No abstract

The charged environment within a dense plasma leads to the phenomenon of ionization potential depression (IPD) for ions embedded in the plasma. Accurate predictions of the IPD effect are of crucial importance for modeling atomic processes occurring within dense plasmas. Several theoretical models have been developed to describe the IPD effect, with frequently discrepant predictions. Only recently, first experiments on IPD in Al plasma have been performed with an x-ray free-electron laser (XFEL), where their results were found to be in disagreement with the widely-used IPD model by Stewart and Pyatt. Another experiment on Al, at the Orion laser, showed disagreement with the model by Ecker and Kröll. This controversy shows a strong need for a rigorous and consistent theoretical approach to calculate the IPD effect. Here we propose such an approach: a two-step Hartree-Fock-Slater model. With this parameter-free model we can accurately and efficiently describe the experimental Al data and validate the accuracy of standard IPD models. Our model can be a useful tool for calculating atomic properties within dense plasmas with wide-ranging applications to studies on warm dense matter, shock experiments, planetary science, inertial confinement fusion and studies of non-equilibrium plasmas created with XFELs.

We investigate ultrafast (fs) electron dynamics in a liquid hydrogen sample, isochorically and volumetrically heated to a moderately coupled plasma state. Thomson scattering measurements using 91.8 eV photons from the free-electron laser in Hamburg (FLASH at DESY) show that the hydrogen plasma has been driven to a nonthermal state with an electron temperature of 13 eV and an ion temperature below 0.1 eV, while the free-electron density is 2:8 Â 10 20 cm À3 . For dense plasmas, our experimental data strongly support a nonequilibrium kinetics model that uses impact ionization cross sections based on classical free-electron collisions. The investigation of warm dense matter (WDM) is one of the grand challenges of contemporary physics [1]. WDM is a plasma state characterized by moderate-tostrong interparticle coupling which takes place at freeelectron temperatures of several eV and free-electron densities around solid density [1]. It is present in many physical environments, such as planetary interiors [2,3], gravitationally collapsing protostellar disks, laser matter interaction and particularly during the implosion of an inertial confinement fusion capsule [4]. While in the astrophysical context WDM exists under stable conditions, in the laboratory it is achieved only as a transient state bridging condensed matter and hot plasma regimes. Here, we report on the first investigation of the nonequilibrium transition of hydrogen from a liquid to a moderately coupled plasma on the fs time scale, induced by highly intense soft-x-ray irradiation. This is an important step towards the investigation of strongly-coupled plasmas which are within reach of current light sources such as the Linac Coherent Light Source (LCLS). Our measurement enables unprecedented direct tests of nonequilibrium statistical models beyond mean field theories in a regime where collision and relaxation processes are dominant [5][6][7].The use of x-ray scattering for the investigation of dense, strongly-coupled plasmas was successfully demonstrated in the past decade [5,[7][8][9][10][11]. This technique is the x-ray analog of optical Thomson scattering (TS) [12] and enables the experimental determination of plasma parameters in dense systems where optical light cannot penetrate. While previous experiments were carried out using highenergy laser facilities, the advent of soft-and hard-x-ray free-electron lasers (FELs) makes ultrashort high brightness beams available for this type of research [13,14]. This Letter reports on ultrafast heating of liquid hydrogen and TS measurement of dense plasma parameters using softx-ray FEL radiation. For the first time, nonequilibrium distributions are observed and the underlying relaxation dynamics are compared with kinetic models showing electron relaxation times in the order of 20 fs, thus, shorter than the pulse duration.The scattering taking place is collective TS, which is characterized by a spectrally blue and red shifted response due to collective electron motion, plasmons, and nearly elastic scattering due t...

Saturable absorption is a phenomenon readily seen in the optical and infrared wavelengths. It has never been observed in core-electron transitions owing to the short lifetime of the excited states involved and the high intensities of the soft X-rays needed. We report saturable absorption of an L-shell transition in aluminium using record intensities over 10 16 W cm −2 at a photon energy of 92 eV. From a consideration of the relevant timescales, we infer that immediately after the X-rays have passed, the sample is in an exotic state where all of the aluminium atoms have an L-shell hole, and the valence band has approximately a 9 eV temperature, whereas the atoms are still on their crystallographic positions. Subsequently, Auger decay heats the material to the warm dense matter regime, at around 25 eV temperatures. The method is an ideal candidate to study homogeneous warm dense matter, highly relevant to planetary science, astrophysics and inertial confinement fusion. Saturable absorption, the decrease in the absorption of light with increasing intensity, is a well-known effect in the visible and near-visible region of the electromagnetic spectrum 1 , and is a widely exploited phenomenon in laser technology. Although there are many ways to induce this effect, in the simplest two-level system it will occur when the population of the lower, absorbing level is severely depleted, which requires light intensities sufficiently high to overcome relaxation from the upper level. Here, we report on the production of saturable absorption of a metal in the soft X-ray regime by the creation of highly uniform warm dense conditions, a regime that is of great interest in high-pressure science 2,3 , the geophysics of large planets 4,5 , astrophysics 6 , plasma production and inertial confinement fusion 7 . Furthermore, the process by which the saturation of the absorption occurs will lead, after the X-ray pulse, to the storage of about 100 eV per atom, which in turn evolves to a warm dense state. This manner of creation is unique as it requires intense, subpicosecond, soft X-rays. As such, it has not hitherto been observed in this region of the spectrum, owing both to the lack of high-intensity sources, and the rapid recombination times associated with such high photon energies. However, with the advent of new fourth-generation X-ray light sources, including the free-electron laser in Hamburg 8 (FLASH), soft X-ray intensities that have previously remained the province of high-power optical lasers can now be produced. Experiments at such high intensities using gas jets have already exhibited novel absorption phenomena 9 , and the possibility of irradiating solid samples with intense soft and hard X-rays has aroused interest as a possible means of producing warm dense matter (WDM) at known atomic densities 10,11 .We present the first measurements of the absorption coefficient of solid samples subject to subpicosecond soft X-ray pulses with intensities up to and in excess of 10 16 W cm −2 , two orders of magnitude higher than could ...

Collective Thomson scattering with extreme ultraviolet light or x rays is shown to allow for a robust measurement of the free electron density in dense plasmas. Collective excitations like plasmons appear as maxima in the scattering signal. Their frequency position can directly be related to the free electron density. The range of applicability of the standard Gross-Bohm dispersion relation and of an improved dispersion relation in comparison to calculations based on the dielectric function in random phase approximation is investigated. More important, this well-established treatment of Thomson scattering on free electrons is generalized in the Born-Mermin approximation by including collisions. We show that, in the transition region from collective to noncollective scattering, the consideration of collisions is important.

We calculate the dynamic structure factor (DSF) in warm dense beryllium by means of ab initio molecular dynamics simulations. The dynamic conductivity is derived from the Kubo-Greenwood formula, and a Drudelike behaviour is observed. The corresponding dielectric function is used to determine the DSF. Since the ab initio approach is so far only applicable for wavenumbers k = 0, the k-dependence of the dielectric function is modelled via the Mermin ansatz. We present the results for the dielectric function and DSF of warm dense beryllium and compare these with perturbative treatments such as the Born-Mermin approximation. We found considerable differences between the results of these approaches; this underlines the need for a first-principles determination of the DSF of warm dense matter. Gesellschaft checked with benchmarking experiments. A versatile and reliable tool for this purpose is x-ray Thomson scattering (XRTS) [1,2], which gains the plasma parameters directly from the dynamic structure factor (DSF) [3]. X-rays penetrate dense matter, and intense x-ray sources are now available to perform scattering experiments. For instance, intense x-ray pulses are produced either by energetic optical lasers or by free-electron lasers in the soft or hard x-ray regimes.X-rays emitted from laser-produced plasmas [4,5] can probe the region from warm dense matter (WDM) [6,7] with temperatures of several eV and densities close to solid density [8][9][10] up to compressed matter well above solid density and at electron temperatures of 0.1 eV and several tens of eV [11][12][13][14][AQ ID=Q1]. The outstanding applications of XRTS are, e.g., in-flight measurements of laser-driven implosion dynamics of inertial confinement fusion capsules [15] and of radiative heating and cooling dynamics of matter [16], both on ns time scales. Plasmas in the near-solid density regime have also been probed by combining optical lasers (pump) and soft x-rays (probe) [17].The plasma parameters electron temperature T e , free-electron density n e and the mean ionization state Z can be derived by analyzing the XRTS signal. The electron temperature can be inferred from the universal detailed balance relation, whereas the electron density follows from the plasmon dispersion in the collective scattering mode [8,18,19].XRTS experiments have been performed on beryllium for different conditions [8,10,12,20]. It is a potential ablator material in inertial confinement fusion capsules [21] and also relevant for astrophysics considering the neutrino capture reactions in the Sun and the problem of neutrino oscillations [22]. In this paper, we determine the DSF for uncompressed beryllium (u-Be [10]: T e = 12 eV, ρ = 1.85 g cm −3 ) and threefold compressed beryllium (c-Be [12]: T e = 13 eV, ρ = 5.5 g cm −3 ), thereby studying the effect of strong correlations characterized by electron coupling parameters e = e 2 4π 0 k B T e 4πn e 3

By use of high intensity XUV radiation from the FLASH free-electron laser at DESY, we have created highly excited exotic states of matter in solid-density aluminum samples. The XUV intensity is sufficiently high to excite an inner-shell electron from a large fraction of the atoms in the focal region. We show that soft-x-ray emission spectroscopy measurements reveal the electronic temperature and density of this highly excited system immediately after the excitation pulse, with detailed calculations of the electronic structure, based on finite-temperature density functional theory, in good agreement with the experimental results.

We report the creation of solid-density aluminum plasma using free-electron laser (FEL) radiation at 13.5nm wavelength. Ultrashort pulses were focused on a bulk Al target, yielding an intensity of 2x10;{14}Wcm;{2} . The radiation emitted from the plasma was measured using an xuv spectrometer. Bremsstrahlung and line intensity ratios yield consistent electron temperatures of about 38eV , supported by radiation hydrodynamics simulations. This shows that xuv FELs heat up plasmas volumetrically and homogeneously at warm-dense-matter conditions, which are accurately characterized by xuv spectroscopy.