arXiv subjects

Pierre de Buyl

Publications and source records attributed to Pierre de Buyl.

27 records · Page 2Linked to original sources

arXiv2012

Vlasov dynamics of 1D models with long-range interactions

Gravitational and electrostatic interactions are fundamental examples of systems with long-range interactions. Equilibrium properties of simple models with long-range interactions are well understood and exhibit exotic behaviors : negative specific heat and inequivalence of statistical ensembles for instance. The understanding of the dynamical evolution in the case of long-range interacting systems still represents a theoretical challenge. Phenomena such as out-of-equilibrium phase transitions or quasi-stationary states have been found even in simple models. The purpose of the present thesis is to investigate the dynamical properties of systems with long-range interactions, specializing on one-dimensional models. The appropriate kinetic description for these systems is the Vlasov equation. A numerical simulation tool for the Vlasov equation is developed. A detailed study of the out-of-equilibrium phase transition occuring in the Free-Electron Laser is performed and the transition is analyzed with the help of Lynden-Bell's theory. Then, the presence of stretching and folding in phase space for the Hamiltonian Mean-Field model is studied and quantified from the point of view of fluid dynamics. Finally, a system of uncoupled pendula for which the asymptotic states are similar to the ones of the Hamiltonian Mean-Field model is introduced. Its asymptotic evolution is predicted via both Lynden-Bell's theory and an exact computation. This system displays a fast initial evolution similar to the violent relaxation found for interacting systems. Moreover, an out-of-equilibrium phase transition is found if one imposes a self-consistent condition on the system. In summary, the present thesis discusses original results related to the occurence of quasi-stationary states and out-of-equilibrium phase transitions in 1D models with long-range interaction.

cond-mat.stat-mech↗

arXiv2011

Self-consistent inhomogeneous steady states in Hamiltonian mean field dynamics

Long-lived quasistationary states, associated with stationary stable solutions of the Vlasov equation, are found in systems with long-range interactions. Studies of the relaxation time in a model of $N$ globally coupled particles moving on a ring, the Hamiltonian Mean Field model (HMF), have shown that it diverges as $N^γ$ for large $N$, with $γ\simeq 1.7$ for some initial conditions with homogeneously distributed particles. We propose a method for identifying exact inhomogeneous steady states in the thermodynamic limit, based on analysing models of uncoupled particles moving in an external field. For the HMF model, we show numerically that the relaxation time of these states diverges with $N$ with the exponent $γ\simeq 1$. The method, applicable to other models with globally coupled particles, also allows an exact evaluation of the stability limit of homogeneous steady states. In some cases it provides a good approximation for the correspondence between the initial condition and the final steady state.

cond-mat.stat-mech↗

arXiv2011

Phase transitions of quasistationary states in the Hamiltonian Mean Field model

The out-of-equilibrium dynamics of the Hamiltonian Mean Field (HMF) model is studied in presence of an externally imposed magnetic field h. Lynden-Bell's theory of violent relaxation is revisited and shown to adequately capture the system dynamics, as revealed by direct Vlasov based numerical simulations in the limit of vanishing field. This includes the existence of an out-of-equilibrium phase transition separating magnetized and non magnetized phases. We also monitor the fluctuations in time of the magnetization, which allows us to elaborate on the choice of the correct order parameter when challenging the performance of Lynden-Bell's theory. The presence of the field h removes the phase transition, as it happens at equilibrium. Moreover, regions with negative susceptibility are numerically found to occur, in agreement with the predictions of the theory.

cond-mat.stat-mech↗

arXiv2011

On the effectiveness of mixing in violent relaxation

Relaxation processes in collisionless dynamics lead to peculiar behavior in systems with long-range interactions such as self-gravitating systems, non-neutral plasmas and wave-particle systems. These systems, adequately described by the Vlasov equation, present quasi-stationary states (QSS), i.e. long lasting intermediate stages of the dynamics that occur after a short significant evolution called "violent relaxation". The nature of the relaxation, in the absence of collisions, is not yet fully understood. We demonstrate in this article the occurrence of stretching and folding behavior in numerical simulations of the Vlasov equation, providing a plausible relaxation mechanism that brings the system from its initial condition into the QSS regime. Area-preserving discrete-time maps with a mean-field coupling term are found to display a similar behaviour in phase space as the Vlasov system.

cond-mat.stat-mech↗

arXiv2010

Statistical mechanics of collisionless relaxation in a noninteracting system

We introduce a model of uncoupled pendula, which mimics the dynamical behavior of the Hamiltonian Mean Field (HMF) model. This model has become a paradigm for long-range interactions, like Coulomb or dipolar forces. As in the HMF model, this simplified integrable model is found to obey the Vlasov equation and to exhibit Quasi Stationary States (QSS), which arise after a "collisionless" relaxation process. Both the magnetization and the single particle distribution function in these QSS can be predicted using Lynden-Bell's theory. The existence of an extra conserved quantity for this model, the energy distribution function, allows us to understand the origin of some discrepancies of the theory with numerical experiments. It also suggests us an improvement of Lynden-Bell's theory, which we fully implement for the zero field case.

cond-mat.stat-mech↗

arXiv2010

A digital oscilloscope setup for the measurement of a transistor's characteristics

The measure of the characteristics of a transistor is an important step in an introductory electronics course. We propose to use a digital oscilloscope with a USB connection to perform a measurement of the characteristic curves with no additional custom circuitry. The setup is presented alongside with code that allows the importation and analysis of the results with open-source software.

physics.ed-ph↗

arXiv2010

Models with short and long-range interactions: phase diagram and reentrant phase

We study the phase diagram of two different Hamiltonians with competiting local, nearest-neighbour, and mean-field couplings. The first example corresponds to the HMF Hamiltonian with an additional short-range interaction. The second example is a reduced Hamiltonian for dipolar layered spin structures, with a new feature with respect to the first example, the presence of anisotropies. The two examples are solved in both the canonical and the microcanonical ensemble using a combination of the min-max method with the transfer operator method. The phase diagrams present typical features of systems with long-range interactions: ensemble inequivalence, negative specific heat and temperature jumps. Moreover, in a given range of parameters, we report the signature of phase reentrance. This can also be interpreted as the presence of azeotropy with the creation of two first order phase transitions with ensemble inequivalence, as one parameter is varied continuously.

cond-mat.stat-mech↗

arXiv2010

Out-of-equilibrium mean-field dynamics of a model for wave-particle interaction

The out-of-equilibrium mean-field dynamics of a model for wave-particle interaction is investigated. Such a model can be regarded as a general formulation for all those applications where the complex interplay between particles and fields is known to be central, e.g., electrostatic instabilities in plasma physics, particle acceleration and free-electron lasers (FELs). The latter case is here assumed as a paradigmatic example. A transition separating different macroscopic regimes is numerically identified and interpreted by making use of the so-called violent relaxation theory. In the context of free-electron lasers, such a theory is showed to be effective in predicting the saturated regime for energies below the transition. The transition is explained as a dynamical switch between two metastable regimes, and is related to the properties of a stationary point of an entropic functional.

physics.plasm-ph↗

arXiv2009

Numerical resolution of the Vlasov equation for the Hamiltonian Mean-Field model

We present in this paper detailed numerical Vlasov simulations of the Hamiltonian Mean-Field model. This model is used as a representative of the class of systems under long-range interactions. We check existing results on the stability of the homogeneous situation and analyze numerical properties of the semi-Lagrangian time-split algorithm for solving the Vlasov equation. We also detail limitations due to finite resolution of the method.

physics.comp-ph↗