Microfield distributions in strongly coupled two-component plasmas

The electric microfield distribution at charged particles is studied for two-component electron-ion plasmas using molecular dynamics simulation and theoretical models. The particles are treated within classical statistical mechanics using an electron-ion Coulomb potential regularized at distances less than the de Broglie length to take into account the quantum-diffraction effects. The potential-of-mean-force (PMF) approximation is deduced from a canonical ensemble formulation. The resulting probability density of the electric microfield satisfies exactly the second-moment sum rule without the use of adjustable parameters. The correlation functions between the charged radiator and the plasma ions and electrons are calculated using molecular dynamics simulations and the hypernetted-chain approximation for a two-component plasma. It is shown that the agreement between the theoretical models for the microfield distributions and the simulations is quite good in general.Comment: 18 figures. Submitted to Phys. Rev.

Isospin relaxation time in heavy-ion collisions at intermediate energies

Using an isospin-dependent transport model, we have studied the isospin and momentum relaxation times in the heavy residues formed in heavy-ion collisions at intermediate energies. It is found that only at incident energies below the Fermi energy, chemical or thermal equilibrium can be reached before dynamical instability is developed in the heavy residues. Also, the isospin relaxation time is shorter (longer) than that for momentum at beam energies lower (higher) than the Fermi energy.Comment: 8 pages Latex + 2 ps Figs.; Phys. Rev. C in pres

A POSSIBLE MODEL FOR MULTINUCLEON TRANSFER

La relation entre l'angle de diffusion et la perte d'énergie dans les réactions de transfert de plusieurs nucléons est expliquée par une étude des éléments de matrice de transition quasi-classiques. Pour le processus de collision conduisant au transfert de plusieurs nucléons un mécanisme est proposé dans lequel les collisions élémentaires ont lieu entre les nucléons dans la région de recouvrement des noyaux.The dependence between the scattering angle and the energy loss in multinucleon transfer reactions is explained by a study of the quasi-classical transition matrix elements. For the collision process leading to multinucleon transfer a mechanism is proposed, in which elementary collisions take place between the nucleons in the overlap region of the nuclei

ELECTRONIC RECOMBINATION IN AN ANISOTROPIC CLASSICAL MAXWELLIAN PLASMA

Le coefficient de recombinaison de l'électron premier est un exemple d'un processus atomique apparaissant dans un plasma non-équilibre anisotrope et neutre consistant d'électrons et d'ions nus. Le plasma, on présume, se trouve dans un état stationnaire de quasi-équilibre et chaque espèces des particules a trois températures différentes selon les trois directions spatiales. En général ça devrait être un problème propre de la théorie cinétique de plasma. Mais l'état de quasi-équilibre permet de faire des généralisations du théorème de fluctuation dissipation (ce que s'applique normalement seulement pour les états d'équilibre), pour ces états de non-équilibre et anisotropes aussi.The coefficient of recombination of the first electron is taken as an example for an atomic process embedded in a nonequilibrium anisotropic neutral plasma of stripped ions and electrons. The plasma is assumed to be in a stationary state of quasi-equilibrium with each particle species having three temperatures along each direction of space. Though this is a genuine problem of kinetic plasma theory the quasi equilibrium allows to make use of generalizations of the fluctuation-dissipation-theorem (FDT), which normally is only applied in thermal equilibrium, even up to this anisotropic nonequilibrium state

Microfield fluctuations and radiative transitions in laser-generated strongly coupled plasmas

Previously unaccessible plasma states of matter can be produced by irradiation with powerful lasers. The interactions within the plasma shift and distort the spectral lines corresponding to radiative transitions. The shape of the spectra is determined by two frequencies characterizing the fluctuations of the electric microfield in the plasma and the Stark splitting of the lines. Traditionally the fluctuations due to the motion of the ions are considered as sufficiently slow and weak to be treated in a linear quasi-static approximation, while the electrons are accounted for in an instantaneous impact approximation. Here also the intermediate regimes and strongly correlated ions are investigated. For that purpose the microfield fluctuations are calculated by molecular dynamics computer simulations. They are then used as input in a numerical solution of the time-dependent Schrödinger equation for the radiating electron. The shape of the Ly$_{\alpha}$–line in H and in Al is investigated in the intermediate regime. The calculations are in agreement with recent experiments on the Ly$_{\alpha}$ and Ly$_{\gamma}$ lines in Al

Dielectric linear response of magnetized electrons: Drag force on ions

The drag force on ions moving in a magnetized electron plasma is calculated in dielectric linear response. Various representations of the dielectric function $\varepsilon (\vec{k}, \omega)$ are investigated for their suitability to display the limits for an infinite and a vanishing magnetic field. While the influence of the magnetic field is negligible in certain regions of $\vec{k}$-space, it introduces in other regions a strong oscillatory structure in the dielectric function. This requires a careful treatment of the multidimensional integrations necessary for the drag force. The contributions from oscillatory integrands are treated by the saddle point method. Explicit results are obtained for the dependence of the drag force on the magnetic field, the direction of motion of the ion relative to the magnetic field, the shielding in the electron plasma, its density and the anisotropy of the electron temperature. The importance of the collective response of the electrons is investigated for limiting cases of the magnetic field. The validity of the linearization of the dielectric theory is checked by comparison with results obtained by numerical simulation of the nonlinear Vlasov-Poisson equation. For strong magnetic fields and low ion velocities, the simulations rather agree with the complementary binary collision model than with linear response

SMALL VIBRATIONS ABOUT EXCITED STATES

Les équations de l'approximation de phase aléatoire pour de petites vibrations autour des états excités sont établies. La dépendence de la température des excitations collectives sera examinée. Nous appliquons ce formalisme à l'état de base ainsi qu'au premier état excité du noyau 90Zr. Par cela nous allons confirmer l'hypothèse de Brink.We derive Modified RPA-equations for small vibrations around excited states. The temperature dependence of collective excitations is examined. The formalism is applied to the ground state and the first excited state of 90Zr in order to confirm the Brink hypothesis