Wolf Kraeft - Academia.edu (original) (raw)

Papers by Wolf Kraeft

Physical Review E, 2017

The properties of a bound electron system immersed in a plasma environment are strongly modified ... more The properties of a bound electron system immersed in a plasma environment are strongly modified by the surrounding plasma. The modification of an essential quantity, the ionization energy, is described by the electronic and ionic self-energies including dynamical screening within the framework of the quantum statistical theory. Introducing the ionic dynamical structure factor as the indicator for the ionic microfield, we demonstrate that ionic correlations and fluctuations play a critical role in determining the ionization potential depression. This is in particular true for mixtures of different ions with large mass and charge asymmetry. The ionization potential depression is calculated for dense aluminum plasmas as well as for a CH plasma and compared to the experimental data and more phenomenological approaches used so far.

Lecture Notes in Computer Science, 2001

Physical Review E, 1995

From the Bethe-Salpeter equation for the two-particle (proton-electron) Green function, an effect... more From the Bethe-Salpeter equation for the two-particle (proton-electron) Green function, an effective Schrodinger wave equation can be derived for a hydrogen atom in a hydrogen plasma, which describes the perturbation of atomic energy levels and eigenstates by many-particle plasma effects (Pauli blocking, exchange and dynamic self-energy, and interaction-potential correction due to dynamic screening). Taking full account of dynamic screening by the random-phase approximation dielectric function, we solved the effective wave equation for nondegenerate plasmas. For bound atomic states, the plasma effects nearly compensate one another and the energy levels depend only weakly on density. In contrast, the lowering of the continuum edge is not diminished by such compensation, so that the bound states successively merge into the continuum with increasing plasma density. As our results show, reliable calculations have to incorporate dynamic screening, since the use of static screening (which greatly facilitates calculations) may lead to substantial errors, even at low densities.

Physical Review E, 2002

The mean value of the kinetic energy of a quantum plasma is investigated in Hartree-Fock and Mont... more The mean value of the kinetic energy of a quantum plasma is investigated in Hartree-Fock and Montroll-Ward approximations using the method of thermodynamic Green's functions. Usually, one finds the kinetic energy to be larger than that of an ideal plasma due to the interaction between the particles in the system. However, also the opposite case is possible, i.e., a decrease of the kinetic energy compared to that of the ideal gas. This special correlation effect is found for temperatures of about 10 6 K and densities between 10 21 and 10 26 cm Ϫ3. Here, the single-particle distribution function is shifted towards smaller momenta, and the binary distribution is changed.

Contributions to Plasma Physics, 2015

Warm dense matter is of interest for modeling the interiors of planets and Brown Dwarfs. Correspo... more Warm dense matter is of interest for modeling the interiors of planets and Brown Dwarfs. Corresponding pumpprobe experiments are performed at free electron laser facilities such as FLASH, LCLS or the future XFEL in Hamburg. X-ray Thomson scattering is of special interest to extract the plasma parameters. In order to explain or predict the X-ray Thomson scattering spectra, simulations on the structural properties of plasmas are performed. While ab initio simulations are computationally expensive, semi-classical approaches can deliver results fast for pair distribution functions and static structure factors, even for dense systems. We solve the Ornstein-Zernike equation within the hypernetted chain approximation for dense multi-component plasmas using the classical-map method. This approach proposes to treat the quantum features of the electrons using an adapted temperature for the electron system while the ions are treated classically. Results for pair distribution functions and static structure factors are presented for dense hydrogen, beryllium, carbon and CH plasmas.

Phys. Rev. E, 1996

In the framework of real-time Green's functions, the general kinetic equations are investigated i... more In the framework of real-time Green's functions, the general kinetic equations are investigated in a firstorder gradient expansion. Within this approximation, the problem of the reconstruction of the two-time correlation functions from the one-time Wigner function was solved. For the Wigner function, a cluster expansion is found in terms of a quasiparticle distribution function. In equilibrium, this expansion leads to the well-known generalized Beth-Uhlenbeck expression of the second virial coefficient. As a special case, the T-matrix approximation for the self-energy is investigated. The quantum kinetic equation derived thus has, besides the ͑Markovian͒ Boltzmann collision integral, additional terms due to the retardation expansion which reflect memory effects. Special interest is paid to the case that bound states exist in the system. It is shown that the bound state contribution, which can be introduced via a bilinear expansion of the two-particle T matrix, follows from the first-order retardation term in the general kinetic equation. The full Wigner function is now a sum of one function describing the unbound particles and another one for the bound state contribution. The latter two functions have to be determined from a coupled set of kinetic equations. In contrast to the quantum Boltzmann equation, energy and density of a nonideal system are conserved. ͓S1063-651X͑96͒11009-6͔

Quantum Statistics of Charged Particle Systems, 1986

The Astrophysical Journal, 1998

Misunderstandings have occurred regarding the conclusions of the paper "" On Density-dependent an... more Misunderstandings have occurred regarding the conclusions of the paper "" On Density-dependent and Temperature-dependent Ground-State and Continuum E †ects in the Equation of State for Stellar Interiors ÏÏ by S. Arndt, W. & A. Nayfonov (1998, ApJ, 498, 349). On occasion, its results have Da ppen, been interpreted as if it showed basic Ñaws in the general theory of dynamical screening. The aim of this comment is to emphasize the context in which the conclusions of the paper must be understood in order to avoid misinterpretations.

Physics of Plasmas, 2000

In this paper, nonequilibrium properties of strongly coupled plasmas are considered. Usually, suc... more In this paper, nonequilibrium properties of strongly coupled plasmas are considered. Usually, such problems are dealt with using Boltzmann– or Lenard–Balescu-type equations. However, for the application to strongly coupled plasmas, these equations exhibit several shortcomings. So, it is not possible (i), to describe the short time kinetics, (ii), to recover the correct (energy) conservation laws and thermodynamics, and, (iii), to account for the formation or destruction of bound states. Therefore, the kinetics of strongly coupled plasmas is considered starting from the Kadanoff–Baym equations, which are known to overcome the above limitations. This is demonstrated by a numerical solution of the two-time Kadanoff–Baym equations in second Born approximation. To be able to discuss approximations which are physically more interesting, it is advantageous to proceed to the time diagonal Kadanoff–Baym equations. In first order gradient expansion, generalizations of the Boltzmann and of the...

Physical Review E, 2004

In spite of the simple structure of hydrogen, up to now there is no unified theoretical and exper... more In spite of the simple structure of hydrogen, up to now there is no unified theoretical and experimental description of hydrogen at high pressures. Recent results of Z-pinch experiments show a large deviation from those obtained by laser driven ones. Theoretical investigations including ab initio computer simulations show considerable differences at such extreme conditions from each other and from experimental values. We apply the reaction ensemble Monte Carlo technique on one hand and a combination of the hypernetted chain approximation with the mass action law on the other to study the behavior of dense hydrogen at such conditions. The agreement between both methods for the equation of state and for the Hugoniot curve is excellent. Comparison to other methods and experimental results is also performed.

physica status solidi (b), 1975

Physica A: Statistical Mechanics and its Applications, 1986

ABSTRACT

Journal of Physics: Condensed Matter, 1996

High Energy Density Physics, 2009

Collective X-ray Thomson scattering has become a versatile tool for the diagnostics of dense plas... more 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 found discrepancies larger than 10% between the mean electron density and the effective density as well as between the mean temperature and the effective temperature.

Le Journal de Physique IV, 2000

It is shown how single and two particle properties enter the equation of state of dense plasmas. ... more It is shown how single and two particle properties enter the equation of state of dense plasmas. Results are given for weakly nonideal plasmas which are of interest for helioseismology and inertial fusion.

Zeitschrift f�r Physik, 1968

If a quantum-mechanical potential is introduced the calculation of the quantummechanical binary d... more If a quantum-mechanical potential is introduced the calculation of the quantummechanical binary distribution function for a system with Coulomb interaction is reduced to the well-known mathematical formalism of classical statistical mechanics in the case of n23< 1 (2 being the thermal wavelength). The two-particle quantummechanical potential is determined by the two-particle Slater sum. In this paper we calculated the two-particle Slater sum using an expansion according to e z and the resolvent formalism. From the binary distribution function the correlation energy and the free energy as well were determined up to order e 6. Symmetry effects were taken into account.

Beiträge aus der Plasmaphysik, 1970

Using the resolvent representation of the binary Boltzmann operator the second virial coefficient... more Using the resolvent representation of the binary Boltzmann operator the second virial coefficient is expressed by a contour integral over the Jost function of the two‐particle scattering problem. This new formula is useful for many general investigations and also for practical calculations if the Jost functions for the scattering problem are known. As special examples real gases with hard core potentials and fully ionized plasmas with Coulomb interactions are treated. For the plasma case the second virial coefficient is derived without approximations with respect to the interaction parameter. The contribution of the bound states to the free energy is discussed and compared with the effect of ionic association to the classical free energy and the conductivity.

Contributions to Plasma Physics, 2005

We present a detailed analysis of temperature‐dependent effective quantum pair potentials. These ... more We present a detailed analysis of temperature‐dependent effective quantum pair potentials. These potentials are derived from first‐principle path integral Monte Carlo simulations and are accurate even at strong coupling and partial ionization. They can be efficiently used in molecular‐dynamics (MD) simulations to obtain accurate thermodynamic and dynamic properties of strongly coupled hydrogen down to the temperatures of about 60 000 K. Furthermore, using spin‐dependent pair potentials, dynamic structure factors and spin‐density correlation functions were calculated for different values of coupling and degeneracy. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)