Biomedical subjects

R Redmer

Publications and source records attributed to R Redmer.

15 recordsLinked to original sources

PubMed2006

Collisional absorption in aluminum.

The interaction of ultrashort laser pulses with matter is a topic of growing interest. In particular, recent developments on free-electron lasers have opened an unexplored field in which many interesting physical phenomena are to be expected. Since hydrodynamic descriptions of the interaction process need a microscopic "input," a quantum statistical theory of energy absorption by matter is required. We present a kinetic theory of collisional absorption in dense plasmas and analyze the electron-ion collision frequency in warm dense aluminum in dependence on laser frequency and temperature.

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PubMed2002

Electrical conductivity of nonideal carbon and zinc plasmas: experimental and theoretical results.

Electrical conductivities of nonideal carbon and zinc plasmas have been measured in this paper. The plasma is produced by vaporizing a wire placed in a glass capillary within some hundred nanoseconds. In the case of carbon, vaporization occurs with good reproducibility when utilizing a preheating system. The particle density is in the range of n=(1-10) x 10(21) cm(-3). The plasma temperature, which is obtained by fitting a Planck function to the measured spectrum, is between 7-15 kK. Plasma radius and behavior of the plasma expansion were studied with a streak, a framing or an intensified charge coupled device camera. We compare the measured electrical conductivities with theoretical results, which were obtained solving quantum kinetic equations for the nonideal partially ionized plasmas. In this approach, the transport cross sections are calculated on the level of a T-matrix approximation using effective potentials. The plasma composition is determined from a system of coupled mass action laws with nonideality corrections.

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PubMed2001

Isentropes and Hugoniot curves for dense hydrogen and deuterium.

Multiple-shock experiments with fluid hydrogen have shown that a transition from semiconducting behavior to metal-like conductivity occurs at pressures (p) of about 140 GPa and temperatures (T) near 3000 K. We model the p-T pathway by Hugoniot curves (initial shock) and isentropes (subsequent shocks). For the calculation of these curves we apply an expression for the free energy developed recently for dense hydrogen and deuterium plasma in the regions of partial dissociation and partial ionization. Furthermore, we discuss the relations between Hugoniot curves, isentropes and the coexistence line of the plasma phase transition.

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PubMed2000

Metallization of hydrogen using heavy-ion-beam implosion of multilayered cylindrical targets.

Employing a two-dimensional simulation model, this paper presents a suitable design for an experiment to study metallization of hydrogen in a heavy-ion beam imploded multilayered cylindrical target that contains a layer of frozen hydrogen. Such an experiment will be carried out at the upgraded heavy-ion synchrotron facility (SIS-18) at the Gesellschaft für Schwerionenforschung, Darmstadt by the end of the year 2001. In these calculations we consider a uranium beam that will be available at the upgraded SIS-18. Our calculations show that it may be possible to achieve theoretically predicted physical conditions necessary to create metallic hydrogen in such experiments. These include a density of about 1 g/cm(3), a pressure of 3-5 Mbar, and a temperature of a few 0.1 eV.

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PubMed2000

Long-wavelength limit of the dynamical local-field factor and dynamical conductivity of a two-component plasma

A systematic approach to the optical conductivity is given within a dielectric function formalism. The response function as well as the dynamical local-field factor G(k-->,omega) of an electron-ion plasma can be expressed in terms of determinants of equilibrium correlation functions which allow for a perturbative treatment. The dynamical collision frequency nu(omega)=-iomega(2)(pl)G(0,omega)/omega for fully ionized weakly coupled plasmas is evaluated in the low-density limit. A renormalization function is given to describe the effects of higher moments of the distribution function, thus the Spitzer formula is reproduced in the static limit. The existence of the third moment sum rule can be shown analytically. Numerical calculations are presented for the dynamical conductivity of hydrogen plasmas at solar core conditions.

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PubMed2000

Transport coefficients for dense metal plasmas

Thermoelectric transport coefficients of metal plasmas are calculated within the linear response theory applied previously to determine the electrical conductivity of Al and Cu plasmas [R. Redmer, Phys. Rev. E 59, 1073 (1999)]. We consider temperatures of 1-3 eV and densities of 0.001-1 g/cm(3) as relevant in rapid wire evaporation experiments. The plasma composition is calculated considering higher ionization stages of atoms up to 5+, and solving the respective system of coupled mass action laws. Interactions between charged particles are treated on T matrix level. Results for the electrical conductivity of various metal plasmas are in reasonable agreement with experimental data. Thermal conductivity and thermopower are also given. In addition, we compare with experimental data for temperatures up to 25 eV and liquidlike densities.

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PubMed1999

Simulations of fluid hydrogen: comparison of a dissociation model with tight-binding molecular dynamics.

We compare the results of two complementary approaches, tight-binding molecular-dynamics simulations and a dissociation model, for determining the characteristics of dense, fluid hydrogen at pressures extending to megabars and temperatures to 10 000 K. Two tight-binding models were examined: one parametrization emphasized crystalline, molecular, and fluid properties, the other focused more on the intricate molecular interactions involving up to four hydrogen atoms. The two tight-binding cases and the dissociation model agree reasonably well for a variety of properties, including the equation of state, dissociation degree, and proton pair-correlation functions. In simulations of recently reported laser-driven shock experiments, the tight-binding and dissociative models predict different maximum compressions of four and five, respectively. We discuss the sensitivities of the models as well as give estimates for the region of validity of the chemical picture (dissociation model) and the accuracy of the dynamical picture (tight-binding simulations) in cases where molecular hydrogen still dominates the physical behavior.

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PubMed1999

Response function including collisions for an interacting fermion gas.

The response function of an interacting fermion gas is considered in the entire (k-->,omega) space. Applying a generalized linear response theory, it is expressed in terms of determinants of equilibrium correlation functions, which allow for a systematic perturbative treatment. The relation to dynamical local-field factors is given. As a special case, the dielectric function is evaluated for two-component (hydrogen) plasmas at arbitrary degeneracies. Collisions are treated in Born approximation leading to a (k-->,omega)-dependent collision integral. The link to the dynamical conductivity is given in the long-wavelength limit. Sum rules are discussed.

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PubMed1996