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In this article, we would like to pay tribute to Gabor Kalman, outlining his contribution to a model widely used in dense plasma physics: the high‐temperature Thomas–Fermi model. The approach of Ruoxian Ying and Kalman relies on the separation of the bound and free electrons, a physically reasonable definition of the bound electrons, a description of the source density in the Poisson equation through the electron–ion and ion–ion pair correlation functions and a determination of the degree of ionization from the minimization of the total free energy. We also report on different approximations of the function Φ$$ \varPhi $$, which is a cornerstone of the original Thomas‐Femi model. [ABSTRACT FROM AUTHOR]

In this work, we prepare a simulation framework for a high‐accuracy numerical study of electron–ion temperature relaxation in nonideal (strongly coupled) plasmas. The existing relaxation rate theories require either parameter selection or some pre‐knowledge of the electron–ion correlation functions and effective interaction potentials. This makes non‐equilibrium classical and quantum molecular dynamics simulations a crucial stage in the study of energy transfer rates. We begin by revisiting the classical molecular dynamics simulations of a system of equally charged particles with different masses on a neutralizing background. We accurately simulate this simple ab‐initio (parameterless) system with controlled precision in terms of number of particles, mass ratio, and energy convergence. The predictions for the equally charged system are compared to the previous simulations and theories, which are reproduced with higher accuracy. We also perform a series of classical molecular dynamics simulations of the system of oppositely charged particles with the corrected Kelbg potential based on the quantum statistical approach. We analyze the differences and similarities between the same‐charge and opposite‐charge systems. Some remarks about the forthcoming application of quantum simulations with the help of WPMD or WPMD‐DFT methods are given. [ABSTRACT FROM AUTHOR]

Warm dense matter—an exotic, highly compressed state on the border between solid and plasma phases is of high current interest, in particular for compact astrophysical objects, high‐pressure laboratory systems, and inertial confinement fusion. For many applications, the interaction of quantum plasmas with energetic particles is crucial. Moreover, often the system is driven far out of equilibrium. In that case, there is high interest in time‐dependent simulations to understand the physics, in particular, during thermalization. Recently a novel many‐particle technique, the G1–G2 scheme was presented with reference to the study by Schlünzen et al. (2020) which allows for first‐principle simulations of the time evolution of interacting quantum systems. Here we apply this scheme to a spatially uniform dense quantum plasma [ABSTRACT FROM AUTHOR]

Günter Kelbg did remarkable early work in the field of quantum statistical physics, in particular for dense quantum plasmas. In 2022 we celebrated his 100th birthday. On this occasion, we give a brief overview of his main scientific achievements in the field of quantum plasmas, complemented by some biographical background of his research and teaching environment at Rostock University. Kelbg's main achievement is the derivation of a regularized quantum pair potential. We demonstrate that it is still of high relevance for molecular dynamics and quantum Monte Carlo simulations of dense plasmas. [ABSTRACT FROM AUTHOR]

We present extensive new ab initio path integral Monte Carlo (PIMC) results for an electron gas at warm dense matter conditions that is subject to multiple harmonic perturbations. In addition to the previously investigated nonlinear effects at the original wave number (Dornheim et al., Phys. Rev. Lett., 2020, 125, 085001) and the excitation of higher harmonics (Dornheim et al., Phys. Rev. Res., 2021, 3, 033231), the presence of multiple external potentials leads to mode‐coupling effects, which constitute the dominant nonlinear effect and lead to a substantially more complicated density response compared to linear response theory. One possibility to estimate mode‐coupling effects from a PIMC simulation of the unperturbed system is given in terms of generalized imaginary‐time correlation functions that have been recently introduced by Dornheim et al. (J. Chem. Phys., 2021, 155, 054110). In addition, we extend our previous analytical theory of the nonlinear density response of the electron gas in terms of the static local field correction (Dornheim et al., 2020, Phys. Rev. Lett., 125, 235001), which allows for a highly accurate description of the PIMC results with negligible computational cost. [ABSTRACT FROM AUTHOR]

Warm dense matter (WDM), an exotic, highly compressed state of matter between solid and plasma phases, is of high current interest, in particular for astrophysics and inertial confinement fusion. For the latter, in particular the propagation of compression shocks is crucial. The main unknown in the shock propagation in WDM is the behaviour of the electrons since they are governed by correlations, quantum and spin effects that need to be accounted for simultaneously. Here we describe the shock dynamics of the warm dense electron gas using a quantum hydrodynamic model. From the numerical hydrodynamic simulations, we observe that the quantum Bohm pressure induces shear force that weakens the formation and strength of the shock. In addition, the Bohm pressure induces an electron density response that takes the form of oscillations. This is confirmed by the theoretical analysis of the early stage of the shock formation. Our theoretical and numerical analyses allow us to identify characteristic dimensionless shock propagation parameters at which the effect of the Bohm force is important. [ABSTRACT FROM AUTHOR]

The screening of a test charge by partially degenerate non‐ideal free electrons at conditions related to warm dense matter and dense plasmas is investigated using linear response theory and the local field correction based on ab initio Quantum Monte‐Carlo simulations data. The analysis of the obtained results is performed by comparing to the random phase approximation and the Singwi–Tosi–Land–Sjölander approximation. The applicability of the long‐wavelength approximation for the description of screening is investigated. The impact of electronic exchange‐correlations effects on structural properties and the applicability of the screened potential from linear response theory for the simulation of the dynamics of ions are discussed. [ABSTRACT FROM AUTHOR]

In a recent Letter [T. Dornheim et al., Phys. Rev. Lett. 125, 085001 (2020)], we have presented the first ab initio results for the nonlinear density response of electrons in the warm dense matter (WDM) regime. In the present work, we extend these efforts by carrying out extensive new path integral Monte Carlo simulations of a ferromagnetic electron gas that is subject to an external harmonic perturbation. This allows us to unambiguously quantify the impact of spin‐effects on the nonlinear density response of the warm dense electron gas. In addition to their utility for the description of WDM in an external magnetic field, our results further advance our current understanding of the uniform electron gas as a fundamental model system, which is important in its own right. [ABSTRACT FROM AUTHOR]

The uniform electron gas (UEG) is one of the key models for the understanding of warm dense matter—an exotic, highly compressed state of matter between solid and plasma phases. The difficulty in modelling the UEG arises from the need to simultaneously account for Coulomb correlations, quantum effects, and exchange effects, as well as finite temperature. The most accurate results so far were obtained from quantum Monte Carlo (QMC) simulations with a variety of representations. However, QMC for electrons is hampered by the fermion sign problem. Here, we present results from a novel fermionic propagator path integral Monte Carlo in the restricted grand canonical ensemble. The ab initio simulation results for the spin‐resolved pair distribution functions and static structure factor are reported for two isotherms θ=1,2 (T in the units of the Fermi temperature). Furthermore, we combine the results from the linear response theory in the Singwi‐Tosi‐Land‐Sjölander scheme with the QMC data to remove finite‐size errors in the interaction energy. We present a new corrected parametrization for the interaction energy v(rs,θ) and the exchange–correlation free energy fxc(rs,θ) in the thermodynamic limit, and benchmark our results against the restricted path integral Monte Carlo by Brown et al. [Phys. Rev. Lett. 110, 146405 (2013)] and configuration path integral Monte Carlo/permutation‐blocking path integral Monte Carlo by Dornheim et al. [Phys. Rev. Lett. 117, 115701 (2016)]. [ABSTRACT FROM AUTHOR]

We present the story of full three‐dimensional ab initio simulation techniques of dense plasmas based on the Kohn–Sham realization of the density functional theory, starting from early attempts using the Car–Parrinello method to the most recent approaches based on density matrix. We recall the decisive role played by two experiments, one on the Nova laser and the other at a much smaller scale, with pulsed electrical discharges. We emphasize that the essential roles of the Physics of Non‐Ideal Plasmas (PNP) and Strongly Coupled Coulomb Systems (SCCS) conference series were most results, and simulation tools were presented and discussed under the benevolent presence of Vladimir Fortov. [ABSTRACT FROM AUTHOR]

Warm dense matter (WDM) is an exotic state on the border between condensed matter and dense plasmas. Important occurrences of WDM include dense astrophysical objects, matter in the core of our Earth, and matter produced in strong compression experiments. As of late, x‐ray Thomson scattering has become an advanced tool to diagnose WDM. The interpretation of the data requires model input for the dynamic structure factor S(q, ω) and the plasmon dispersion ω(q). Recently, the first ab initio results for S(q, ω) of the homogeneous warm dense electron gas were obtained from path integral Monte Carlo simulations (Dornheim et al., Phys. Rev. Lett., 121, 255001, 2018). Here, we analyse the effects of correlations and finite temperature on the dynamic dielectric function and the plasmon dispersion. Our results for the plasmon dispersion and damping differ significantly from the random‐phase approximation and from earlier models of the correlated electron gas. Moreover, we show when commonly used weak damping approximations break down and how the method of complex zeroes of the dielectric function can solve this problem for WDM conditions. [ABSTRACT FROM AUTHOR]

Data from recent laser‐shock experiments or from simulations using density functional theory (DFT), molecular dynamics (MD), path integrals etc. that are available for warm dense carbon are compared with the corresponding results predicted using the neutral pseudo‐atom [NPA] method. The NPA results are in good agreement not only with the 3–12 g/cm3 regimes that have been studied via path‐integral Monte Carlo methods but even at low densities and low temperatures where transient covalent bonding dominates ionic correlations. Thus, the “prepeak” due to the C–C bond at ∼1.4 Å and other features found in the pair distribution function g[r] from DFT + MD simulations at 0.86 eV and 3.7 g/cm3 and other densities etc. are recovered accurately in the NPA + HNC (hypernetted‐chain) calculations. The exploration of regimes not covered by previous studies is presented together with evidence of an unusual phase transition of carbon in the liquid state to a vapour, as seen from the loss of ion–ion correlations. An abrupt decrease in the number of free electrons per ion occurs simultaneously. A metallic liquid with strong ionic correlations arising from transient C–C bonds becomes a metallic vapour. This occurs when carbon at the density of ∼1.0 g/cm3 and mean ionization Z = 4 transits abruptly to a disordered mono‐atomic vapour at 7 eV, with Z ≃ 3. The behaviour of the pressure, compressibility and the electrical conductivity as well as physical quantities available from X‐ray Thomson scattering are presented across the tentatively proposed phase transition. [ABSTRACT FROM AUTHOR]

The equation of state, composition, and electrical conductivity are calculated for the supercritical ionized fluid of aluminium. For calculation, we used the previously proposed chemical plasma model called “3+”‐component. The model includes atoms, immersed in jellium, and thermal electrons and ions. Part of the bound states electron density is referred to as jellium. The density of electron jellium increases with the compression of atomic gas and does not depend on temperature directly. The emergence of jellium can be termed the process of ionization in the full sense of the word. The conductivity of dense vapours is determined by the sum of the conductivity of the thermal electrons, calculated according to the Frost formula, and the conductivity of jellium, calculated according to the Regel–Ioffe formula. At compression, the electrical conductivity passes through the minimum from the conductivity of thermal electrons to the conductivity of electrons of jellium accordingly. Calculations of the equation of state and the electrical conductivity of supercritical metal vapours agree well with physical and numerical experimental data. [ABSTRACT FROM AUTHOR]

We review the development of high‐energy‐density physics (HEDP) driven by intense particle beams, and give an account of the current status and the anticipated developments in the near future. Since progress of this field is strongly coupled to the technical development of high‐current accelerators, we report on the recent results in specific areas of high‐current accelerators relevant to HEDP. These are related to dynamic vacuum problems and the activation of accelerator structural material by high‐current, high‐energy particle beams, and we give an example of dense plasma diagnostics with high‐energy protons. The reported results have been achieved at existing machines at the Gesellschaft für Schwerionenforschung (GSI‐Darmstadt), the Institute of Theoretical and Experimental Physics in Moscow (ITEP‐Moscow), and the Institute of Modern Physics (IMP‐Lanzhou), and they are relevant for facilities under construction such as the FAIR facility in Darmstadt and the High Intensity Accelerator Facility (HIAF) proposed in China. [ABSTRACT FROM AUTHOR]

Planetary physics research is an important part of the high energy density ( HED) physics programme at the Facility for Antiprotons and Ion Research ( FAIR) at Darmstadt. In this paper, we report numerical simulations of a proposed experiment named LAboratory PLAnetary Sciences ( LAPLAS). These simulations show that in such experiments, an Fe sample can be imploded to extreme physical conditions that are expected to exist in the interior of the Earth and in the interior of more massive rocky planets named, super-Earths. The LAPLAS experiments will thus provide very valuable information on the equation-of-state ( EOS) and transport properties of HED Fe, which will help the scientists to understand the structure and evolution of the planets in our solar system and of the extrasolar system planets. [ABSTRACT FROM AUTHOR]

This paper presents numerical simulations of the thermodynamic and hydrodynamic response of a solid copper cylindrical target that is subjected to the full impact of one future circular collider (FCC) ultra-relativistic proton beam. The target is facially irradiated so that the beam axis coincides with the cylinder axis. The simulations have been carried out employing an energy deposition code, FLUKA, and a 2D hydrodynamic code, BIG2, iteratively. The simulations show that, although the static range of a single FCC proton and its shower in solid copper is ∼1.5 m, the full beam may penetrate up to 350 m into the target as a result of hydrodynamic tunnelling. Moreover, simulations also show that a major part of the target is converted into high energy density ( HED) matter, including warm dense matter ( WDM) and strongly coupled plasma. [ABSTRACT FROM AUTHOR]

The uniform electron gas is a key model system in the description of matter, including dense plasmas and solid-state systems. However, the simultaneous occurrence of quantum, correlation, and thermal effects makes the theoretical description challenging. For these reasons, over the last half century, many analytical approaches have been developed, the accuracy of which has remained unclear. We have recently obtained the first ab initio data for the exchange correlation free energy of the uniform electron gas, which now provides the opportunity to assess the quality of the mentioned approaches and parameterizations. Particular emphasis is placed on the warm, dense matter regime, where we find significant discrepancies between the different approaches. [ABSTRACT FROM AUTHOR]

The ion potential in the warm dense matter regime exhibits wake effects due to streaming degenerate electrons and has been discussed previously [Phys. Rev. E 91, 023102 (2015)]. Here, we extend the analysis with particular focus on anomalous wake effects that is (i) the collision-induced wake amplification, and (ii) the non-monotonic temperature dependence of the wake amplitude. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

We present in this work, a thermodynamic perturbation theory for equation of state of hydrogen and helium in the warm dense regime. The system is modeled as a mixture of classical point ions and quantum electrons. A perturbation series for Helmholtz free energy and correlation functions of the ions and electrons as a function of density and temperature is proposed. Combining the classical thermodynamic perturbation theory and the orbitial-free quantum hyper-netted chain theory, a systematic procedure to obtain the terms of the perturbation series is developed. The ion-ion correlations are treated within the hyper-netted chain approximation and the ion-electron correlations are treated within the Thomas-Fermi-Dirac-Weizsäcker approximation. The method has been applied to obtain isotherms of hydrogen and helium in the warm dense regime. The isotherms are compared with available ab-initio data and the results are analyzed. A good agreement with ab-initio data has been observed for pressures greater than one Mbar. Advantages and limitations of the present method are discussed along with possible future improvements. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Non-equilibrium two-temperature warm dense metals consist of the ion subsystem that is subjected to structural transitions and involved in the mass transfer, and the electron subsystem that in various pulsed experiments absorbs energy and then evolves together with ions to equilibrium. Definition of pressure in such non-equilibrium systems causes certain controversy. In this work we make an attempt to clarify this definition that is vital for proper description of the whole relaxation process. Using the density functional theory we analyze on examples of Al and Au electronic pressure components in warm dense metals. Appealing to the Fermi gas model we elucidate a way to find a number of free delocalized electrons in warm dense metals. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

When heated at constant volume (isochoric heating) a hot and dense plasma (10-5000 eV, 1-50 g/cm3) exhibits the same persistent microscopic structure over a wide range of temperatures as intuited long time ago by Laughlin [1]. In this steady-state regime, which depends on the chosen density and on the atomic number, the static structure is essentially independent of the temperature and results in the subtle balance between ionization and temperature leading to a constant coupling between ions. This behavior, suggested by simulations, is confirmed by an analysis in the framework of the Thomas-Fermi scaling laws and is driven by the ionization dynamics which regulates the coupling between ions and electrons. A simple fit is derived allowing for predicting the occurrence of this self-organized regime: the Γ-plateau. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

We develop ion-ion pair potentials for Al, Na and K for densities and temperatures relevant to the warm-densematter (WDM) regime. Furthermore, we emphasize non-equilibrium states where the ion temperature T i differs from the electron temperature T e. This work focuses mainly on ultra-fast laser-metal interactions where the energy of the laser is almost exclusively transferred to the electron sub-system over femtosecond time scales. This results in a two-temperature system with T e > T i and with the ions still at the initial room temperature T i = T r. First-principles calculations, such as density functional theory (DFT) or quantum Monte Carlo, are as yet not fully feasible for WDM conditions due to lack of finite- T features, e.g. pseudopotentials, and extensive CPU time requirements. Simpler methods are needed to study these highly complex systems. We propose to use two-temperature pair potentials U ii( r, T i, T e) constructed from linear-response theory using the non-linear electron density n ( r) obtained from finite- T DFT with a single ion immersed in the appropriate electron fluid. We compute equilibrium phonon spectra at T r which are found to be in very good agreement with experiments. This gives credibility to our non-equilibrium phonon dispersion relations which are important in determining thermophysical properties, stability, energy-relaxation mechanisms and transport coefficients. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

High energy lasers are a unique tool to create high pressure states above 10 Mbar at ns time scales, which allow to study material properties under these extreme conditions. These conditions are, for example, comparable with planetary cores, where material properties play an important role for the properties and evolution of a planet. The rapid compression allows also to study dynamic effects of phase transitions as compression times are comparable to relaxation times. We will present recent results of laser compression of iron reaching conditions of so called 'super-Earth' cores. A description of the compression schemes as well as present and future diagnostics is presented. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Diffraction corrected effective and Coulomb electron-ion interactions are worked out for helium-like ions in plasmas of warm dense matter concern, through explicit wave functions derived within a suitable quantum defect frame work. Very significant departures from the corresponding and usual hydrogen-like expressions are reported. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Warm dense matter conductivity and reflectivity are investigated by means of density functional theory. Both one- and two-temperature cases are considered. One-temperature mode is related to equilibrium state where temperature of electrons and ions are equal. As an example of one-temperature system xenon plasma is studied. The reflectivity of shock-compressed dense xenon plasma is calculated and compared with experimental data. Two-temperature mode is associated with different temperature of electrons and ions. The thermal conductivity of aluminium and gold in such mode is examined. The comparison of obtained results with theoretical model based on Boltzmann equation is conducted. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

We discuss simplified formulations for the electrical conductivity [1] and compare them with experimental results of various expanded metals recently published [2]. We use formulations designed for fully ionized hydrogen in a broader range of non-fully ionized materials of various atomic number through the use of an effective coupling parameter based on estimations of the ionization. The ionizations are deduced from average-atom calculations and compared with faster Thomas-Fermi-Dirac estimations. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Warm dense matter is the state between the heated condensed matter and plasma. The importance of the development of warm dense matter theoretical description is determined by the fact that such conditions may arise in the variety of different scientific and industrial applications. For instance, warm dense matter is formed: in the matter impacted by femto- and picoseconds laser pulses; in nuclear materials at the formation of radiation track, etc. In these phenomena, the initial state of the system is a two-temperature state and the electron temperature may be several orders higher than the ion one. In this work, the attempt of development of the united atomistic model of a warm dense matter is carried out. The special consideration is given to the twotemperature effects and the influence of the electron pressure on the behavior of ions. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]