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Cristel Chandre

Publications and source records attributed to Cristel Chandre.

At least 37 records · Page 2Linked to original sources
arXiv2019

First principles gyrokinetic analysis of electromagnetic plasma instabilities

A two-fold analysis of electromagnetic core tokamak instabilities in the framework of the gyrokinetic theory is presented. First principle theoretical foundations of the gyrokinetic theory are used to explain and justify the numerical results obtained with the global electromagnetic particle-in-cell code Orb5 whose model is derived from the Lagrangian formalism. The energy conservation law corresponding to the Orb5 model is derived from the Noether theorem and implemented in the code as a diagnostics for energy balance and conservation verification. An additional Noether theorem based diagnostics is implemented in order to analyse destabilising mechanisms for the electrostatic and the electromagnetic Ion Temperature Gradient (ITG) instabilities in the core region of the tokamak. The transition towards the Kinetic Ballooning Modes (KBM) at high electromagnetic $β$ is also investigated.

physics.plasm-ph↗
arXiv2019

Inclusion of Coulomb effects in laser-atom interactions

We investigate the role of the Coulomb interaction in strong field processes. We find that the Coulomb field of the ion makes its presence known even in highly intense laser fields, in contrast to the assumptions of the strong field approximation. The dynamics of the electron after ionization is analyzed with four models for an arbitrary laser polarization: the Hamiltonian model in the dipole approximation, the strong field approximation, the Coulomb-corrected strong field approximation and the guiding center. These models illustrate clearly the Coulomb effects, in particular Coulomb focusing and Coulomb asymmetry. We show that the Coulomb-corrected strong field approximation and the guiding center are complementary, in the sense that the Coulomb-corrected strong field approximation describes well short time scale phenomena (shorter than a laser cycle) while the guiding center is well suited for describing long time scale phenomena (longer than a laser cycle) like Coulomb-driven recollisions and Rydberg state creation.

physics.atom-ph↗
arXiv2018

Formation of RbCs dimers using an elliptically polarized laser pulse

We consider the formation of RbCs by an elliptically polarized laser pulse. By varying the ellipticity of the laser for sufficiently large laser intensity, we see that the formation probability presents a strong dependence, especially around ellipticity 1/ $\sqrt$ 2. We show that the analysis can be reduced to the investigation of the long-range interaction between the two atoms. The formation is mainly due to a small momentum shifts induced by the laser pulse. We analyze these results using the Silberstein's expressions of the polarizabilities, and show that the ellipticity of the field acts as a control knob for the formation probability, allowing significant variations of the dimer formation probability at a fixed laser intensity, especially in the region around an ellipticity of 1/ $\sqrt$ 2.

physics.atom-ph↗
arXiv2018

Variational formulation of classical and quantum models for intense laser pulse propagation

We consider the theoretical description of intense laser pulses propagating through gases. Starting from a first-principles description of both the electromagnetic field and the electron motion within the gas atoms, we derive a hierarchy of reduced models. We obtain a parallel set of models, where the atomic electrons are treated classically on the one hand, and quantum-mechanically on the other. By working consistently in either a Lagrangian formulation or a Hamiltonian formulation, we ensure that our reduced models preserve the variational structure of the parent models. Taking advantage of the Hamiltonian formulation, we deduce a number of conserved quantities of the reduced models.

physics.plasm-ph↗
arXiv2018

Second order nonlinear gyrokinetic theory : From the particle to the gyrocenter

A gyrokinetic reduction is based on a specific ordering of the different small parameters characterizing the background magnetic field and the fluctuating electromagnetic fields. In this tutorial, we consider the following ordering of the small parameters: $ε\_B=ε\_δ^2$ where $ε\_B$ is the small parameter associated with spatial inhomogeneities of the background magnetic field and $ε\_δ$ characterizes the small amplitude of the fluctuating fields. In particular, we do not make any assumption on the amplitude of the background magnetic field. Given this choice of ordering, we describe a self-contained and systematic derivation which is particularly well suited for the gyrokinetic reduction, following a two-step procedure. We follow the approach developed in [Sugama, Physics of Plasmas 7, 466 (2000)]:In a first step, using a translation in velocity, we embed the transformation performed on the symplectic part of the gyrocentre reduction in the guiding-centre one. In a second step, using a canonical Lie transform, we eliminate the gyroangle dependence from the Hamiltonian. As a consequence, we explicitly derive the fully electromagnetic gyrokinetic equations at the second order in $ε\_δ$.

physics.plasm-ph↗
arXiv2018

Capturing photoelectron motion with guiding fictitious particles

Photoelectron momentum distributions (PMDs) from atoms and molecules undergo qualitative changes as laser parameters are varied. We present a model to interpret the shape of the PMDs. The electron's motion is guided by a fictitious particle in our model, clearly characterizing two distinct dynamical behaviors: direct ionization and rescattering. As laser ellipticity is varied, our model reproduces the bifurcation in the PMDs seen in experiments.

nlin.CD↗
arXiv2017

Hierarchy of second-order gyrokinetic Hamiltonian models for particle-in-cell codes

The reduced-particle model is the central element for the systematic derivation of the gyrokinetic Vlasov-Maxwell equations from first principles. Coupled to the fields inside the gyrokinetic field-particle Lagrangian, the reduced-particle model defines polarization and magnetization effects appearing in the gyrokinetic Maxwell equations. It is also used for the reconstruction of the gyrokinetic Vlasov equation from the particle characteristics. Various representations of reduced-particle models are available according to the choice of the gyrokinetic phase space coordinates. In this paper, the Hamiltonian representation of the reduced particle dynamics at an order suitable for the implementation in particle-in-cell simulations is explicitly derived from the general reduction procedure. The second-order (with respect to the fluctuating electromagnetic fields), full Finite Larmor Radius (FLR) Hamiltonian gyrokinetic particle model as well as the second-order model suitable specifically for the long-wavelength approximation (i.e., containing up to the second-order FLR corrections), are derived and compared to the model recently implemented in the particle-in-cell code ORB5. We show that the same long-wavelength approximate equations can also be derived by taking the proper limit of the full FLR model.

physics.plasm-ph↗
arXiv2017

Driving the formation of the RbCs dimer by a laser pulse. A nonlinear dynamics approach

We study the formation of the RbCs molecule by an intense laser pulse using nonlinear dynamics. Under the Born-Oppenheimer approximation, the system is modeled by a two degree of freedom rovibrational Hamiltonian, which includes the ground electronic potential energy curve of the diatomic molecule and the interaction of the molecular polarizability with the electric field of the laser. As the laser intensity increases, we observe that the formation probability first increases and then decreases after reaching a maximum. We show that the analysis can be simplified to the investigation of the long-range interaction between the two atoms. We conclude that the formation is due to a very small change in the radial momentum of the dimer induced by the laser pulse. From this observation, we build a reduced one dimensional model which allows us to derive an approximate expression of the formation probability as a function of the laser intensity.

nlin.CD↗
arXiv2017

Single and double ionization of magnesium by electron impact: A classical study

We consider electron impact-driven single and double ionization of magnesium in the 10-100 eV energy range. Our classical Hamiltonian model of these (e, 2e) and (e, 3e) processes sheds light on their total cross sections and reveals the underlying ionization mechanisms. Two pathways are at play in single ionization: Delayed and direct. In contrast, only the direct process is observed in double ionization, ruling out the excitation-autoionization channel. We also provide evidence that the so-called TS2 (Two-Step 2) mechanism predominates over the TS1 (Two-Step 1) mechanism, in agreement with experiments.

physics.atom-ph↗
arXiv2016

Conservative dissipation: How important is the Jacobi identity in the dynamics?

Hamiltonian dynamics are characterized by a function, called the Hamiltonian, and a Poisson bracket. The Hamiltonian is a conserved quantity due to the anti-symmetry of the Poisson bracket. The Poisson bracket satisfies the Jacobi identity which is usually more intricate and more complex to comprehend than the conservation of the Hamiltonian. Here we investigate the importance of the Jacobi identity in the dynamics by considering three different types of conservative flows in R3 : Hamiltonian, almost-Poisson and metriplectic. The comparison of their dynamics reveals the importance of the Jacobi identity in structuring the resulting phase space.

nlin.CD↗
arXiv2016

Kinematics of fluid particles on the sea surface. Hamiltonian theory

We derive the John-Sclavounos equations describing the motion of a fluid particle on the sea surface from first principles using Lagrangian and Hamiltonian formalisms applied to the motion of a frictionless particle constrained on an unsteady surface. The main result is that vorticity generated on a stress-free surface vanishes at a wave crest when the horizontal particle velocity equals the crest propagation speed, which is the kinematic criterion for wave breaking. If this holds for the largest crest, then the symplectic two-form associated with the Hamiltonian dynamics reduces instantaneously to that associated with the motion of a particle in free flight, as if the surface did not exist. Further, exploiting the conservation of the Hamiltonian function for steady surfaces and traveling waves we show that particle velocities remain bounded at all times, ruling out the possibility of the finite-time blowup of solutions.

physics.ao-ph↗
arXiv2015

Hamiltonian reductions of the one-dimensional Vlasov equation using phase-space moments

We consider Hamiltonian closures of the Vlasov equation using the phase-space moments of the distribution function. We provide some conditions on the closures imposed by the Jacobi identity. We completely solve some families of examples. As a result, we show that imposing that the resulting reduced system preserves the Hamiltonian character of the parent model shapes its phase space by creating a set of Casimir invariants as a direct consequence of the Jacobi identity.

nlin.CD↗
arXiv2015

Photon plasma--wave interaction via Compton scattering

The Kompaneets theory of photon kinetic evolution due to the Compton effect in the absence of absorption and emission is extended to the case of the Vlasov plasma wave oscillations. Under the assumption that the electron distribution function at equilibrium is perturbed by a solution of the linearised Vlasov equation in the long-wavelength limit, a solution of the Kompaneets kinetic equation for the photon distribution function is found and discussed.

nlin.CD↗
arXiv2015

Hamiltonian fluid reductions of drift-kinetic equations and the correspondence with water-bag distribution functions

Hamiltonian models for the first three moments of the drift-kinetic distribution function, namely the density, the fluid velocity and the parallel pressure, are derived from the Hamiltonian structure of the drift-kinetic equations. The link with the water-bag closure is established, showing that, unlike the one-dimensional Vlasov equations, these solutions are the only Hamiltonian fluid reductions for the drift-kinetic equation. These models are discussed through their equations of motion and their Casimir invariants.

physics.plasm-ph↗
arXiv2015

Hamiltonian fluid closures of the Vlasov-Amp{è}re equations: from water-bags to N moment models

Moment closures of the Vlasov-Amp{è}re system, whereby higher moments are represented as functions of lower moments with the constraint that the resulting fluid system remains Hamiltonian, are investigated by using water-bag theory. The link between the water-bag formalism and fluid models that involve density, fluid velocity, pressure and higher moments is established by introducing suitable thermodynamic variables. The cases of one, two and three water-bags are treated and their Hamiltonian structures are provided. In each case, we give the associated fluid closures and we discuss their Casimir invariants. We show how the method can be extended to an arbitrary number of fields, i.e., an arbitrary number of water-bags and associated moments. The thermodynamic interpretation of the resulting models is discussed. Finally, a general procedure to derive Hamiltonian N-field fluid models is proposed.

physics.plasm-ph↗
arXiv2014

Higher order Hamiltonian fluid reduction of Vlasov equation

From the Hamiltonian structure of the Vlasov equation, we build a Hamiltonian model for the first three moments of the Vlasov distribution function, namely, the density, the momentum density and the specific internal energy. We derive the Poisson bracket of this model from the Poisson bracket of the Vlasov equation, and we discuss the associated Casimir invariants.

nlin.CD↗
arXiv2013

Recollision scenario without tunneling : Role of the ionic core potential

The standard model of strong laser physics, the recollision scenario, omits the ionic core potential after tunneling. Strikingly, although the Coulomb interaction drives all stages of recollision, the maximum energy the electrons bring back to the core is found by ignoring it. We resolve this long-standing paradox by showing that this good agreement stems from a fortuitous cancellation at high intensities. Instead of the three step model, we find that the Coulomb interaction can be fully integrated into a purely classical scenario that explains recollisions without invoking tunneling.

physics.atom-ph↗