A. Wierling

Inverse bremsstrahlung of hot, weakly coupled plasmas

The free–free absorption coefficient for radiation in hot, weakly coupled plasmas is determined from a systematic approach to the dynamical conductivity. Based on a generalized linear-response theory, it is expressed in terms of determinants of equilibrium correlation functions permitting a perturbative treatment. Within a Gould–DeWitt-type approach, dynamical screening is accounted for by a partial summation of loop diagrams, while strong collisions are treated by a ladder approximation. Known expressions for the absorption coefficient are reproduced when considering certain limits. A comparison is made with simulation results.

Influence of local-field corrections on Thomson scattering in collision-dominated two-component plasmas

The dynamic structure factor, which determines the Thomson scattering spectrum, is calculated via an extended Mermin approach. It incorporates the dynamical collision frequency as well as the local-field correction factor. This allows to study systematically the impact of electron-ion collisions as well as electron-electron correlations due to degeneracy and short-range interaction on the characteristics of the Thomson scattering signal. As such, the plasmon dispersion and damping width is calculated for a two-component plasma, where the electron subsystem is completely degenerate. Strong deviations of the plasmon resonance position due to the electron-electron correlations are observed at increasing Brueckner parameters r(s). These results are of paramount importance for the interpretation of collective Thomson scattering spectra, as the determination of the free electron density from the plasmon resonance position requires a precise theory of the plasmon dispersion. Implications due to different approximations for the electron-electron correlation, i.e., different forms of the one-component local-field correction, are discussed.

Reflectivity of shock compressed xenon plasma

Experimental results [1] for the reflection coefficient of shock-compressed dense xenon plasmas at pressures of 1.6 – 17 GPa and temperatures around 30 000 K using a laser beam with λ = 1.06 μm are compared with calculations based on different theoretical approaches to the dynamical collision frequency. It is found that a reasonable description can be given assuming a spatial electron density profile corresponding to a finite width of the shock wave front of about 2 · 10–6 m.

Lindhard dielectric function in the relaxation-time approximation and generalized linear response theory

Abstract Within a generalized linear response theory, an expression for the dielectric function is derived which allows to include the effect of collisions. Former attempts to include collisions into the Lindhard dielectric function by a relaxation time are discussed and systematic improvements are outlined.

Reflectivity in shock wave fronts of xenon

New results for the reflection coefficient of shock-compressed dense xenon plasmas at pressures of 1.6–20 GPa and temperatures around 30 000 K are interpreted. Reflectivities typical of metallic systems are found at high densities. A consistent description of the measured reflectivities is achieved if a finite width of the shock wave front is considered. Several mechanisms to give a microscopic explanation for a finite extension of the shock front are discussed.

Bremsstrahlung vs. Thomson scattering in VUV-FEL plasma experiments

Abstract We determine the spectral photon yield from a hot dense plasma irradiated by VUV-FEL light in a Thomson scattering experiment. The Thomson signal is compared to the emission background mainly caused by bremsstrahlung photons. We determine experimental conditions that allow for a signal-to-background ratio larger than unity. By derivation of the Thomson and the bremsstrahlung spectrum from linear response theory we present a consistent quantum statistical approach to both processes. This allows for a systematic treatment of medium and quantum effects such as dynamical screening and strong collisions. Results are presented for the threshold FEL-intensity as a function of density and temperature. We show that the account for quantum effects leads to larger thresholds as compared to previous work.

Extended Mermin-like Dielectric Function for a Two-Component Plasma

The Mermin dielectric function is extended by including energy conservation. Within the framework of the relaxation time ansatz, the Zubarev approach to linear response theory is used which allows to incorporate the exact Hamiltonian dynamics of the relevant degrees of freedom. Analytical expressions for the dielectric function in the entire (k, ω)-plane are given for a two-component plasma.

Quantum-statistical T-matrix approach to line broadening of hydrogen in dense plasmas

The electronic self‐energy ∑e is an important input in a quantum‐statistical theory for spectral line profile calculations. It describes the influence of plasma electrons on bound state properties. In dense plasmas, the effect of strong, i.e. close, electron‐emitter collisions can be considered by three‐particle T‐matrix diagrams. These digrams are approximated with the help of an effective two‐particle T‐matrix, which is obtained from convergent close‐coupling calculations with Debye screening. A comparison with other theories is carried out for the 2p level of hydrogen at kBT = 1 eV and ne = 2⋅1023 m−3, and results are given for ne = 1⋅1025 m−3.

Analysing brilliance spectra of a laser-induced carbon plasma

Brilliance spectra of a carbon plasma generated by subpicosecond high intensity laser pulses are analysed (Wilhein et al 1998 J. Opt. Soc. Am. 15 1235). The plasma parameters such as electron density and temperature are determined using a plasma slab model. Synthetic carbon He-α and He-β line profiles are calculated for the inferred plasma parameters by using thermodynamic Greens function, based on a microscopic quantum statistical approach assuming local thermal equilibrium. Self-absorption is taken into account considering one-dimensional radiation transport equation. The comparison between the measured spectrum and our calculated synthetic profile is good for He-α line (C V 1s2–1s2p), while discrepancies are found in the case of He-β line (C V 1s2–1s3p).

Optical properties and one-particle spectral function in non-ideal plasmas

A basic concept to calculate physical features of non-ideal plasmas, such as optical properties, is the spectral function which is linked to the self-energy. We calculate the spectral function for a non-relativistic hydrogen plasma in GW-approximation. In order to go beyond GW approximation, we include self-energy and vertex correction to the polarization function in lowest order. Partial compensation is observed. The relation of our approach to GW and GW calculations in other fields, such as the band-structure ca lculations in semiconductor physics, is discussed. From the spectral function we derive the absorption coefficient due to inverse bremsstrahlung via the polarization function. As a result, a significant reduction of the absorption as compared to the Bethe-Heitler formula for bremsstrahlung is obtained.