Search

Nature Communications 12(1), 1638 (2021). doi:10.1038/s41467-021-21756-6, Key insights in materials at extreme temperatures and pressures can be gained by accurate measurements that determine the electrical conductivity. Free-electron laser pulses can ionize and excite matter out of equilibrium on femtosecond time scales, modifying the electronic and ionic structures and enhancing electronic scattering properties. The transient evolution of the conductivity manifests the energy coupling from high temperature electrons to low temperature ions. Here we combine accelerator-based, high-brightness multi-cycle terahertz radiation with a single-shot electro-optic sampling technique to probe the evolution of DC electrical conductivity using terahertz transmission measurements on sub-picosecond time scales with a multi-undulator free electron laser. Our results allow the direct determination of the electron-electron and electron-ion scattering frequencies that are the major contributors of the electrical resistivity., Published by Nature Publishing Group UK, [London]

X-ray phase-contrast imaging of micron-scale objects and features requires high spatial coherence. These highly spatially coherent x-rays can be generated at fourth generation light sources or by passing laser-produced x-rays through a narrow slit. By coupling these sources to an isochoric heating platform, we can measure the dynamic changes in density gradients at the interface between two warm dense matter (WDM) samples with micron-scale spatial resolution. The high spatial coherence gives rise to refractive and diffractive features at the interface 1 , 2 , 3 , allowing for precise measurement of dynamic processes in WDM, including transport properties and measuring the equation of state. We will present an overview of successful experiments at both the Omega Laser Facility and the European XFEL that demonstrate our platform's high efficacy and spatial resolution for characterizing density gradients in dynamically evolving WDM systems.

Studies have shown significant discrepancies between the recent experiment [Berg et al., Phys. Rev. Lett. 120, 055002 (2018)PRLTAO10.1103/PhysRevLett.120.055002] and current theoretical calculations on the electron-impact ionization cross section of ions in warm dense magnesium. Here, we present a systematic study the effects of the ionic correlations and free-electron screening on the electron-impact ionization of ions in warm dense matter. The ionic correlation and the free-electron screening effects yield additional Hermitian terms to the calculation of the ionic central-force-field potential, which significantly change the electronic structure compared with that of the isolated ion. In calculating the electron-impact ionization, we describe the impact and ionized electrons using a damped-distorted wave function, which considers the momentum relaxation of free electrons due to collisions with other free electrons and ions. We reproduce the electron-impact ionization process for Mg^{7+} in the solid-density plasma and increase the ionization cross section by one order of magnitude compared with that of the isolated ion, which excellently agrees with the experimental result of Berg et al.

Three main paths have been selected within the ELI Beamlines research program for transforming driving laser pulses into brilliant beams of short wavelength radiation: High-order harmonic generation in gases, Plasma X-ray sources, and sources based on relativistic electron beams accelerated in laser-plasma. For each of these research areas, dedicated beamlines are built to provide a unique combination of X-ray sources to the user community. The employment of these beamlines has a well-defined balance between fundamental science and applications in different fields of science and technology. Besides those beamlines, a plasma betatron radiation source driven by the PW-class HAPLS laser system is being commissioned in the plasma physics platform to serve as a unique diagnostic tool for dense plasma and warm dense matter probing.

International audience; Second-harmonic (SH) radiation spectrum is an important diagnostic in nonlinear laser-plasma interactions.

Small and large amplitude linear and nonlinear ion acoustic and Langmuir waves are investigated for plasma with the generalized equation of state (EoS). This EoS covers a wide range of energy-density conditions from dilute classical regime up to the relativistically degenerate matter. Investigation shows that the linear dispersion and localized excitations of ion acoustic waves are significantly affected by the plasma parameters. It is also found that the criterion for ordinary soliton existence is significantly different for fully degenerate and nondegenerate plasma regimes with given electron temperature. In the nondegenerate regime, bright solitons form only for Mach numbers above a critical value which is only a function of the fluid temperature.

We assess the relative importance of ionic structure on the opacity of dense plasmas by using the potential of mean force as a scattering potential within the Kubo-Greenwood formalism. We compare results from the potential of mean force, which includes realistic ionic structure, to results using an average atom potential, which includes only a crude treatment of ionic structure. Comparisons with less approximate but more expensive DFT-MD simulations for aluminum plasma reveal that the mean force generally improves agreement for DC conductivity. We also see improvement when applying the mean force to free-free transitions, whereas for bound-bound and bound-free transitions the mean force leads to poorer agreement on transition energies. Further, we assess the impact of accounting for correlations within the plasma at the temperature and density conditions relevant to iron opacity measurements at Sandia’s Z machine facility [Bailey et al., Nature 517:56-59, 2015] and find that these correlations do not account for the discrepancy between the measurements and leading opacity calculations.

We have employed an exploding-wire compression method to measure the electrical conductivity of diamond-like carbon (DLC) in a warm dense matter (WDM) state. We generated a DLC in the WDM state using shock compression driven by an exploding-wire discharge within a rigid capillary. To elucidate the generation of DLC in WDM, we performed a 1-D magnetohydrodynamic simulation. Using the numerical results, we estimated the electrical conductivity of the DLC plasma. By comparing the time-evolutions of the voltage and current for gold and gold + DLC samples, we demonstrated that the voltage–current evolution is different in the two cases. From the absorption spectroscopy, we estimated the DLC temperature to be 8000–9000 K. By comparing the results of the experiment with those of the numerical simulation, we determined the electrical conductivity of the DLC plasma to be 106 S/m. This is relatively high compared with the electrical conductivity of the conventional carbon plasma, but the experimental results are similar to the theoretical values for diamond at solid density.

We numerically model the ablation process of a 25- $\mu \text{m}$ -thick aluminum foil driven by a pulsed-power machine that provides a 1-MA peak current in a 100-ns zero-to-peak rise time. The extended magnetohydrodynamics simulation is a discontinuous Galerkin code with Cartesian coordinates in 3-D and with 25- $\mu \text{m}$ spatial resolution. We investigate the influence of an external magnetic field normal to the foil surface, $B_{z}$ . During the foil ablation, $B_{z}=1$ T causes more nonuniform distributions of density and current compared to $B_{z}=0$ T. $B_{z}=4$ T delays the generation of surface plasma relative to the 0- and 1-T cases. The understanding of a material’s ablation as it undergoes transition from the solid to plasma phases requires detailed knowledge of a material’s equation of state and conductivity. This paper of warm dense matter and how instabilities propagate from a solid material to plasma motivates improvements to both numerical simulations and experimental diagnostics.

The European physical journal / D 75, 224 (2021). doi:10.1140/epjd/s10053-021-00235-z, In this paper, we report on the Boltzmann kinetic equation approach adapted for simulations of warm dense matter created by irradiation of bulk gold with intense ultrashort X-ray pulses. X-rays can excite inner-shell electrons, which triggers creation of deep-lying core holes. Their relaxation, especially in heavier elements such as gold (atomic number Z=79) takes complicated pathways, involving collisional processes, and leading through a large number of active configurations. This number can be so high that solving a set of evolution equations for each configuration becomes computationally inefficient, and another modeling approach should be used instead. Here, we use the earlier introduced ���predominant excitation and relaxation path��� approach. It still uses true atomic configurations but limits their number by restricting material relaxation to a selected set of predominant pathways for material excitation and relaxation. With that, we obtain time-resolved predictions for excitation and relaxation in X-ray irradiated bulk of gold, including the respective change of gold optical properties. We compare the predictions with the available data from high-energy-density experiments. Their good agreement indicates ability of the Boltzmann kinetic equation approach to describe warm dense matter created from high-Z materials after their irradiation with X rays, which can be validated in future experiments., Published by Springer, Heidelberg

The low-density limit of the electrical conductivity $\sigma(n,T)$ of hydrogen as the simplest ionic plasma is presented as function of temperature T and mass density n in form of a virial expansion of the resistivity. Quantum statistical methods yield exact values for the lowest virial coefficients which serve as benchmark for analytical approaches to the electrical conductivity as well as for numerical results obtained from density functional theory based molecular dynamics simulations (DFT-MD) or path-integral Monte Carlo (PIMC) simulations. While these simulations are well suited to calculate $\sigma(n,T)$ in a wide range of density and temperature, in particular for the warm dense matter region, they become computationally expensive in the low-density limit, and virial expansions can be utilized to balance this drawback. We present new results of DFT-MD simulations in that regime and discuss the account of electron-electron collisions by comparing with the virial expansion., Comment: Supplementary Material will be made available upon journal publication

A numerical model for laser-matter interactions in the warm dense matter regime is presented with broad applications, e.g., ablation, thermionic emission, and radiation. A unique approach is adopted, in which a complete set of collisional and transport data is calculated using a quantum model and incorporated into the classical two-temperature model for the electron and lattice-ion temperatures. The data set was produced by the average atom model that combines speed, conceptual simplicity, and straightforward numerical development. Such data are suitable for use in the warm dense matter regime, where most of the laser-matter interactions at moderate intensities occur, thus eliminating deficiencies of previous models, e.g., interpolation between solid and ideal plasma regimes. In contrast to other works, we use a more rigorous definition of solid and plasma states of the metal, based on the physical condition of the lattice, crystalline (ordered) versus melted (disordered), rather than a definition based on electron temperature. The synergy between the two-temperature and average atom models has been demonstrated on a problem involving heating and melting of the interior of Al by a short-pulse laser with duration 0.1--1 ps and laser fluences $1\ifmmode\times\else\texttimes\fi{}{10}^{3}\ensuremath{-}3\ifmmode\times\else\texttimes\fi{}{10}^{4}\phantom{\rule{0.16em}{0ex}}\mathrm{J}/{\mathrm{m}}^{2}(0.1\text{--}3\phantom{\rule{0.16em}{0ex}}\mathrm{J}/\mathrm{c}{\mathrm{m}}^{2})$. The melting line, which separates the solid and plasma regimes, has been tracked in time and space. The maximum melting depth has been determined as a function of laser fluence: ${l}_{\mathrm{melt}}(\ensuremath{\mu}\mathrm{m})\ensuremath{\cong}4\ifmmode\times\else\texttimes\fi{}{10}^{3}F\phantom{\rule{4pt}{0ex}}(\frac{\mathrm{J}}{{\mathrm{m}}^{2}})$.

There is growing interest in warm dense matter (WDM), an exotic state on the border between condensed matter and plasmas. Due to the simultaneous importance of quantum and correlation effects, WDM is complicated to treat theoretically. A key role has been played by ab initio path integral Monte Carlo (PIMC) simulations, and recently extensive results for thermodynamic quantities have been obtained. The first extension of PIMC simulations to the dynamic structure factor of the uniform electron gas was reported by Dornheim et al. [Phys. Rev. Lett. 121, 255001 (2018)]. This was based on an accurate reconstruction of the dynamic local field correction. Here we extend this concept to other dynamical quantities of the warm dense electron gas including the dynamic susceptibility, the dielectric function, and the conductivity.

Warm dense matter (WDM)---an extreme state with high temperatures and densities that occurs e.g. in astrophysical objects---constitutes one of the most active fields in plasma physics and materials science. These conditions can be realized in the lab by shock compression or laser excitation, and the most accurate experimental diagnostics is achieved with lasers and free electron lasers which is theoretically modeled using linear response theory. Here, we present first \textit{ab initio} path integral Monte Carlo results for the nonlinear density response of correlated electrons in WDM and show that for many situations of experimental relevance nonlinear effects cannot be neglected.

We present a new model of electron transport in warm and hot dense plasmas which combines the quantum Landau-Fokker-Planck equation with the concept of mean-force scattering. We obtain electrical and thermal conductivities across several orders of magnitude in temperature, from warm dense matter conditions to hot, nondegenerate plasma conditions, including the challenging crossover regime between the two. The small-angle approximation characteristic of Fokker-Planck collision theories is mitigated to good effect by the construction of accurate effective Coulomb logarithms based on mean-force scattering, which allows the theory to remain accurate even at low temperatures, as compared with high-fidelity quantum simulation results. Electron-electron collisions are treated on equal footing as electron-ion collisions. Their accurate treatment is found to be essential for hydrogen, and is expected to be important to other low-Z elements. We find that electron-electron scattering remains influential to the value of the thermal conductivity down to temperatures somewhat below the Fermi energy. The accuracy of the theory seems to falter only for the behavior of the thermal conductivity at very low temperatures due to a subtle interplay between the Pauli exclusion principle and the small-angle approximation as they pertain to electron-electron scattering. Even there, the model is in fair agreement with ab initio simulations., Comment: 17 pages, 7 figures (2 new, 1 updated)

Understanding many processes, e.g., fusion experiments, planetary interiors, and dwarf stars, depends strongly on microscopic physics modeling of warm dense matter and hot dense plasma. This complex state of matter consists of a transient mixture of degenerate and nearly free electrons, molecules, and ions. This regime challenges both experiment and analytical modeling, necessitating predictive ab initio atomistic computation, typically based on quantum mechanical Kohn-Sham density functional theory (KS-DFT). However, cubic computational scaling with temperature and system size prohibits the use of DFT through much of the warm dense matter regime. A recently developed stochastic approach to KS-DFT can be used at high temperatures, with the exact same accuracy as the deterministic approach, but the stochastic error can converge slowly and it remains expensive for intermediate temperatures ($l50\text{ }\text{ }\mathrm{eV}$). We have developed a universal mixed stochastic-deterministic algorithm for DFT at any temperature. This approach leverages the physics of KS-DFT to seamlessly integrate the best aspects of these different approaches. We demonstrate that this method significantly accelerated self-consistent field calculations for temperatures from 3 to 50 eV, while producing stable molecular dynamics and accurate diffusion coefficients.

The effective one-component plasma (EOCP) model has provided an efficient approach to obtaining many important thermophysical parameters of hot dense matter [J. Cl\'erouin, et al., Phys. Rev. Lett. 116, 115003 (2016)]. In this paper, we perform extensive quantum molecular dynamics (QMD) simulations to determine the equations of state, ionic structures, and ionic transport properties of neon and krypton within the warm dense matter (WDM) regime where the density ( $\ensuremath{\rho}$ ) is up to 12 $\mathrm{g}/{\mathrm{cm}}^{3}$ and the temperature ($T$) is up to 100 kK. The simulated data are then used as a benchmark to explicitly evaluate the EOCP and Yukawa models. It is found that, within present $\ensuremath{\rho}$-T regime, the EOCP model can excellently reproduce the diffusion and viscosity coefficients of neon and krypton due to the fact that this model defines a system which nearly reproduces the actual physical states of WDM. Therefore, the EOCP model may be a promising alternative approach to reasonably predicting the transport behaviors of matter in WDM regime at lower QMD computational cost. The evaluation of Yukawa model shows that the consideration of the energy level broadening effect in the average atom model is necessary. Finally, with the help of EOCP model, the Stokes-Einstein relationships about neon and krypton are discussed, and fruitful plasma parameters as well as a practical $\ensuremath{\rho}$-T-dependent formula of the effective coupling parameter are obtained. These results not only provide valuable information for future theoretical and experimental studies on dense neon and krypton but also reveal the applicability of the EOCP model and the limitation of the Yukawa model in WDM regime and further support the continuing search for a unified description of ionic transport in dense plasma.

Capillary discharge lasers with output at wavelength 46.9 nm offer a route to laboratory investigations of warm dense matter and may have an impact in the micro-ablation of solid samples of relevance to microelectromechanical systems. The commissioning and operation of a capillary discharge laser with 50 \(\upmu \)J output per pulse is presented alongside initial results of target ablation obtained by focusing the laser output. Studies have been undertaken relevant to the operation of the capillary laser. The capillary laser discharges a current pulse of up to 40 kAmps through an argon plasma, which is initially singly ionized via radio-frequency heating.

The idea behind this article is the work of physicists on the study of the properties of warm dense matter (WDM) and thermonuclear fusion based on the reaction of muon-catalyzed melting (μCF). According to our calculations, when the magma moves through the magma channel, hydraulic shocks occur, reaching the power of several Terawatts, which is enough to “ignite” WDM. With the subsequent mandatory expansion of WDM, a collision of electrons and positrons with the formation of muons will occur according to the well-known scheme: e- + e + → µ- + µ +. "Born" muons will replace electrons in atoms with the formation of mesoatoms. Since the Bohr radius is inversely proportional to the mass of the particle moving around the atomic nucleus, and the muon mass mμ = 206.7 me is approximately 200 times greater than the electron mass, the size of the newly formed atom orbital will be ~ 200 times smaller than the electron one. Plus, replacing electrons with muons will leads to a decrease in the diameter of protons by ~ 10%. These facts will allow the atoms to approach to a distance less than two mesoatomic units [(~ 2aμ = 2h2 / mμe2 ~ 5⋅10−13m.) - 10%], which will lead to the start of the classical reaction μCF, to the fusion of atomic nuclei and the start of the reaction natural thermonuclear fusion, the energy of which may well be the source of mysterious deep-focus earthquakes.

The free-free opacity in plasmas is fundamental to our understanding of energy transport in stellar interiors and for inertial confinement fusion research. However, theoretical predictions in the challenging dense plasma regime are conflicting and there is a dearth of accurate experimental data to allow for direct model validation. Here we present time-resolved transmission measurements in solid-density Al heated by an XUV free-electron laser. We use a novel functional optimization approach to extract the temperature-dependent absorption coefficient directly from an oversampled pool of single-shot measurements, and find a pronounced enhancement of the opacity as the plasma is heated to temperatures of order of the Fermi energy. Plasma heating and opacity enhancement are observed on ultrafast timescales, within the duration of the femtosecond XUV pulse. We attribute further rises in the opacity on ps timescales to melt and the formation of warm dense matter.

Ultrafast laser pulses spatially shaped as Bessel beams in dielectrics create high aspect ratio plasma channels whose relaxation can lead to the formation of nanochannels. We report a strong enhancement of the nanochannel drilling efficiency with illumination by double pulses separated by a delay between 10 and 500 ps. This enables the formation of nanochannels with diameters down to 100 nm. Experimental absorption measurements demonstrate that the increase of drilling efficiency is due to an increase of the confinement of the energy deposition. Nanochannel formation corresponds to a drastic change in absorption of the second pulse, demonstrating the occurrence of a phase change produced by the first pulse. This creates a highly absorbing, long-living state. Our measurements show that it is compatible with the semi-metallization of warm dense glass which takes place within a timescale of

With the recent realization of kilotesla quasistatic magnetic fields, the interaction of a laser with magnetized solids enters an unexplored new regime. In particular, a circularly polarized (CP) laser pulse may propagate in a highly magnetized plasma of any high density without encountering cutoff reflection in the whistler mode. With this, we propose a scheme for producing uniform warm dense matter (WDM) by direct laser heating with a CP laser irradiating onto the target along the magnetic field. It is shown by particle-in-cell simulations, which include advanced ionization dynamics and collision dynamics, moderately intense right-hand CP laser light at ${10}^{15}\phantom{\rule{0.28em}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$ can propagate in solid aluminum and heat it efficiently to the 100 eV level within picoseconds. By using two laser pulses irradiating from two sides of a thin solid target, uniform heating to WDM can be achieved. This provides a controllable way to create WDM at different temperatures.

Within the generalized linear response theory, the dynamical conductivity of charged particle systems is obtained from equilibrium correlation functions. Using the force–force correlation function approach, interband transitions in addition to phonon interaction and umklapp processes are taken into account for warm dense matter. For solid-density aluminum and copper plasmas, the results are shown for dynamical collision frequency, dynamical conductivity, and opacity as functions of laser frequency accounting for interband absorption. Calculated opacities for aluminum plasmas are compared with results of first-principles density functional theory–molecular dynamic (DFT-MD) simulations and recent experiments. The problem of underestimating the size of opacities by theoretical models in the vicinity of the L edge is discussed.

We present computationally simple parameter-free pair potentials useful for solids, liquids, and plasmas at arbitrary temperatures. They successfully treat warm-dense matter (WDM) systems like carbon or silicon with complex tetrahedral or other structural bonding features. Density functional theory asserts that only one-body electron densities and one-body ion densities are needed for a complete description of electron–ion systems. Density functional theory (DFT) is used here to reduce both the electron many-body problem and the ion many-body problem to an exact one-body problem, namely, that of the neutral pseudoatom (NPA). We compare the Stillinger–Weber (SW) class of multi-center potentials, the embedded-atom approaches, and N-atom DFT, with the one-atom DFT approach of the NPA to show that many-ion effects are systematically included in this one-center method via one-body exchange-correlation functionals. This computationally highly efficient one-center DFT-NPA approach is contrasted with the usual N-center DFT calculations that are coupled with molecular dynamics simulations to equilibrate the ion distribution. Comparisons are given with the pair-potential parts of the SW, “glue” models, and the corresponding NPA pair-potentials to elucidate how the NPA potentials capture many-center effects using single-center one-body densities.

Warm dense matter is difficult to generate since it corresponds to a state of matter which pressure is order of magnitude larger than can be handled by natural materials. A diamond anvil can be used to pressurize matter up to one Gbar, this matter is at high density but at room temperature. High power lasers and heavy ion beams can generate warm dense matter but they cannot confine it long enough to allow measurements of quasi-static transport coefficients such as viscosity or heat conduction. We present here a third method to generate warm dense matter. It uses a pulsed-power driver which current rise time is substantially shortened by using a plasma opening switch, limiting the development of electrothermal instabilities. The switch relies on the implosion of a gas puff Z-pinch which carries most of the discharge current until the pinch reaches the sample. After that, the sample is compressed until it reaches the warm dense matter regime. Three-dimensional magnetohydrodynamics computations show that if the density of the gas is low enough no detectable instabilities (e.g. kinks and sausages modes) impede the remainder of the implosion.

ELI-Beamlines (ELI-BL), one of the three pillars of the Extreme Light Infrastructure endeavour, will be in a unique position to perform research in high-energy-density-physics (HEDP), plasma physics and ultra-high intensity (UHI) (>1022W/cm2) laser–plasma interaction. Recently the need for HED laboratory physics was identified and the P3 (plasma physics platform) installation under construction in ELI-BL will be an answer. The ELI-BL 10 PW laser makes possible fundamental research topics from high-field physics to new extreme states of matter such as radiation-dominated ones, high-pressure quantum ones, warm dense matter (WDM) and ultra-relativistic plasmas. HEDP is of fundamental importance for research in the field of laboratory astrophysics and inertial confinement fusion (ICF). Reaching such extreme states of matter now and in the future will depend on the use of plasma optics for amplifying and focusing laser pulses. This article will present the relevant technological infrastructure being built in ELI-BL for HEDP and UHI, and gives a brief overview of some research under way in the field of UHI, laboratory astrophysics, ICF, WDM, and plasma optics. Keywords: High-energy-density-physics, Ultra-high-intensity, Warm dense matter, Laboratory astrophysics, High repetition rate lasers, Plasma optics, Inertial confinement fusion, Laser–plasma interaction, Relativistic plasmas, PACS Codes: 52.25.Fi, 52.27.Ep, 52.27.Ny, 52.35.Mw, 52.38.Fz, 52.38.Dx, 52.38.Kd, 52.38.Mf, 52.57.-z, 52.65.-y, 52.72.+v

Ionization in experiments where solid targets are irradiated by high irradiance extreme ultra-violet (EUV) lasers is examined. Free electron degeneracy effects on ionization in the presence of a high EUV flux of radiation is shown to be important. Overlap of the physics of such plasmas with plasma material under compression in indirect inertial fusion is explored. The design of the focusing optics needed to achieve high irradiance (up to 1014 Wcm−2) using an EUV capillary laser is presented.

A model for the free-free opacity of dense plasmas is presented. The model uses a previously developed average atom model, together with the Kubo-Greenwood model for optical conductivity. This, in turn, is used to calculate the opacity with the Kramers-Kronig dispersion relations. Comparisons to other methods for dense deuterium results in excellent agreement with DFT-MD simulations, and reasonable agreement with a simple Yukawa screening model corrected to satisfy the conductivity sum rule. Comparisons against the very recent experiments of Kettle et al. for dense aluminum also reveal very good agreement, in contrast to existing models. Weaknesses in the model are also highlighted.

Warm dense matter (WDM) belongs to a part of high energy density physics, which includes both extensive and rich physics phenomena. It is an important state of the evolution and presence of matters in inertial confinement fusion (ICF), heavy-ion fusion, Z-pinch processes and so on. In particular, thermodynamic, optical, and radiated characteristic of warm dense matter plays an important role in determining for the macro fluid movement of matter and determining for the energy transportation and transfer in the interactions of radiated field with matter in the evolution process. Therefore, further investigation of the properties of warm dense matter and precision improvements on its related parameters, such as equations of state and radiation transportation, are of science significant and applied background in many research fields such as ICF, Z-pinch, earth’s and planetary interior structure. The important research progress in the techniques of production, diagnostics, and simulation of warm dense matter under the laboratory conditions are briefly introduced and reviewed. The topics on the techniques of creating WDM were discussed, including experimental capabilities and facilities enabling the synthesis and confinement of warm dense states. These experimental capabilities include energetic materials, short pulse and high-energy-density lasers, ion beams, static high-pressure diamond-anvil cells, radiation- synchrotron sources, and mechanical impact techniques such as gas-gun launchers. Advanced diagnostics required for the characterization and interrogation of warm dense states were employ in these experiments accordingly. A general review on the theoretical approaches and computational capabilities enabling the prediction of the thermodynamic properties of matter in the warm dense regime were given. These approaches include quantum-based finite-temperature methods based on density functional theory, finite-temperature average-atom method, molecular dynamics, and various plasma physics-based theoretical approaches. In our National Key Laboratory for Shock Wave and Detonation Physics (LSD), warm dense matter was created by multiple shock reverberation technique. The multi-shock compressed states were directly determined by the multi-wavelength channel optical transience radiance pyrometry (MCOP), Doppler pins system (DPS), streak optical pyrometer (SOP), and spectrum analyser. The gas sample is confined between a 304 steel baseplate at the impact end and a composite window at the other end. The strong plain shock wave was produced using the flyer plate impact accelerated up to about several km/s by a two-stage light gas gun on the target baseplate and introduced into the plenum gas sample, which was pre-compressed from environmental pressure to 20–40 MPa. The optical radiation histories recorded by two sets of MCOPs were used to determine shock velocity. Simultaneously, the particle velocity profiles of gas sample-window interface were measured with four DPSs and the time-resolved spectrum was determined by SOP. The states of multi-shock compression gas sample were determined from the measured shock velocities combining the particle velocity profiles. The multi-shock temperatures were obtained from the measured radiation histories and spectrum of warm dense matter. We performed the experiments on warm dense helium, deuterium, argon, and xenon to reach to above one hundred GPa. The experimental results are used to validate our developed self-consistent fluid variational theoretical model, to check the existing WDM theoretical model, and to create new theoretical ones. Finally, some suggestions, summary, and outlook in the future development tendency and direction of warm dense matter are presented.

High energy density physics and astrophysics require reliable methods for determining the equation of state of warm dense matter. At high temperatures (above 106 K or 100 eV), path integral Monte Carlo (PIMC) as a first-principles method is a useful option because of its accuracy and efficiency. Previous developments in PIMC implemented free-particle nodes to study plasmas comprised of heavy (Z ≤ 10) elements and constructed equations of state in tandem with those from molecular dynamics (MD) simulations based on density functional theory (DFT), whose applicability is limited to low temperatures (up to ∼ 106 K). Recent PIMC method developments employed a localized, Hartree-Fock nodal surface, allowing for a better description of bound states in warm dense silicon. In this work, we use the localized nodal scheme to study warm dense sodium at 2-fold ambient density. We demonstrate that PIMC and DFT-MD produce a coherent equation of state and discuss the electronic structure of the plasma.

We survey quite extensively the present research status of ion-stopping in dense plasmas of potential importance for initial confinement fusion (ICF) driven by intense and heavy ion beams, and also for warm dense matter (WDM). First, we put emphasis on every possible mechanism involved in the shaping of the ion projectile effective charge, while losing energy in a target plasma with classical ions and partially degenerate electrons. Then, we switch to ion stopping by target bound electrons, taking detailed account of mean excitation energies. Free electron stopping has already been given a lot of attention in former works [C. Deutsch et al., Recent Res. Devel. Plasma 1 (2000) 1–23; Open Plasma Phys. J. 3 (2010) 88–115]. Then, we extend the usual standard stopping model (SSM) framework to nonlinear stopping including a treatment of the Z3 Barkas effect and a confronting comparison of Bloch and Bohr Coulomb logarithms. Finally, we document low velocity ion slowing down (LVISD) in single ion plasmas as well as in binary ionic mixtures (BIM), in connection with specific ICF fuels.

X-ray Thomson scattering experiments in the soft and hard x-ray regime yield information on fundamental parameters of high-density systems. Pump-probe experiments with variable time delay provide insight into the excitation and relaxation dynamics in dense plasmas. On short time scales, a local thermodynamic equilibrium description might not be sufficient. Besides nonequilibrium effects on the electron distribution function, spatial inhomogeneities influence the scattering signal. Generalizing previous approaches of Belyi [Phys. Rev. E 97, 053204 (2018)2470-004510.1103/PhysRevE.97.053204] and Kozlowski et al. [Sci. Rep. 6, 24283 (2016)2045-232210.1038/srep24283], we discuss implications for Thomson scattering spectra for inhomogeneous plasmas in the warm dense matter regime based on a gradient expansion within real-time Green's-functions theory. Especially in the collective scattering regime, Thomson scattering spectra are modifed substantially by spatial inhomogeneities. Within a first-order gradient expansion, the dispersion relation for plasmons is determined. In particular, the ratio of the heights of the plasmon peaks is changed which prevents a simple estimation of the plasma temperature from the detailed balance relation.

The Scottish Centre for the Application of Plasma-based Accelerators (SCAPA) is a research centre dedicated to providing high energy particle beams and high peak brightness radiation pulses for users across all scientific and engineering disciplines. A pair of Ti:Sapphire femtosecond laser systems (40 TW peak power at 10 Hz pulse repetition rate and 350 TW at 5 Hz, respectively) are the drivers for a suite of laser-plasma accelerator beamlines housed across a series of radiation shielded areas. The petawatt-scale laser delivers 45 W of average power that establishes it as the world leader in its class. The University of Strathclyde has had an operational laser wakefield accelerator since 2007 as the centrepiece of the ongoing Advanced Laser Plasma High-energy Accelerators towards X-rays (ALPHA-X) project. SCAPA, which is a multipartner venture under the auspices of the Scottish Universities Physics Alliance, continues the dedicated beamline approach pioneered by ALPHA-X and represents a significant expansion in the UK's experimental capability at the university level in laser-driven acceleration. The new centre supports seven radiation beamlines across three concrete shielded bunkers that each nominally specialise in a different aspect of fundamental laser-plasma interaction physics and radiation sources: GeVscale electron beams, MeV/c proton and ion beams, X-rays, gamma rays and so on. Development of application programmes based on these sources cover a wide range of fields including nuclear physics, radiotherapy, space radiation reproduction, warm dense matter, high field physics and radioisotope generation.

We present first-principles calculations of the rate of energy exchanges between electrons and ions in nonequilibrium warm dense plasmas, liquid metals, and hot solids, a fundamental property for which various models offer diverging predictions. To this end, a Kubo relation for the electron-ion coupling parameter is introduced, which includes self-consistently the quantum, thermal, nonlinear, and strong coupling effects that coexist in materials at the confluence of solids and plasmas. Most importantly, like other Kubo relations widely used for calculating electronic conductivities, the expression can be evaluated using quantum molecular dynamics simulations. Results are presented and compared to experimental and theoretical predictions for representative materials of various electronic complexity, including aluminum, copper, iron, and nickel.

We present a theory for the rate of energy exchange between electrons and ions -- also known as the electron-ion coupling factor -- in physical systems ranging from hot solid metals to plasmas, including liquid metals and warm dense matter. The paper provides the theoretical foundations of a recent work [J. Simoni and J. Daligault, Phys. Rev. Lett. {\bf 122}, 205001 (2019)], where first-principles quantum molecular dynamics calculations based on this theory were presented for representative materials and conditions. We first derive a general expression for the electron-ion coupling factor that includes self-consistently the quantum mechanical and statistical nature of electrons, the thermal and disorder effects, and the correlations between particles. We show that our theory reduces to well-known models in limiting cases. In particular, we show that it simplifies to the standard electron-phonon coupling formula in the limit of hot solids with lattice and electronic temperatures much greater than the Debye temperature, and that it extends the electron-phonon coupling formula beyond the harmonic phonon approximation. For plasmas, we show that the theory readily reduces to well-know Spitzer formula in the hot plasma limit, to the Fermi golden rule formula in the limit of weak electron-ion interactions, and to other models proposed to go beyond the latter approximation. We explain that the electron-ion coupling is particularly well adapted to averaged atom models, which offer an effective way to include non-ideal interaction effects to the standard models and at a much reduced computational cost in comparison to first-principles quantum molecular dynamics simulations., Comment: 0 figure

Single-shot terahertz time-domain spectroscopy is developed experimentally to measure precisely the conductivity of warm dense golden plasma with temporal resolution while the AB initial principle-based theoretical models and simulation approaches are established to predict exactly the structures and the dynamics of warm dense matter.

We discuss the possibility of obtaining highly precise measurements of the ionization potential depression in dense plasmas with spectrally resolved X-ray scattering, while simultaneously determining the electron temperature and the free electron density. A proof-of-principle experiment at the Linac Coherent Light Source, probing isochorically heated carbon samples, demonstrates the capabilities of this method and motivates future experiments at X-ray free electron laser facilities.

We study electron heating and stopping power in warm dense matter as formed in interactions of sub-picosecond high-intensity lasers with solid bulk targets. In such interactions, an intense beam of forward moving relativistic electrons is created, inducing a compensating return current and generating characteristic Kα x-ray radiation along the propagation path. The theoretical calculations presented here are inspired by, and tested against, a previously published study that provides bulk-temperature and absolutely calibrated Kα radial profiles. By using Monte Carlo simulations, the experimental data allow for inferring the flux of the relativistic electrons, which is a crucial input for the target heating calculations. For the latter, a “rigid beam” model is employed, describing the central, nearly homogeneous, part of the target. The comparison with the experiment shows a fairly good agreement. For the conditions analyzed, we find that the effect of the return current is dominant both in the target heating and in the beam stopping.

The photorecombination continuum emission of solid-density plasma can create a featureless spectral continuum with high intensity.

X-ray absorption spectroscopy is a well-accepted diagnostic for experimental studies of warm dense matter. It requires a short-lived X-ray source of sufficiently high emissivity and without characteristic lines in the spectral range of interest. In the present work, we discuss how to choose an optimum material and thickness to get a bright source in the wavelength range 2 Å-6 Å (∼2 keV to 6 keV) by considering relatively low-Z elements. We demonstrate that the highest emissivity of solid aluminum and silicon foil targets irradiated with a 1-ps high-contrast sub-kJ laser pulse is achieved when the target thickness is close to 10 μm. An outer plastic layer can increase the emissivity even further.

The study of warm dense matter is very important for the evolution of celestial bodies and inertial confinement fusion, which often contains a mixture of multiple elements and different charge-state ions. The ionic structure and distribution of different charge-states directly affect the diagnosis and physical properties of warm dense matter. At the same time, the influence of high-temperature dense plasma on the ionic structure should be considered when we study the physical properties from the first-principle calculation of electron structure. In the present work, the radial distribution functions of multiple charge-state ions (gold, carbon-hydrogen mixture, and aluminum) are developed in the hypernetted-chain approximation, and elastic x-ray scattering of different charge-state ions are calculated in the warm dense matter regime. Firstly, the electron structure of different charge-state ions is self-consistently computed in the ionic sphere, in which the ion-sphere radii are determined by the plasma density and their charges. And then the ionic fraction is obtained by solving the modified Saha equation, with the interactions among different charge-state ions taken into account, and ion-ion pair potentials are obtained by Yukawa model. Finally, the ion features of x-ray elastic scattering for Al are calculated on the basis of electronic distribution around the nuclei and ionic radial distribution function. By comparing the results of different charge-sate ions with the result of mean charge-sate ion, it is shown that different statistical methods can affect the physical properties which are dependent on the electronic and ionic structure.

A unified framework for describing dense plasma screenings from warm dense matter to hot dense matter, including gradient correction and exchange-correlation, has been recently proposed by Stanton and Murillo [Phys. Rev. E 91,033104 (2015)]. With this unified description of linear screening, the energy levels, absorption oscillator strengths, and radiative transition probabilities of hydrogen-like plasmas are reported for a wide range of critical screening parameters (plasma temperatures and densities), where the bound level nl merges into continuum. The importance of both finite temperature gradient correction and exchange correlation increases with the increasing plasma coupling strength or the Coulomb coupling parameter Γ, and the effect of exchange correlation is generally greater than that of finite temperature gradient correction. Further results from the weakly coupled and strongly coupled limits or Debye plasmas and dense quantum plasmas also show consistency with those from the unified framework, respectively.