma rest-mass energy. We identify outstanding
properties: (i) For relativistic inflow magnetizations (here ), the reconnection outflows are dominated by thermal agitation instead of
bulk kinetic energy. (ii) At large inflow electron magnetization (), the reconnection electric field is sustained more by bulk inertia than by
thermal inertia. It challenges the thermal-inertia-paradigm and its
implications. (iii) The inflows feature sharp transitions at the entrance of
the diffusion zones. These are not shocks but results from particle ballistic
motions, all bouncing at the same location, provided that the thermal velocity
in the inflow is far smaller than the inflow E cross B bulk velocity. (iv)
Island centers are magnetically isolated from the rest of the flow, and can
present a density depletion at their center. (v) The reconnection rates are
slightly larger than in non-relativistic studies. They are best normalized by
the inflow relativistic Alfv\'en speed projected in the outflow direction,
which then leads to rates in a close range (0.14-0.25) thus allowing for an
easy estimation of the reconnection electric field.Comment: Submitted to A&
The energetics of relativistic magnetic reconnection: ion-electron repartition and particle distribution hardness
Collisionless magnetic reconnection is a prime candidate to account for
flare-like or steady emission, outflow launching, or plasma heating, in a
variety of high-energy astrophysical objects, including ones with relativistic
ion-electron plasmas. But the fate of the initial magnetic energy in a
reconnection event remains poorly known: what is the amount given to kinetic
energy, the ion/electron repartition, and the hardness of the particle
distributions? We explore these questions with 2D particle-in-cell simulations
of ion-electron plasmas. We find that 45 to 75% of the total initial magnetic
energy ends up in kinetic energy, this fraction increasing with the inflow
magnetization. Depending on the guide field strength, ions get from 30 to 60%
of the total kinetic energy. Particles can be separated into two populations
that only weakly mix: (i) particles initially in the current sheet, heated by
its initial tearing and subsequent contraction of the islands; and (ii)
particles from the background plasma that primarily gain energy via the
reconnection electric field when passing near the X-point. Particles (ii) tend
to form a power-law with an index , that
depends mostly on the inflow Alfv\'en speed and magnetization
of species , with for electrons to for increasing .
The highest particle Lorentz factor, for ions or electrons, increases roughly
linearly with time for all the relativistic simulations. This is faster, and
the spectra can be harder, than for collisionless shock acceleration. We
discuss applications to microquasar and AGN coronae, to extragalactic jets, and
to radio lobes. We point out situations where effects such as Compton drag or
pair creation are important.Comment: 15 pages, submitted to A&
Apar-T: code, validation, and physical interpretation of particle-in-cell results
We present the parallel particle-in-cell (PIC) code Apar-T and, more
importantly, address the fundamental question of the relations between the PIC
model, the Vlasov-Maxwell theory, and real plasmas.
First, we present four validation tests: spectra from simulations of thermal
plasmas, linear growth rates of the relativistic tearing instability and of the
filamentation instability, and non-linear filamentation merging phase. For the
filamentation instability we show that the effective growth rates measured on
the total energy can differ by more than 50% from the linear cold predictions
and from the fastest modes of the simulation.
Second, we detail a new method for initial loading of Maxwell-J\"uttner
particle distributions with relativistic bulk velocity and relativistic
temperature, and explain why the traditional method with individual particle
boosting fails.
Third, we scrutinize the question of what description of physical plasmas is
obtained by PIC models. These models rely on two building blocks:
coarse-graining, i.e., grouping of the order of p~10^10 real particles into a
single computer superparticle, and field storage on a grid with its subsequent
finite superparticle size. We introduce the notion of coarse-graining dependent
quantities, i.e., quantities depending on p. They derive from the PIC plasma
parameter Lambda^{PIC}, which we show to scale as 1/p. We explore two
implications. One is that PIC collision- and fluctuation-induced thermalization
times are expected to scale with the number of superparticles per grid cell,
and thus to be a factor p~10^10 smaller than in real plasmas. The other is that
the level of electric field fluctuations scales as 1/Lambda^{PIC} ~ p. We
provide a corresponding exact expression.
Fourth, we compare the Vlasov-Maxwell theory, which describes a phase-space
fluid with infinite Lambda, to the PIC model and its relatively small Lambda.Comment: 24 pages, 14 figures, accepted in Astronomy & Astrophysic
Thermodynamique des plasmas denses dans des conditons astrophysiques (hélium et mélange hydrogène-hélium)
La première partie de cette thèse est consacrée à l'étude du diagramme de phase du mélange hydrogène-hélium dans des conditions thermodynamiques correspondant à celles rencontrées dans les planètes géantes ou dans les expériences de chocs laser. La démixtion de ce mélange binaire est notamment invoquée pour expliquer l'excès dans le bilan radiatif de Saturne. Le calcul précis de ce diagramme de phase nécessite la prise en compte du caractère quantique des électrons et des effets de corrélation entre les différentes particules présentes dans le milieu. Nous utilisons la théorie de la fonctionnelle de densité pour traiter ab initio le gaz éléctronique. Les diverses grandeurs thermodynamiques sont alors calculées dans l'ensemble microcanonique à l'aide de simulations de dynamique moléculaire. Aucun potentiel d'interaction a priori n'est requis, toutes les grandeurs thermodynamiques étant ainsi issues des seuls principes fondamentaux de la physique. Nous avons validé cette méthode par le calcul de potentiels chimiques, première étape de la détermination du diagramme de phase complet. Dans la seconde partie de cette thèse, nous présentons le calcul de l'équation d'état de l'hélium dense à l'aide d'une approche chimique. Cette méthode repose sur la minimisation de l'énergie libre d'un mélange d'espèces chimiques. Elle présente l'avantage d'être semi-analytique et est donc numériquement beaucoup plus rapide que la précédente. La contre-partie est la nécessité de s'appuyer sur des potentiels d'interaction calibrés sur des données expérimentales. Nous avons développé cette méthode pour le mélange {He, He+, He puissance 2+}, ce qui nous a permis d'obtenir une équation d'état valable aussi bien dans le domaine correspondant au régime de Saha qu'à celui du plasma totalement inonisé et décrivant de façon convenable le phénomène d'ionisation par pression.LYON-ENS Sciences (693872304) / SudocSudocFranceF
Ab initio Gibbs ensemble Monte Carlo simulations of the liquid–vapor equilibrium and the critical point of sodium
International audienceWe extend the application of the ab initio Gibbs ensemble method to the metallic system by including the contribution of excited electronic states