arXiv subjects

Cristel Chandre

Publications and source records attributed to Cristel Chandre.

At least 73 records · Page 4Linked to original sources

arXiv2010

Recollisions and correlated double ionization with circularly polarized light

It is generally believed that the recollision mechanism of atomic nonsequential double ionization is suppressed in circularly polarized laser fields because the returning electron is unlikely to encounter the core. On the contrary, we find that recollision can and does significantly enhance double ionization, even to the extent of forming a "knee", the signature of the nonsequential process. Using a classical model, we explain two apparently contradictory experiments, the absence of a knee for helium and its presence for magnesium.

physics.atom-ph↗

arXiv2010

Derivation of reduced two-dimensional fluid models via Dirac's theory of constrained Hamiltonian systems

We present a Hamiltonian derivation of a class of reduced plasma two-dimensional fluid models, an example being the Charney-Hasegawa-Mima equation. These models are obtained from the same parent Hamiltonian model, which consists of the ion momentum equation coupled to the continuity equation, by imposing dynamical constraints. It is shown that the Poisson bracket associated with these reduced models is the Dirac bracket obtained from the Poisson bracket of the parent model.

nlin.CD↗

arXiv2009

From recollisions to the knee: A road map for double ionization in intense laser fields

We examine the nature and statistical properties of electron-electron collisions in the recollision process in a strong laser field. The separation of the double ionization yield into sequential and nonsequential components leads to a bell-shaped curve for the nonsequential probability and a monotonically rising one for the sequential process. We identify key features of the nonsequential process and connect our findings in a simplified model which reproduces the knee shape for the probability of double ionization with laser intensity and associated trends.

physics.atom-ph↗

arXiv2009

Reduction of the chaotic transport of impurities in turbulent magnetized plasmas

The chaotic transport of charged particles in a turbulent electrostatic potential sets the conditions of a severe limitation to the plasma confinement in devices such as tokamaks. In this chapter, we consider the motion of impurities driven by the ExB velocity where a strong magnetic field B (which allows for the guiding center approximation) is uniform and constant, and a turbulent electric field is obtained from models or from numerical fluid codes. Hamiltonian dynamics rule the transport properties of these impurities. Therefore a technique to reduce chaotic diffusion in Hamiltonian systems is able to address the issue of reducing the radial transport of impurities under some approximations. The general idea is to build barriers in phase space by a small and apt modification of the Hamiltonian. We show numerically that such perturbations are able to drastically reduce the diffusion of test-particles, and we discuss the robustness of such additional modifications.

nlin.CD↗

arXiv2009

Using Resonances to Control Chaotic Mixing within a Translating and Rotating Droplet

Enhancing and controlling chaotic advection or chaotic mixing within liquid droplets is crucial for a variety of applications including digital microfluidic devices which use microscopic ``discrete'' fluid volumes (droplets) as microreactors. In this work, we consider the Stokes flow of a translating spherical liquid droplet which we perturb by imposing a time-periodic rigid-body rotation. Using the tools of dynamical systems, we have shown in previous work that the rotation not only leads to one or more three-dimensional chaotic mixing regions, in which mixing occurs through the stretching and folding of material lines, but also offers the possibility of controlling both the size and the location of chaotic mixing within the drop. Such a control was achieved through appropriate tuning of the amplitude and frequency of the rotation in order to use resonances between the natural frequencies of the system and those of the external forcing. In this paper, we study the influence of the orientation of the rotation axis on the chaotic mixing zones as a third parameter, as well as propose an experimental set up to implement the techniques discussed.

nlin.CD↗

arXiv2009

Hamiltonian derivation of the Charney-Hasegawa-Mima equation

The Charney-Hasegawa-Mima equation is an infinite-dimensional Hamiltonian system with dynamics generated by a noncanonical Poisson bracket. Here a first principle Hamiltonian derivation of this system, beginning with the ion fluid dynamics and its known Hamiltonian form, is given.

nlin.CD↗

arXiv2009

Strong field double ionization of H2 : Insights from nonlinear dynamics

The uncorrelated (``sequential'') and correlated (``nonsequential'') double ionization of the H2 molecule in strong laser pulses is investigated using the tools of nonlinear dynamics. We focus on the phase-space dynamics of this system, specifically by finding the dynamical structures that regulate these ionization processes. The emerging picture complements the recollision scenario by clarifying the distinct roles played by the recolliding and core electrons. Our analysis leads to verifiable predictions of the intensities where qualitative changes in ionization occur. We also show how these findings depend on the internuclear distance.

nlin.CD↗

arXiv2009

Strong field double ionization: What is under the "knee"?

Both uncorrelated ("sequential") and correlated ("nonsequential") processes contribute to the double ionization of the helium atom in strong laser pulses. The double ionization probability has a characteristic "knee" shape as a function of the intensity of the pulse. We investigate the phase-space dynamics of this system, specifically by finding the dynamical structures that regulate the ionization processes. The emerging picture complements the recollision scenario by clarifying the distinct roles played by the recolliding and core electrons. Our analysis leads to verifiable predictions of the intensities where qualitiative changes in ionization occur, leading to the hallmark "knee" shape.

nlin.CD↗

arXiv2009

Analyzing intramolecular dynamics by Fast Lyapunov Indicators

We report an analysis of intramolecular dynamics of the highly excited planar carbonyl sulfide (OCS) below and at the dissociation threshold by the Fast Lyapunov Indicator (FLI) method. By mapping out the variety of dynamical regimes in the phase space of this molecule, we obtain the degree of regularity of the system versus its energy. We combine this stability analysis with a periodic orbit search, which yields a family of elliptic periodic orbits in the regular part of phase space an a family of in-phase collinear hyperbolic orbits associated with the chaotic regime.

nlin.CD↗

arXiv2009

Analytical results on the magnetization of the Hamiltonian Mean Field model

The violent relaxation and the metastable states of the Hamiltonian Mean-Field model, a paradigmatic system of long-range interactions, is studied using a Hamiltonian formalism. Rigorous results are derived algebraically for the time evolution of selected macroscopic observables, e.g., the global magnetization. The high and low energy limits are investigated and the analytical predictions are compared with direct $N$-body simulations. The method we use enables us to re-interpret the out-of-equilibrium phase transition separating magnetized and (almost) unmagnetized regimes.

nlin.CD↗

arXiv2009

Barriers for the reduction of transport due to the EXB drift in magnetized plasmas

We consider a 1 1/2degrees of freedom Hamiltonian dynamical system, which models the chaotic dynamics of charged test-particles in a turbulent electric field, across the confining magnetic field in controlled thermonuclear fusion devices. The external electric field E = \nabla\bigvee is modeled by a phenomenological potential V and the magnetic field B is considered uniform. It is shown that, by introducing a small additive control term to the external electric field, it is possible to create a transport barrier for this dynamical system. The robustness of this control method is also investigated. This theoretical study indicates that alternative transport barriers can be triggered without requiring a control action on the device scale as in present Internal Transport Barriers (ITB).

math-ph↗

arXiv2009

Abundance of regular orbits and out-of-equilibrium phase transitions in the thermodynamic limit for long-range systems

We investigate the dynamics of many-body long-range interacting systems, taking the Hamiltonian Mean Field model as a case study. We show that an abundance of regular trajectories, associated with invariant tori of the single-particle dynamics, exists. The presence of such tori provides a dynamical interpretation of the emergence of long-lasting out-of-equilibrium regimes observed generically in long-range systems. This is alternative to a previous statistical mechanics approach to such phenomena which was based on a maximum entropy principle. Previously detected out-of-equilibrium phase transitions are also reinterpreted within this framework.

cond-mat.stat-mech↗

arXiv2008

Strong field double ionization : The phase space perspective

We identify the phase-space structures that regulate atomic double ionization in strong ultrashort laser pulses. The emerging dynamical picture complements the recollision scenario by clarifying the distinct roles played by the recolliding and core electrons, and leads to verifiable predictions on the characteristic features of the "knee", a hallmark of the nonsequential process.

nlin.CD↗

arXiv2008

Local control of area-preserving maps

We present a method of control of chaos in area-preserving maps. This method gives an explicit expression of a control term which is added to a given area-preserving map. The resulting controlled map which is a small and suitable modification of the original map, is again area-preserving and has an invariant curve whose equation is explicitly known.

nlin.CD↗

arXiv2008

Ergodicité, collage et transport anomal

Nous nous intéressons à la convergence vers sa moyenne spatiale ergodique de la moyenne temporelle d'une observable d'un flow hamiltonien à un degré et demi de liberté avec espace des phases mixte. L'analyse est faite au travers de l'évolution de la distribution des moyennes en temps fini d'un ensemble de conditions initiales sur la même composante ergodique. Un exposant caractérisant la vitesse de convergence est défini. Les résultats indiquent que pour le système considéré la convergence évolue en $t^α$, avec $α=0.45$ pour alors qu'elle évolue en $t^{1/2}$ lorsque la dynamique est globalement chaotique dans l'espace des phases. De même une loi $α=1-β/2$ reliant cet exposant $α$ à l'exposant caractéristique du deuxième moment associé aux propriétés de transport $β$ est proposée et est vérifiée pour les cas considérés.

nlin.CD↗

arXiv2008

Tailored mixing inside a translating droplet

Tailored mixing inside individual droplets could be useful to ensure that reactions within microscopic discrete fluid volumes, which are used as microreactors in ``digital microfluidic'' applications, take place in a controlled fashion. In this article we consider a translating spherical liquid drop to which we impose a time periodic rigid-body rotation. Such a rotation not only induces mixing via chaotic advection, which operates through the stretching and folding of material lines, but also offers the possibility of tuning the mixing by controlling the location and size of the mixing region. Tuned mixing is achieved by judiciously adjusting the amplitude and frequency of the rotation, which are determined by using a resonance condition and following the evolution of adiabatic invariants. As the size of the mixing region is increased, complete mixing within the drop is obtained.

nlin.CD↗

arXiv2008

Hamiltonian description of a self-consistent interaction between charged particles and electromagnetic waves

The Hamiltonian description of the self-consistent interaction between an electromagnetic plane-wave and a co-propagating beam of charged particles is considered. We show how the motion can be reduced to a one-dimensional Hamiltonian model (in a canonical setting) from the Vlasov-Maxwell Poisson brackets. The reduction to this paradigmatic Hamiltonian model is performed using a Lie algebraic formalism which allows us to remain Hamiltonian at each step of the derivation.

physics.plasm-ph↗

arXiv2007

Control of test particle transport in a turbulent electrostatic model of the Scrape Off Layer

The ${\bm E}\times{\bm B}$ drift motion of charged test particle dynamics in the Scrape Off Layer (SOL)is analyzed to investigate a transport control strategy based on Hamiltonian dynamics. We model SOL turbulence using a 2D non-linear fluid code based on interchange instability which was found to exhibit intermittent dynamics of the particle flux. The effect of a small and appropriate modification of the turbulent electric potential is studied with respect to the chaotic diffusion of test particle dynamics. Over a significant range in the magnitude of the turbulent electrostatic field, a three-fold reduction of the test particle diffusion coefficient is achieved.

physics.plasm-ph↗