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We report on the design and testing of glass nozzles used to produce liquid sheets. The sheet nozzles use a single converging channel chemically etched into glass wafers by standard lithographic methods. Operation in ambient air and vacuum was demonstrated. The measured sheet thickness ranges over one order of magnitude with the smallest thickness of 250 nm and the largest of 2.5 μm. Sheet thickness was shown to be independent of liquid flow rate, and dependent on the nozzle outlet area. Sheet surface roughness was dependent on nozzle surface finish and was on the order of 10 nm for polished nozzles. Electron transmission data is presented for various sheet thicknesses near the MeV mean free path and the charge pair distribution function for D

Capturing OH(H 3 O + ) in ionized water Recent advances in liquid-phase ultrafast electron diffraction techniques make it possible to observe what has only been theoretically presumed to occur at short times upon interaction of ionizing radiation with liquid water. Lin et al . provide direct evidence for capturing the short-lived radical-cation pair OH(H 3 O + ), which has been hypothesized for years to form after ionization of liquid water but was not structurally resolved previously (see the Perspective by Cao et al .). The authors trace dissociation and subsequent nonradiative structural relaxation around the ionization center. —YS

Water is one of the most important, yet least understood, liquids in nature. Many anomalous properties of liquid water originate from its well-connected hydrogen bond network1, including unusually efficient vibrational energy redistribution and relaxation2. An accurate description of the ultrafast vibrational motion of water molecules is essential for understanding the nature of hydrogen bonds and many solution-phase chemical reactions. Most existing knowledge of vibrational relaxation in water is built upon ultrafast spectroscopy experiments2–7. However, these experiments cannot directly resolve the motion of the atomic positions and require difficult translation of spectral dynamics into hydrogen bond dynamics. Here, we measure the ultrafast structural response to the excitation of the OH stretching vibration in liquid water with femtosecond temporal and atomic spatial resolution using liquid ultrafast electron scattering. We observed a transient hydrogen bond contraction of roughly 0.04 A on a timescale of 80 femtoseconds, followed by a thermalization on a timescale of approximately 1 picosecond. Molecular dynamics simulations reveal the need to treat the distribution of the shared proton in the hydrogen bond quantum mechanically to capture the structural dynamics on femtosecond timescales. Our experiment and simulations unveil the intermolecular character of the water vibration preceding the relaxation of the OH stretch. Liquid ultrafast electron scattering measures structural responses in liquid water with femtosecond temporal and atomic spatial resolution to reveal a transient hydrogen bond contraction then thermalization preceding relaxation of the OH stretch.

Intense ultrafast laser excitation brings materials into highly nonequilibrium states with complex solid–liquid phase transitions. These ultrafast processes yield warm dense matter (WDM) conditions characterized by comparable thermal and Fermi energies, and strong ion–ion coupling. Here, we review recent studies employing mega-electron-volt ultrafast electron diffraction to resolve the structural dynamics of nonequilibrium WDM created by femtosecond laser heating of solids. For the study of warm dense gold, the results showed homogeneous melting that occurs within tens of picoseconds at absorbed energy densities of 0.4–1.4 MJ/kg. These results constrained the electron–ion coupling rate and revealed the melting sensitivity to nucleation seeds. For the study of radiation-damaged tungsten, the results showed a melting transition below the melting temperature. Molecular dynamics simulations suggested that this melting behavior is driven by vacancy defect clusters from radiation damage. These studies provided atomic-level insights into the melting behavior of materials under extreme conditions.

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]

Using in situ electron diffraction techniques, we demonstrate that femtosecond optical excitation above a threshold fluence of the amorphous, as-deposited, phase change material Ge2Sb2Te5 creates l...

We report on the study of electron kinetics induced by intense femtosecond (fs) laser excitation of electrons in the $5d$ band of Au. Changes in the electron system are observed from the temporal evolution of ac conductivity and conduction electron density. The results reveal an increase of electron thermalization time with excitation energy density, contrary to the Fermi-liquid behavior of the decrease of thermalization time associated with the heating of conduction electrons. This is attributed to the severe mitigation of photoexcitation by Auger decay. The study also uncovers the shortening of $5d$ hole lifetime with the increase of photoexcitation rates. These unique findings provide valuable insights for understanding electron kinetics under extreme nonequilibrium conditions.

Water is one of the most important, yet least understood, liquids in nature. Many anomalous properties of liquid water originate from its well-connected hydrogen bond network1, including unusually efficient vibrational energy redistribution and relaxation2. An accurate description of the ultrafast vibrational motion of water molecules is essential for understanding the nature of hydrogen bonds and many solution-phase chemical reactions. Most existing knowledge of vibrational relaxation in water is built upon ultrafast spectroscopy experiments2-7. However, these experiments cannot directly resolve the motion of the atomic positions and require difficult translation of spectral dynamics into hydrogen bond dynamics. Here, we measure the ultrafast structural response to the excitation of the OH stretching vibration in liquid water with femtosecond temporal and atomic spatial resolution using liquid ultrafast electron scattering. We observed a transient hydrogen bond contraction of roughly 0.04 Å on a timescale of 80 femtoseconds, followed by a thermalization on a timescale of approximately 1 picosecond. Molecular dynamics simulations reveal the need to treat the distribution of the shared proton in the hydrogen bond quantum mechanically to capture the structural dynamics on femtosecond timescales. Our experiment and simulations unveil the intermolecular character of the water vibration preceding the relaxation of the OH stretch.

The conversion of light into chemical and mechanical energy mediates many important processes in nature, e.g. vision, photosynthesis and DNA photodamage. To understand the structure-function relationships regulating such processes one must strive to study them in their natural environment, i.e. in the liquid-phase. This presentation reports on the design of a novel Ultrafast Electron Diffraction instrument capable of resolving structural dynamics in liquid samples. The capabilities of this instrument are showcased in the study of water, where its structure was resolved up to the 3rd hydration shell with 0.6 A spatial resolution, and dynamics were resolved with 200 fs resolution.

Defect clusters in ion-irradiated tungsten are characterized by X-ray diffuse scattering. Single crystal tungsten samples are irradiated at 300 K by 0.5–5 MeV copper ions with total doses of 0.2, 0.6, 2, and 5 displacements per atom (DPA). Analysis of the diffuse scattering intensities allows the determination of the concentration of vacancy- and interstitial-type dislocation loops as a function of loop radius for each sample. More vacancies than interstitials are observed in each sample, and the radius of the vacancy loops increases with irradiation dose. For the 0.2DPA sample the average vacancy loop radius is 10.1 ± 3.0 A with a defect density (counting all atoms in the loops) of ( 10.2 ± 1.5 ) × 10 − 4 per tungsten atom, increasing to an average loop radius of 16.1 ± 3.1 A at a density of ( 8.5 ± 1.0 ) × 10 − 4 per tungsten atom for the 5DPA sample.

We studied the formation and attenuation of GHz elastic waves in free-standing nanoporous gold films by MeV ultrafast electron diffraction and finite element simulations. By tracing the evolution of the high frequency acoustic waves in time domain, we found that the bicontinuous nanoporous structure in nanoporous gold films results in three-dimensionally acoustic response with low coherence, leading to fast attenuation of the elastic waves in comparison with solid gold films. The morphologically dominated dynamics indicates the nanoporosity plays an important role in the high-frequency acoustic energy relaxation, which shines a light on the applications of dealloyed nanoporous materials in nanodevices and sensors as GHz and THz acoustic filters and dampers.

Electron scattering on liquid samples has been enabled recently by the development of ultrathin liquid sheet technologies. The data treatment of liquid-phase electron scattering has been mostly reliant on methodologies developed for gas electron diffraction, in which theoretical inputs and empirical fittings are often needed to account for the atomic form factor and remove the inelastic scattering background. The accuracy and impact of these theoretical and empirical inputs has not been benchmarked for liquid-phase electron scattering data. In this work, we present an alternative data treatment method that requires neither theoretical inputs nor empirical fittings. The merits of this new method are illustrated through the retrieval of real-space molecular structure from experimental electron scattering patterns of liquid water, carbon tetrachloride, chloroform, and dichloromethane.

The conversion of light into usable chemical and mechanical energy is pivotal to several biological and chemical processes, many of which occur in solution. To understand the structure–function relationships mediating these processes, a technique with high spatial and temporal resolutions is required. Here, we report on the design and commissioning of a liquid-phase mega-electron-volt (MeV) ultrafast electron diffraction instrument for the study of structural dynamics in solution. Limitations posed by the shallow penetration depth of electrons and the resulting information loss due to multiple scattering and the technical challenge of delivering liquids to vacuum were overcome through the use of MeV electrons and a gas-accelerated thin liquid sheet jet. To demonstrate the capabilities of this instrument, the structure of water and its network were resolved up to the 3 rd hydration shell with a spatial resolution of 0.6 Å; preliminary time-resolved experiments demonstrated a temporal resolution of 200 fs.

Time-dependent and constituent-specific spectral changes in soft near edge X-ray spectroscopy (XAS) of an [Fe/MgO]$_8$ metal/insulator heterostructure upon laser excitation are analyzed at the O K-edge with picosecond time resolution. The oxygen absorption edge of the insulator features a uniform intensity decrease of the fine structure at elevated phononic temperatures, which can be quantified by a simple simulation and fitting procedure presented here. Combining X-ray absorption spectroscopy with ultrafast electron diffraction measurements and ab initio calculations demonstrate that the transient intensity changes in XAS can be assigned to a transient lattice temperature. Thus, the sensitivity of transient near edge XAS to phonons is demonstrated., 10 pages, 6 figures

The electrical conductivity of water under extreme temperatures and densities plays a central role in modeling planetary magnetic fields. Experimental data are vital to test theories of high-energy-density water and assess the possible development and presence of extraterrestrial life. These states are also important in biology and chemistry studies when specimens in water are confined and excited using ultrafast optical or free-electron lasers (FELs). Here we utilize femtosecond optical lasers to measure the transient reflection and transmission of ultrathin water sheet samples uniformly heated by a 13.6 nm FEL approaching a highly conducting state at electron temperatures exceeding 20 000 K. The experiment probes the trajectory of water through the high-energy-density phase space and provides insights into changes in the index of refraction, charge carrier densities, and AC electrical conductivity at optical frequencies. At excitation energy densities exceeding 10 MJ/kg, the index of refraction falls to n = 0.7, and the thermally excited free-carrier density reaches n$_e$ = 5 × 10$^{27}$ m$^{−3}$, which is over an order of magnitude higher than that of the electron carriers produced by direct photoionization. Significant specular reflection is observed owing to critical electron density shielding of electromagnetic waves. The measured optical conductivity reaches 2 × 10$^4$ S/m, a value that is one to two orders of magnitude lower than those of simple metals in a liquid state. At electron temperatures below 15 000 K, the experimental results agree well with the theoretical calculations using density-functional theory/molecular-dynamics simulations. With increasing temperature, the electron density increases and the system approaches a Fermi distribution. In this regime, the conductivities agree better with predictions from the Ziman theory of liquid metals.

Radiation damage lowers the melting point of metal tungsten, an effect that needs to be reckoned with for fusion reactors., Materials exposed to extreme radiation environments such as fusion reactors or deep spaces accumulate substantial defect populations that alter their properties and subsequently the melting behavior. The quantitative characterization requires visualization with femtosecond temporal resolution on the atomic-scale length through measurements of the pair correlation function. Here, we demonstrate experimentally that electron diffraction at relativistic energies opens a new approach for studies of melting kinetics. Our measurements in radiation-damaged tungsten show that the tungsten target subjected to 10 displacements per atom of damage undergoes a melting transition below the melting temperature. Two-temperature molecular dynamics simulations reveal the crucial role of defect clusters, particularly nanovoids, in driving the ultrafast melting process observed on the time scale of less than 10 ps. These results provide new atomic-level insights into the ultrafast melting processes of materials in extreme environments.

Monomers and dimers of spherical gold nanoparticles (NPs) exhibit highly uniform plasmonic properties at the single-particle level due to their high structural homogeneity (precision plasmonics). R...

The element specificity of soft X-ray spectroscopy makes it an ideal tool for analyzing the microscopic origin of ultrafast dynamics induced by localized optical excitation in metal-insulator heterostructures. Using [Fe/MgO]$_n$ as a model system, we perform ultraviolet pump/soft X-ray probe experiments, which are sensitive to all constituents of these heterostructures, to probe both electronic and lattice excitations. Complementary ultrafast electron diffraction experiments independently analyze the lattice dynamics of the Fe constituent, and together with ab initio calculations yield comprehensive insight into the microscopic processes leading to local relaxation within a single constituent or non-local relaxation between two constituents. Besides electronic excitations in Fe, which are monitored at the Fe L$_3$ absorption edge and relax within 1 ps by electron-phonon coupling, soft X-ray analysis identifies a change at the oxygen K absorption edge of the MgO layers which occurs within 0.5 ps. This ultrafast energy transfer across the Fe-MgO interface is mediated by high-frequency, interface vibrational modes, which are excited by hot electrons in Fe and couple to vibrations in MgO in a mode-selective, non-thermal manner. A second, slower timescale is identified at the oxygen K pre-edge and the Fe L$_3$ edge. The slower process represents energy transfer by acoustic phonons and contributes to thermalization of the entire heterostructure. We thus find that the interfacial energy transfer is associated with non-equilibrium behavior in the phonon system. Because our experiments lack signatures of charge transfer across the interface, we conclude that phonon-mediated processes dominate the competition of electronic and lattice excitations in these non-local, non-equilibrium dynamics., 12 pages, 10 figures

We present a new measurement of lattice disassembly times in femtosecond-laser-heated polycrystalline Au nanofoils. The results are compared with molecular dynamics simulations incorporating a highly optimized, embedded-atom-method interatomic potential. For absorbed energy densities of $0.9--4.3\text{ }\text{ }\mathrm{MJ}/\mathrm{kg}$, the agreement between the experiment and simulation reveals a single-crystal-like behavior of homogeneous melting and corroborates the applicability of the interatomic potential in the nonequilibrium warm dense matter regime. For energy densities below $0.9\text{ }\text{ }\mathrm{MJ}/\mathrm{kg}$, the measurement is consistent with nanocrystal behavior where melting is initiated at the grain boundaries.

The nanosize effect on electron–phonon interactions in free-electron noble metals has been a subject of intense discussion because of their important applications in physics, chemistry, and biomedicine. However, the interference from supports of dispersive nanoparticulate metals has led to controversial observations. We utilize freestanding, bicontinuous nanoporous gold (NPG) films to investigate electron–phonon interaction dynamics using ultrafast MeV electron diffraction. Compared to solid gold films, NPG shows faster electron–phonon interaction and thus noticeably higher electron–phonon coupling constant. The results demonstrate that surface states of electrons and phonons play an important role in electron–phonon coupling of nanostructured materials.

Golden ultrafast melting Understanding fast melting of metals is important for applications such as welding and micromachining. However, fast melting leaves simulation as the only option for probing the process. Mo et al. performed ultrafast electron diffraction experiments on laser-pulsed gold films. This allowed detailed mapping of the melting process, which proceeds through two distinct regimes while the bonding behavior changes in unexpected ways. The results require adding new physical processes to high-energy melting models. Science , this issue p. 1451

We apply time-resolved MeV electron diffraction to study the electron-lattice energy relaxation in thin film Au-insulator heterostructures. Through precise measurements of the transient Debye-Waller-factor, the mean-square atomic displacement is directly determined, which allows to quantitatively follow the temporal evolution of the lattice temperature after short pulse laser excitation. Data obtained over an extended range of laser fluences reveal an increased relaxation rate when the film thickness is reduced or the Au-film is capped with an additional insulator top-layer. This behavior is attributed to a cross-interfacial coupling of excited electrons in the Au film to phonons in the adjacent insulator layer(s). Analysis of the data using the two-temperature-model taking explicitly into account the additional energy loss at the interface(s) allows to deduce the relative strength of the two relaxation channels. CA Sokolowski-Tinten

Time-resolved measurements of the ionization states of warm dense aluminum via K-shell absorption spectroscopy are demonstrated using betatron radiation generated from laser wakefield acceleration as a probe. The warm dense aluminum is generated by irradiating a free-standing nanofoil with a femtosecond optical laser pulse and was heated to an electron temperature of ∼20-25 eV at a close-to-solid mass density. Absorption dips in the transmitted x-ray spectrum due to the Al^{4+} and Al^{5+} ions are clearly seen during the experiments. The measured absorption spectra are compared to simulations with various ionization potential depression models, including the commonly used Stewart-Pyatt model and an alternative modified Ecker-Kroll model. The observed absorption spectra are in approximate agreement with these models, though indicating a slightly higher state of ionization and closer agreement for simulations with the modified Ecker-Kroll model.

Central to the calculation of structural, dynamical and thermal properties of matter is the interatomic potential that is little known particularly for non-simple metals under extreme conditions. A direct manifestation of interatomic potential is lattice stability that can readily be determined by observing the disassembly of an excited sample. Here we report the study of fs-laser excited Au. In our experiment, 30nm-thick freestanding gold foils are irradiated by 400nm, 45fs (FWHM) laser pulses to produce isochoric heated warm dense states with excitation energy densities ranging from ∼0.5–7.5MJ/kg (1010-1.5×1011J/m3). The disassembly of the sample is monitored with Frequency Domain Interferometry (FDI) using an 800nm, chirped laser probe pulse. The first finding is the dependence of disassembly time on excitation energy density. This provides the first test of predictions obtained from MD simulations1 using a glue model interatomic potential2. An equally significant finding is the appearance of surface melting that precedes bulk melting at energy densities above 3MJ/kg. Interestingly, the melting of two uppermost surface layers in Au has also been predicted in the glue model simulations2. The melting occurs at ∼100K below the bulk melting temperature and remains confined to the two layers. It should be noted that our results are not only of fundamental importance for understanding and modeling the properties of warm dense matter, but also key to the understanding of high intensity laser ablation of solids.

We report on the experimental and simulated characterization of proton acceleration from double-pulse irradiation of um-scale foil targets with varying preplasma conditions. Temporally separated pulses of less than a picosecond in duration have been shown to increase the conversion efficiency of laser energy to MeV protons1. The experiment utilized two 700 fs, 1054 nm pulses, separated by 1 to 5 ps; total beam energy was 100 J, with 5–20% of the total energy contained within the first pulse. In contrast to the ultraclean beams used in previous experiments1, prepulse energies on the order of 10 mJ were present. The resulting significant preplasma appears to have a moderating effect on the double pulse enhancement. Proton beam measurements were made with radiochromic film stacks and magnetic spectrometers.

Using time-resolved electron diffraction the electron-lattice equilibration in laser-excited thin Bi-films has been investigated. Our data reveal a pronounced thickness-dependence which is attributed to cross-interfacial coupling of hot electrons in the Bi-film to substrate phonons.

We have developed a single-shot mega-electronvolt ultrafast-electron-diffraction system to measure the structural dynamics of warm dense matter. The electron probe in this system is featured by a kinetic energy of 3.2 MeV and a total charge of 20 fC, with the FWHM pulse duration and spot size at sample of 350 fs and 120 μm respectively. We demonstrate its unique capability by visualizing the atomic structural changes of warm dense gold formed from a laser-excited 35-nm freestanding single-crystal gold foil. The temporal evolution of the Bragg peak intensity and of the liquid signal during solid-liquid phase transition are quantitatively determined. This experimental capability opens up an exciting opportunity to unravel the atomic dynamics of structural phase transitions in warm dense matter regime.

Electron-lattice coupling strength governs the energy transfer between electrons and the lattice and is important for understanding the material behavior under highly non-equilibrium conditions. Here we report the results of employing time-resolved electron diffraction at MeV energies to directly study the electron-lattice coupling strength in 40-nm-thick polycrystalline copper excited by femtosecond optical lasers. The temporal evolution of lattice temperature at various pump fluence conditions were obtained from the measurements of the Debye-Waller decay of multiple diffraction peaks. We observed the temperature dependence of the electron-lattice relaxation time which is a result of the temperature dependence of electron heat capacity. Comparison with two-temperature model simulations reveals an electron-lattice coupling strength of (0.9 ± 0.1) × 1017 W/m3/K for copper.

We measure temporal and spectral characteristics of soft X-ray lasers based on capillary discharge, employing flat-field and Rowland spectrographs for analyzing the spectrum of a Ne-like Ar soft Xray laser and a monochromator for analyzing its temporal behavior on the 46.9 nm laser line. The monochromator provides the time resolution for a single wavelength; therefore, the pulse width and the onset time of the laser pulse can be reliably measured. In spite of the fact that the flat field spectrograph is relatively difficult to adjust (due to the long distance between the entrance slit and the grating), we obtained a fine spectrum with a fairly high precision. In contrast, the Rowland spectrograph is widely used, easily adjusted, and provides instantaneously visible results, but its detection precision is just between that of the monochromator and the flat-field spectrograph. Each of these three types of devices is characterized by its own strong point for the accurate detection and study of the spectrum of a Ne-like Ar laser at J = 0− 1 and wavelength 46.9 nm.

Ne-like Ar 46.9 nm soft X-ray laser has been realized by the main current with different rise time. In this paper, the spectrum of capillary discharge Ne-like Ar 46.9 nm soft X-ray laser has been detected using a Rowland spectrograph, a new method of changing the rise time of the main current is also researched when the other conditions are fixed. The rise time will be changed when altering the inductance of the main switch, which will influence the time of lasing onset and the energy of the laser output. The time of lasing onset will be postponed and the energy of the laser will increase when the induction of the main switch increases. The energy of the laser increase by 78.5% when the rise time is changed from 38 to 55 ns.

To raise the utilization ratio of the energy power, and meanwhile, guarantee the waveform of the main discharge current to obtain X-ray laser, our capillary discharged Ne-like Ar soft X-ray laser system adopts Blumleim transmission line (BTL) and a 300 kV-Marx generator; and by using our self-developed intrinsic-prepulse elimination device and extrinsic-prepulse generator, we successfully removed the intrinsic prepulse of BTL, and thus eliminated its deteriorative effect towards Z-pinch plasma. By this means, we obtained stable X-ray laser output; and the voltage transmission rate of our system nearly reaches 122%, which provides a guarantee for the miniaturization of the power source and the whole system.

Under the condition of the low main current pulse, the capillary discharge soft X-ray laser is obtained. The laser energy of the same discharge current amplitude at different Ar pressures and of different current amplitudes at the same Ar pressure are compared, respectively. At different Ar pressures, the amplitude of the main current pulse evidently impacts the soft X ray, especially at a pressure higher than 30 Pa. In order to obtain the laser output at a high Ar pressure, the amplitude should be increased. However, at the same Ar pressure, it might have an optimum current, in which case the laser spike is the highest and the most stationary. The laser achieving time at different amplitudes is also discussed. As far as we know, this is the first time the influence of the amplitude of the main current pulse has been analyzed.

Energy loss in the transport of a beam of relativistic electrons in warm dense aluminum is measured in the regime of ultrahigh electron beam current density over 2×10^{11} A/cm^{2} (time averaged). The samples are heated by shock compression. Comparing to undriven cold solid targets, the roles of the different initial resistivity and of the transient resistivity (upon target heating during electron transport) are directly observable in the experimental data, and are reproduced by a comprehensive set of simulations describing the hydrodynamics of the shock compression and electron beam generation and transport. We measured a 19% increase in electron resistive energy loss in warm dense compared to cold solid samples of identical areal mass.

We have developed a new approach to characterize betatron X-ray radiation that is excited by the wiggling of relativistic electrons inside laser wakefield driven cavities. This approach can measure the average critical energy, Ec, of the betatron radiation by using the reflection off a grazing incidence mirror (ROGIM). This technique relies on the reflectivity change of a high-Z-coated mirror with the grazing angle of incidence for the betatron X-rays. The average Ec measured with this approach is in agreement with the one measured with a photon-counting X-ray CCD. This approach suggests that grazing incidence mirrors coupled to X-ray detectors can be used as online monitors of Ec in betatron applications.

At the ALLS laser facility1 we have explored and optimized the wakefield acceleration of electrons to the GeV energy range and now are beginning to explore the application of the Betatron x-rays generated in such a wakefield as an ultrashort probe pulse for the study of transient heating of warm dense matter2. Such broadband Betatron X-ray radiation, with femtosecond pulse duration, provides a new technique to diagnose femtosecond to picosecond transitions in the atomic structure as material transforms from solid state to Warm Dense Matter state. This can be done by observing the K-shell related absorption lines as a function of time which are directly related to the ionization states of the matter.

We have developed a laser wakefield generated X-ray probe to directly measure the temporal evolution of the ionization states in warm dense aluminum by means of absorption spectroscopy. As a promising alternative to the free electron excited X-ray sources, Betatron X-ray radiation, with femtosecond pulse duration, provides a new technique to diagnose femtosecond to picosecond transitions in the atomic structure. The X-ray probe system consists of an adjustable Kirkpatrick-Baez (KB) microscope for focusing the Betatron emission to a small probe spot on the sample being measured, and a flat Potassium Acid Phthalate Bragg crystal spectrometer to measure the transmitted X-ray spectrum in the region of the aluminum K-edge absorption lines. An X-ray focal spot size of around 50 μm was achieved after reflection from the platinum-coated 10-cm-long KB microscope mirrors. Shot to shot positioning stability of the Betatron radiation was measured resulting in an rms shot to shot variation in spatial pointing on the sample of 16 μm. The entire probe setup had a spectral resolution of ~1.5 eV, a detection bandwidth of ~24 eV, and an overall photon throughput efficiency of the order of 10(-5). Approximately 10 photons were detected by the X-ray CCD per laser shot within the spectrally resolved detection band. Thus, it is expected that hundreds of shots will be required per absorption spectrum to clearly observe the K-shell absorption features expected from the ionization states of the warm dense aluminum.

Summary form only given. Laser wakefield acceleration (LWFA) is a promising approach to realize table-top accelerators. The injection process into the wakefield bubble to some extent determines the charge, divergence, energy gain as well as the energy distribution of the accelerated electrons. Traditionally, self injection using pure helium or hydrogen gas as the interaction medium was employed to accelerate the electrons. However, very high laser powers are required to achieve self injection at the low densities which are compatible with acceleration to GeV energies. Recently, a new technique, ionization induced injection, which takes advantage of the large ionization potential difference between the inner and outer shell electrons of trace atoms in the plasma, has been demonstrated to generate electron beams beyond 1 GeV at lower threshold laser powers than self injection would require [1].

Summary form only given. Fast Ignition [1] holds the promise of improved efficiency and reduced laser energy requirements for Laser Fusion Energy systems. The main approach proposed to date is by coupling a beam of 1 to 2 MeV electrons from the laser interaction spot to a 40 micron spot in the compressed fuel core using a metal cone insert to get close to the compressed core [2]. However, multi-millijoule level laser prepulse can create extended preplasmas within the cone, effectively moving the electron generation source region far back from the cone tip and core [3]. By employing second harmonic pulses much reduced levels of prepulse can be achieved and at the same time colder electron distribution can be obtained, closer to those required ultimately for Fast Ignition.

We report the generation of a saturated capillary discharge soft X-ray laser obtained with low current of 14 kA. The laser is generated at the initial pressure of 18 Pa, with the gain coefficient of 0.58 cm−1.

We show through experiments that a transition from laser wakefield acceleration (LWFA) regime to a plasma wakefield acceleration (PWFA) regime can drive electrons up to energies close to the GeV level. Initially, the acceleration mechanism is dominated by the bubble created by the laser in the nonlinear regime of LWFA, leading to an injection of a large number of electrons. After propagation beyond the depletion length, leading to a depletion of the laser pulse, whose transverse ponderomotive force is not able to sustain the bubble anymore, the high energy dense bunch of electrons propagating inside bubble will drive its own wakefield by a PWFA regime. This wakefield will be able to trap and accelerate a population of electrons up to the GeV level during this second stage. Three dimensional particle-in-cell simulations support this analysis and confirm the scenario.

We have developed a new experimental platform at the Linac Coherent Light Source (LCLS) which combines simultaneous angularly and spectrally resolved x-ray scattering measurements. This technique offers a new insights on the structural and thermodynamic properties of warm dense matter. The < 50 fs temporal duration of the x-ray pulse provides near instantaneous snapshots of the dynamics of the compression. We present a proof of principle experiment for this platform to characterize a shock-compressed plastic foil. We observe the disappearance of the plastic semi-crystal structure and the formation of a compressed liquid ion-ion correlation peak. The plasma parameters of shock-compressed plastic can be measured as well, but requires an averaging over a few tens of shots.

Laser wakefield acceleration of 500 MeV to 1 GeV electron bunches has been demonstrated using ionization injection in mixtures of 4% to 10% of CO2 in He. 80 TW laser pulses were propagated through 5 mm gas jet targets at electron densities of 0.4−1.5×1019cm−3. Ionization injection led to lower density thresholds, a higher total electron charge, and an increased probability of producing electrons above 500 MeV in energy compared to self-injection in He gas alone. Electrons with GeV energies were also observed on a few shots and indicative of an additional energy enhancement mechanism.

A Kirkpatrick-Baez X-ray microscope has been developed for use on the Titan laser facility at the Lawrence Livermore National Laboratory in Fast Ignition experiments. It was developed as a broadband alternative to narrow band Bragg crystal imagers for imaging Kα emission from tracer layers. A re-entrant design is employed which allows for alignment from outside the chamber. The mirrors are coated with Pt and operate at a grazing incident angle of 0.5° providing higher resolution than an equal brightness pinhole and sufficient bandwidth to image thermally shifted characteristic Kα emission from heated Cu tracer layers in Fast Ignition experiments. The superpolished substrates (

Quasimonoenergetic electron beams with maximum energy >0.5 GeV and 2 mrad divergence have been generated in pure nitrogen gas via wakefield acceleration with 80 TW, 30 fs laser pulses. Long low energy tail features were typically observed due to continuous ionization injection. The measured peak electron energy decreased with the plasma density, agreeing with the predicted scaling for electrons. The experiments showed a threshold electron density of 3×1018cm-3 for self-trapping. Our experiments suggest that pure Nitrogen is a potential candidate gas to achieve GeV monoenergetic electrons using the ionization induced injection scheme for laser wakefield acceleration.