Th. Bornath

Thomson scattering from near-solid density plasmas using soft x-ray free electron lasers

We propose a collective Thomson scattering experiment at the VUV free electron laser facility at DESY (FLASH) which aims to diagnose warm dense matter at near-solid density. The plasma region of interest marks the transition from an ideal plasma to a correlated and degenerate many-particle system and is of current interest, e.g. in ICF experiments or laboratory astrophysics. Plasma diagnostic of such plasmas is a longstanding issue. The collective electron plasma mode (plasmon) is revealed in a pump-probe scattering experiment using the high-brilliant radiation to probe the plasma. The distinctive scattering features allow to infer basic plasma properties. For plasmas in thermal equilibrium the electron density and temperature is determined from scattering off the plasmon mode.

Quantum Kinetic Theory for Laser Plasmas. Dynamical Screening in Strong Fields

A quantum kinetic theory for correlated charged-particle systems in strong time-dependent electromagnetic fields is developed. Our approach is based on a systematic gauge-invariant nonequilibrium Greens functions formulation. Extending our previous analysis [1] we concentrate on the selfconsistent treatment of dynamical screening and electromagnetic fields which is applicable to arbitrary nonequilibrium situations. The resulting kinetic equation generalizes previous results to quantum plasmas with full dynamical screening and includes many-body effects. It is, in particular, applicable to the interaction of dense plasmas with strong electromagnetic fields, including laser fields and x-rays. Furthermore, results for the modification of the plasma screening and the longitudinal field fluctuations due to the electromagnetic field are presented.

Two-temperature relaxation in nonideal partially ionized plasmas

Evolution equations for the coupled relaxation of densities and temperatures for the components in nonideal partially ionized plasmas are given. In these equations many‐body effects, such as screening, self‐energy, and lowering of the binding energy, are included. The coupled equations are solved numerically for a hydrogen plasma consisting of electrons, protons, and atoms. Impact ionization, three‐body recombination, and elastic processes are taken into account. Thermal relaxation times are determined and the results are compared with those from the literature. The influence of many‐body effects on the evolution process are discussed. In some cases, a significantly increased lifetime of the two‐temperature regime is found.

Inverse bremsstrahlung heating rate in atomic clusters irradiated by femtosecond laser pulses

In the interaction of atomic clusters with femtosecond laser pulses, nanoplasmas with high density and high temperature are created. The heating is mainly determined by inverse bremsstrahlung (IB) due to electron-ion collisions. In many approaches for the calculation of the IB heating rate such as the Born approximation, large-angle scattering events are underestimated. However, rescattering events of an electron on the same atomic ion play an important role because they increase the amount of energy exchanged between the electrons and the laser field. In noble gas plasmas, the electron-ion interaction is often considered to take place between point-like particles. For typical noble gas clusters studied in experiments, one is advised to take into account not only the screening by the surrounding plasma medium but also the inner structure of the ions what can be accomplished by the use of appropriate model potentials. In the present paper, the IB heating rate is calculated from the classical simulation of individual electron trajectories. Results are presented for xenon clusters and argon clusters with different degree of ionization. Especially for higher energies, the consideration of the ionic structure increases the heating rate compared with the scattering on point-like particles. The Born approximation, however, overestimates this effect.In the interaction of atomic clusters with femtosecond laser pulses, nanoplasmas with high density and high temperature are created. The heating is mainly determined by inverse bremsstrahlung (IB) due to electron-ion collisions. In many approaches for the calculation of the IB heating rate such as the Born approximation, large-angle scattering events are underestimated. However, rescattering events of an electron on the same atomic ion play an important role because they increase the amount of energy exchanged between the electrons and the laser field. In noble gas plasmas, the electron-ion interaction is often considered to take place between point-like particles. For typical noble gas clusters studied in experiments, one is advised to take into account not only the screening by the surrounding plasma medium but also the inner structure of the ions what can be accomplished by the use of appropriate model potentials. In the present paper, the IB heating rate is calculated from the classical simulation of ...

Thomson scattering in dense plasmas with density and temperature gradients

Abstract Collective X-ray Thomson scattering has become a versatile tool for the diagnostics of dense plasmas. Assuming homogeneous density and temperature throughout the target sample, these parameters can be determined directly from the plasmon dispersion and the ratio of plasmon amplitudes via detailed balance. In inhomogeneous media, the scattering signal is an average of the density and temperature dependent scattering cross-section weighted with the density and temperature profiles. We analyse Thomson scattering spectra in the XUV range from near solid density hydrogen targets generated by free electron laser radiation. The influence of plasma inhomogeneities on the scattering spectrum is investigated by comparing density and temperature averaged scattering signals to calculations assuming homogeneous targets. We find discrepancies larger than 10% between the mean electron density and the effective density as well as between the mean temperature and the effective temperature.

Hypernetted Chain Calculations for Two-Component Plasmas

We have performed HNC calculations for dense beryllium plasma as studied experimentally using x-ray Thomson scattering, recently. We treated non-equilibrium situations with different electron and ion temperatures which are relevant in pump-probe experiments on ultra-short time scales. To consider quantum effects adequately, we used effective pair potentials to describe the interactions. Results are compared with classical as well as quantum corrected Debye model calculations. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantum kinetic equations, memory effects, conservation laws

Abstract In the framework of real-time Greens functions, a general non-Markovian Boltzmann equation including initial correlations, full time retardation (memory) and self energy is considered. This equation conserves the total (kinetic plus potential) energy. Two approximations of this very general equation are investigated: (i) the first order expansion with respect to the retardation and (ii) the first Born approximation for the scattering T-matrix (non-Markovian Landau equation). The influence of memory and damping effects on the relaxation of the one-particle distribution and of the kinetic energy is demonstrated by a numerical analysis.

Reaction rates for dense nonideal alkali plasmas

The quantum statistical theory of collisional rate coefficients of nonideal plasmas, developed in some recent papers for hydrogen, is applied to calculate the impact ionization and three-body recombination coefficients for the valence electron of the atoms in Li-, Na-, K-, Rb- and Cs-plasmas. The many-body effects are included by static screening in the quasiparticle energy shifts and in the effective potentials. Because the ionization of alkali atoms represents a multielectron problem, an effective two-body electron-ion potential is derived using the method of close coupling equations of quantum scattering theory. The latter is replaced by a suited two-parametric pseudopotential taking into account the shielding effects of the inner shell atomic electrons. Finally, numerical results are given for the ionization cross sections and the rate coefficients.

Quantum kinetic theory of plasmas in strong laser fields

A kinetic theory for quantum many-particle systems in time-dependent electromagnetic fields is developed based on a gauge-invariant formulation. The resulting kinetic equation generalizes previous results to quantum systems and includes many-body effects. It is, in particular, applicable to the interaction of strong laser fields with dense correlated plasmas.

Population kinetics of dense hydrogen-like plasmas

The population kinetics of dense hydrogen-like plasmas is investigated within a quantum kinetic approach. Recombining plasmas are considered for the isothermal, the isoenergetic and the adiabatic case. In the latter cases, additionally to the rate equations, balance equations for the temperatures of the plasma species are used. The influence of many-body effects on the relaxation is discussed. Comparisons are given with a recent experiment on laser-produced carbon plasmas.