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The advent of high-power lasers has provided insights into laboratory high energy density (>1011 J/m3) physics. In particular, the properties of warm dense matter (WDM) with temperatures of 104–106 K and near-solid densities is a research area that has garnered significant interest recently. However, owing to the high temperatures and pressures associated with WDM, the measurement of fundamental properties is difficult, and insufficient data has been a significant setback in WDM research. Herein, we review recent developments in time-resolved X-ray absorption spectroscopy with synchrotron and X-ray free electron lasers for WDM research. Various physical properties, such as atomic bonding, electronic structures, electron–phonon coupling, and thermal conductivity of various elements in WDM conditions are investigated via this noble X-ray technique at various time scales from 100 ps to 100 fs.

The plasma opacities of the widely used X-pinch target materials (Cu, Mo, W) are calculated over a wide temperature and density range (T = 10–3–102 keV, ρ = 1016–1024 cm−3) by using the collisional radiative code FLYCHK. Including the scattering effect, the FLYCHK opacity of the mid-Z element copper is in good agreement with the Los Alamos opacity code ATOMIC in the broad T–ρ range corresponding to typical X-pinch plasma conditions. In a strongly coupled region, a few corrections, such as the degeneracy effect for the free–free opacity, may be required to improve accuracies. The absence of a Δn = 0 transition and the simple scattering opacity formula in FLYCHK also cause the characteristics of the FLYCHK opacity data. The plasma opacities of the high-Z elements molybdenum and tungsten are also calculated. These results can be used as basic inputs for various radiative hydrodynamic simulations not only for X-pinch plasmas but for different types of high-energy–density plasma research.

Free-electron laser pulse-based X-ray absorption spectroscopy measurements on warm dense copper are presented. The incident X-ray pulse energies were measured with a detector assembly consisting of a photocathode membrane and microchannel plates, and the transmitted energies were measured simultaneously with a photodiode detector. The precision of the absorption measurements was evaluated. For a warm dense copper foil irradiated by an intense femtosecond laser pulse, the enhanced X-ray absorption below the L 3-edge, followed by the rapid evolution of highly excited Fermi liquid within a picosecond, were successfully measured. This result demonstrates a unique capability to study femtosecond non-equilibrium electron–hole dynamics in extreme states of matter.

The electron-ion relaxation dynamics in warm dense iron were investigated by time-resolved x-ray absorption near-edge spectroscopy (XANES). A novel analysis, combining ab initio density functional theory (DFT) and two temperature model (TTM) simulations, was developed to calculate the x-ray absorption spectra as a function of delay time. Here we present experimental evidence of changes at the XAS L3 edge of iron that are consistent with the reduction of the electron-phonon coupling under warm dense matter conditions. The experimental results are in agreement with the model that takes both the electron (${T}_{e}$) and the ion temperature (${T}_{i}$) dependence of the thermophysical properties into consideration, while models where either constant electron-phonon coupling factor (G) or only ${T}_{e}$-dependent G are used do not agree with the observed relaxation dynamics of iron.

Above the damage threshold of Au nanofoil, experimental and theoretical investigations of the fluence-dependent self-reflection of intense femtosecond laser pulses are presented. Measured reflectance shows not only the fluence-dependency, but also the laser pulse duration dependency, which is barely described by the calculation of dielectric function of high-temperature Au, based on the traditional two-temperature model. A simple hot electron kinetic model, which contains a nonthermalized electronic subsystem cooperated with the two-temperature model, is considered to determine transient electron densities and temperature to calculate the dielectric functions and self-reflection of femtosecond laser pulses. Comparison to experimental data shows the effect of hot electrons with a relaxation time of a few hundreds of femtoseconds on the transient optical properties of gold under a strong excitation.

Laser induced breakdown spectroscopy (LIBS) has been tested to diagnose impurity elements on a Korea Superconducting Tokamak Advanced Research (KSTAR) divertor tile. Spectral lines of various impurity elements such as iron, chromium, and nickel were detected from the divertor surface. The variation of spectra with consecutive laser pulses demonstrates the potential for depth profiling analysis for the deposited impurity layer. The LIBS plasma parameters have been qualitatively determined from analysis of the relative line intensities and linewidths for each element. The validity of this analysis has been checked with atomic spectral simulations.

Degenerate plasmas, in which quantum effects dictate the behavior of free electrons, are ubiquitous on earth and throughout space. Transitions between bound and free electron states determine basic plasma properties, yet the effects of degeneracy on these transitions have only been theorized. Here, we use an x-ray free electron laser to create and characterize a degenerate plasma. We observe a core electron fluorescence spectrum that cannot be reproduced by models that ignore free electron degeneracy. We show that degeneracy acts to restrict the available electron energy states, thereby slowing the rate of transitions to and from the continuum. We couple degeneracy and bound electron dynamics in an existing collisional-radiative code, which agrees well with observations. The impact of the shape of the cross section, and hence the magnitude of the correction due to degeneracy, is also discussed. This study shows that degeneracy in plasmas can significantly influence experimental observables such as the emission spectra, and that these effects can be included parametrically in well-established atomic physics codes. This work narrows the gap in understanding between the condensed-matter and plasma phases, which coexist in myriad scenarios.

The opacity information of a finite-temperature plasma is an important property and requires the population distribution of a given plasma condition. A population kinetic code for plasma spectroscopy, FLYCHK, has been widely used by researchers to study the spectroscopic properties of high-energy-density plasmas under a wide range of conditions. In this study, the FLYCHK calculation of the Planck and Rosseland mean opacities of low- to high-Z elements, such as aluminum (Z = 13), iron (Z = 26), and gold (Z = 79), under a wide temperature and density range (T = 10−3–102 keV, ρ = 10−6–102 g/cc) is reported. This study mainly focused on the quantitative comparisons of FLYCHK opacities with commonly used opacities: ATOMIC and PROPACEOS. Comparisons show that the FLYCHK mean opacities are comparable to other results over a wide range of plasma conditions. Aluminum opacities were analyzed in detail to understand the characteristics of FLYCHK opacity simulations.

With the development of X-ray free electron lasers (XFEL), a novel state of matter of highly transient and non-equilibrium plasma has been created in laboratories. As high intensity X-ray laser beams interact with a solid density target, electrons are ionized from inner-shell orbitals and these electrons and XFEL photons create dense and finite temperature plasmas. In order to study atomic processes in XFEL driven plasmas, the atomic kinetics model SCFLY containing an extensive set of configurations needed for solid density plasmas was applied to study atomic processes of XFEL driven systems. The code accepts the time-dependent conditions of the XFEL as input parameters, and computes time-dependent population distributions and ionization distributions self-consistently with electron temperatures and densities assuming an instantaneous equilibration of electron energies. The methods and assumptions in the atomic kinetics model and unique aspects of atomic processes in XFEL driven plasmas are described.

Physical review letters 109, 245003 (2012). doi:10.1103/PhysRevLett.109.245003, Published by APS, College Park, Md.

We have measured the time evolution of the phase shift at the front and back surfaces of gold nano-foils that have been excited with a 150 fs ( λ = 400 nm) laser pulse. The thickness of the foils (d∼30 nm) is roughly one-third of the ballistic electron transport range at ambient conditions (∼100 nm). At lower pump fluences, the front and back sides of the foil behave similarly, indicating uniform heating by ballistic electrons. As the pump fluence is increased, the phase shift on the front side is higher than that on the back, indicating a significant reduction in the ballistic electron transport range.

X-ray absorption spectroscopy (XAS) is a versatile technique for studying the electronic structure and/or the local geometry of matter, and time-resolved XAS has been developed into a powerful tool for studying the dynamics of materials [1, 2]. The temporal resolution of this technique is usually determined by the X-ray pulse duration in pump and probe geometries. Although this technique has been developed in the synchrotron environment and takes advantage of existing X-ray beamlines, the study of ultrafast dynamics on time scales on the order of a picosecond or faster is limited because typical pulse durations from third-generation light sources is on the order of tens of picoseconds. In addition, XAS typically requires the integration of a large number of X-ray pulses to get quality data. These features make it difficult to apply this technique to research areas in which samples undergo irreversible processes.

X-ray free-electron lasers (XFELs) provide unique opportunities to generate and investigate dense plasmas. The absorption and transmission properties of x-ray photons in dense plasmas are important in characterizing the state of the plasmas. Experimental evidence shows that the transmission of x-ray photons through dense plasmas depends greatly on the incident XFEL intensity. Here, we present a detailed analysis of intensity-dependent x-ray transmission in solid-density aluminum using collisional-radiative population kinetics calculations. Reverse saturable absorption (RSA), i.e., an increase in x-ray absorption with intensity has been observed for photon energies below the K-absorption edge and in the intensity range of 1016–1017 W/cm2 for XFEL photons with 1487 eV. At higher intensities, a transition from RSA to saturable absorption (SA) is predicted; thus, the x-ray absorption decreases with intensity above a threshold value. For XFEL photon energies of 1501 eV and 1515 eV, the transition from RSA to SA occurs at XFEL intensities between 1017–1018 W/cm2. Electron temperatures are predicted to be in the range of 30–50 eV for the given experimental conditions. Detailed population kinetics of the charge states explains the intensity-dependent absorption of x-ray photons and the fast modulation of XFEL pulses for both RSA and SA.

Warm dense conditions in titanium foils irradiated with intense femtosecond laser pulses are diagnosed using an x-ray imaging spectroscopy technique. The line shapes of radially resolved titanium Kα spectra are measured with a toroidally bent GaAs crystal and an x-ray charge-coupled device. Measured spectra are compared with the K-shell emissions modeled using an atomic kinetics - spectroscopy simulation code. Kα line shapes are strongly affected by warm (5-40 eV) bulk electron temperatures and imply multiple temperature distributions in the targets. The spatial distribution of temperature is dependent on the target thickness, and a thin target shows an advantage to generate uniform warm dense conditions in a large area.

K-shell spectra of targets with microstructured features irradiated by an intense femtosecond laser have been studied. Examination of Kα emission from laser irradiated Si targets coated with micron-scale polystyrene spheres indicates that the emission is enhanced by a factor of ∼3 over emission from planar solids. Sphere-coated targets also emit K-shell He-like Si radiation indicating the presence of a hot dense plasma beneath the microspheres. Furthermore, Kα from Ti foils coupled to micro-tipped reentrant pyramid and wedge shaped targets has been studied, however, no significant enhancement of the Kα yield is observed for these kinds of targets. These studies illustrate that, with correct tailoring of the target surface, field enhancements can be used to increase X-ray emission from intensely irradiated targets.

High-intensity x-ray pulses from an x-ray free-electron laser are used to heat and probe a solid-density aluminum sample. The photon-energy-dependent transmission of the heating beam is studied through the use of a photodiode. Saturable absorption is observed, with the resulting transmission differing significantly from the cold case, in good agreement with atomic-kinetics simulations.

We have used the Linac Coherent Light Source to generate solid-density aluminum plasmas at temperatures of up to 180 eV. By varying the photon energy of the x rays that both create and probe the plasma, and observing the $K\mathrm{\text{\ensuremath{-}}}\ensuremath{\alpha}$ fluorescence, we can directly measure the position of the $K$ edge of the highly charged ions within the system. The results are found to disagree with the predictions of the extensively used Stewart-Pyatt model, but are consistent with the earlier model of Ecker and Kr\"oll, which predicts significantly greater depression of the ionization potential.

We use time-resolved x-ray absorption spectroscopy to investigate the unoccupied electronic density of states of warm dense copper that is produced isochorically through the absorption of an ultrafast optical pulse. The temperature of the superheated electron-hole plasma, which ranges from 4000 to 10 000 K, was determined by comparing the measured x-ray absorption spectrum with a simulation. The electronic structure of warm dense copper is adequately described with the high temperature electronic density of state calculated by the density functional theory. The dynamics of the electron temperature is consistent with a two-temperature model, while a temperature-dependent electron-phonon coupling parameter is necessary.

Using ultrafast x-ray technique, we observed the changes of electronic structure of isochorically heated copper. It allows us to determine electron temperature at a few eV regimes and investigate thermo-physical properties of solid-density plasmas.

Evolution of superheated electron hole plasma in the warm dense copper is investigated using time resolved x-ray absorption technique. Experimental data indicate a strong electron-phonon non-equilibrium state and it is described with two-temperature model.

The optical conductivity of a dense femtosecond laser-heated aluminum plasma heated to 0.1-1.5 eV was measured using frequency-domain interferometry with chirped pulses, permitting simultaneous observation of optical probe reflectivity and probe pulse phase shift. Coupled with published models of bound-electron contributions to the conductivity, these two independent experimental data yielded a direct measurement of both real and imaginary components of the plasma conductivity.

The transport of energetic electron beams generated from aluminum foils irradiated by ultraintense laser pulses has been studied by imaging coherent transition radiation from the rear side of the target. Two distinct beams of MeV electrons are emitted from the target rear side at the same time. This measurement indicates that two different mechanisms, namely resonance absorption and $\mathbit{j}\ifmmode\times\else\texttimes\fi{}\mathbit{B}$ heating, accelerate the electrons at the targets front side and drive them to different directions, with different temperatures. This interpretation is consistent with 3D-particle-in-cell simulations.

Coherent transition radiation was observed from aluminum foils irradiated by an ultra-intense laser. Comparison of experiments and theoretical calculations indicated two streams of hot electron micro-pulses were generated by resonance absorption and jtimesB heating.

The ultrafast dynamics in a dense plasma were studied with a single-shot measurement of the time-dependent reflectivity and phase shift at the rear surface of a free standing aluminum foil following femtosecond irradiation.

The dynamics of heat and shock waves in a dense plasma were studied using time-resolved reflectivity measurements of the rear surface of an aluminum foil following femtosecond irradiation at ~5x1013 W/cm2.

An ultrafast x-ray streak camera has been realized using a grazing incidence reflection photocathode. X-rays are incident on a gold photocathode at a grazing angle of 20 degree and photoemitted electrons are focused by a large aperture magnetic solenoid lens. The streak camera has high quantum efficiency, 600fs temporal resolution, and 6mm imaging length in the spectral direction. Its single shot capability eliminates temporal smearing due to sweep jitter, and allows recording of the ultrafast dynamics of samples that undergo non-reversible changes.

The use of cone targets in high intensity laser-plasma experiments has been of recent interest because of their potential use in integrated fast ignition experiments. Simpler experiments provide a good avenue for understanding the underlying physics, however precise control of the alignment along with good pointing accuracy is of crucial importance. While on big laser facilities target alignment is done precisely with several microscopes, it is not always the case on smaller facilities. This can have a detrimental effect on the quality of the results. We have developed and characterized a method for accurate alignment of intense laser pulses into a cone target. This, along with optimal positioning of the focus compared to the tip, efficiently uses the shape of the target to microfocus the laser light and concentrates the hot electrons in the tip, and can mitigate preplasma issues.

X-ray production from the interaction of femtosecond laser pulses focused to relativistic intensity into re-entrant targets etched into silicon has been investigated. Kα and hard x-ray yields were compared when the laser was focused into pyramidal shaped cone targets and wedge shaped targets. Hot electron production is highest in the wedge targets irradiated with transverse polarization, though Kα is maximized with wedge targets and parallel polarization. These results are explained with particle-in-cell simulations.

Purpose: To show the potential of improving image quality with a cleaner, brighter, quasi‐monochromatic X‐ray micro‐source via laser produced plasmas(LPP).Method and Materials: First generation targets consisting of ten micron thick gold formed into free‐standing pyramids have been built. PIC (Particle‐In‐Cell) simulations have been performed in order to validate this target geometry. Preliminary experiments with a Ti‐Sapphire CPA laser have been achieved with these targets. Second generation parabolic cone targets with an optimal angle for electron transport have also been built. This new nano‐fabricated target could optimize X‐ray source characteristics. Results:PIC (Particle‐In‐Cell) simulations show that conical targets optically guide laser light resulting in a higher density of hot electrons at the apex These simulations show a possible ten times augmentation in hot electron density and a three times increase in electron temperature with a conical verses flat target. This increase in collimated suprathermal electrons boosts total photon yield as well as possibly enhancing line emission verses the bremsstrahlung continuum. Preliminary experiments demonstrate a three‐fold higher X‐ray yield and a two‐fold reduction in focal spot with the pyramidal verses the flat target. Furthermore, the geometry of the conical targets not only reduces focal spot size to a few microns and pulse duration to a couple picoseconds, but allows the particles to escape the target perpendicular to the surface resulting in a particle‐free, ultra‐short X‐ray micro‐beam. Conclusion: Comparing LPP X‐ray source parameters to that of a standard X‐ray tube shows substantial improvements in focal spot size, photon flux, spectral range and emission duration. Focusing on target design can provide a cleaner, brighter, quasi‐monochromatic X‐ray source that could improve image quality in any medicaldiagnostic regime. Such advancements show promising applications in mammography and angiography. Conflict of Interest: Research sponsored by DOE/NNSA under University of Nevada Reno grant #DE‐FC52‐01NV14050.

We investigated the hot electron transport in foil and wedge shaped targets irradiated with ultra-intense laser pulses. The results suggest that the electrons are guided by the strong quasi-static electromagnetic fields at the wedge boundary.

A laser-accelerated beam of protons is used to isochorically heat a silicon foil to multi-eV temperatures. Proton dosimetry, self-emission, and synchronized reflective interferometry are used simultaneously to infer internal energy, temperature, and pressure.

We report the simultaneous, single shot, time-resolved measurements of self-emission and free expansion of aluminum isochorically heated by laser generated MeV protons. In this way we measure the equation of state along an isochore.

A novel target design utilizing hollow pyramid geometry is studied as a radiation source of electrons, protons, and Ka X-rays. Experiments compare this source to flat foil and circular cone targets under similar laser conditions.