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The spectrum of the undulator radiation of beamline P01 at Petra III has been measured after passing a multiple reflection channel-cut polarimeter. Odd and even harmonics up to the 15th order, as well as Compton peaks which were produced by the high harmonics in the spectrum, could been measured. These additional contributions can have a tremendous influence on the performance of the polarimeter and have to be taken into account for further polarimeter designs.

Single-photon spectroscopy of pulsed, high-intensity sources of hard X-rays - such as laser-generated plasmas - is often hampered by the pileup of several photons absorbed by the unsegmented, large-volume sensors routinely used for the detection of high-energy radiation. Detectors based on the Timepix chip, with a segmentation pitch of 55 μm and the possibility to be equipped with high-Z sensor chips, constitute an attractive alternative to commonly used passive solutions such as image plates. In this report, we present energy calibration and characterization measurements of such devices. The achievable energy resolution is comparable to that of scintillators for γ spectroscopy. Moreover, we also introduce a simple two-detector Compton polarimeter setup with a polarimeter quality of (98 ± 1)%. Finally, a proof-of-principle polarimetry experiment is discussed, where we studied the linear polarization of bremsstrahlung emitted by a laser-driven plasma and found an indication of the X-ray polarization direction depending on the polarization state of the incident laser pulse.

WOS:000373585800006 PubMed:27176413 An analysis of an electron spectrometer used to characterize fast electrons generated by ultraintense (10(20) W cm(-2)) laser interaction with a preformed plasma of scale length measured by shadowgraphy is presented. The effects of fringing magnetic fields on the electron spectral measurements and the accuracy of density scale-length measurements are evaluated. 2D EPOCH PIC code simulations are found to be in agreement with measurements of the electron energy spectra showing that laser filamentation in plasma preformed by a prepulse is important with longer plasma scale lengths (> 8 mu m). RFBR Research Project [15-32-21121]; RAS Presidium Program for Basic Research [13]; UK EPSRCEngineering & Physical Sciences Research Council (EPSRC) [EP/G054950/1, EP/G056803/1, EP/G055165/1, EP/M022463/1]; Engineering and Physical Sciences Research CouncilEngineering & Physical Sciences Research Council (EPSRC) [EP/M022463/1, EP/G056803/1] The authors gratefully acknowledge the assistance of laser operations, target preparation, and engineering staff at the Central Laser Facility of RAL. Part of the research was supported by RFBR Research Project No. 15-32-21121 and RAS Presidium Program for Basic Research No. 13. This work was in part funded by the UK EPSRC Grants No. EP/G054950/1, No. EP/G056803/1, No. EP/G055165/1, and No. EP/M022463/1.

The linear polarization of the characteristic lines $L{\ensuremath{\alpha}}_{1}$ $(3{d}_{5/2}\ensuremath{\rightarrow}2{p}_{3/2})$ and $L{\ensuremath{\alpha}}_{2}$ $(3{d}_{3/2}\ensuremath{\rightarrow}2{p}_{3/2})$, following inner-shell photoionization of neutral tungsten, is analyzed both experimentally and theoretically. In the experiment, a tungsten target is photoionized by the primary emission of an x-ray tube with incident photon energies in the range of 10.2--30 keV. The $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ components of the emitted fluorescence are measured by using a spectropolarimeter, based on x-ray diffraction at Bragg angles close to ${45}^{\ensuremath{\circ}}$. The degree of linear polarization of the $L{\ensuremath{\alpha}}_{1}$ and $L{\ensuremath{\alpha}}_{2}$ lines is determined to be $+(1.6\ifmmode\pm\else\textpm\fi{}0.5)%$ and $\ensuremath{-}(7\ifmmode\pm\else\textpm\fi{}2)%$, respectively. In addition, this degree of polarization is calculated within the framework of the density-matrix theory as a function of the incident photon energy. These calculations are in good agreement with the experimental results and show only a weak dependence of the degree of polarization on the energy of the incident photoionizing photon.

We studied temperature and Kα yield radial profiles of thin titanium foils as a result of femtosecond high-power laser pulse irradiation at several 1019 W/cm2 by high-resolution x-ray spectroscopy. Laser-accelerated electrons heat the cold solid to bulk temperatures of up to ∼50 eV. The plasma strongly affects the shape of the emitted Kα doublet, which is surveyed by x-ray spectroscopy with both high spectral (E/ΔE ≥ 15,000) and 1D spatial (Δx ≤ 13.5 μm) resolutions. Temperature-dependent spectra modeled by line-shape calculations are compared with Abel-inverted experimental spectra and provide a radial temperature distribution. The radially resolved Kα yield shows a depletion of the Kα1-line at the position of the laser focus. The density gradients induced by prepulses are modeled by hydrodynamic simulations, and density-dependent line-shape models are applied. The x-ray yield as function of foil thickness is explained by partial refluxing of a multi-keV electron distribution inside the foil, supported by Monte-Carlo simulations. Finally, we derive parameters to optimize the peak brilliance of such a laser-driven thin foil x-ray source.

We investigated laser-driven ion acceleration with tightly focused femtosecond laser pulses at an intensity of 5×1019 W/cm2. Targets consisting of thin metal foils either back-coated with a μ m -thick dielectric layer or uncoated have been used. The observations we report show that proton bunches with energy in the MeV range have been produced. Furthermore, the protons emitted from back-coated targets exhibit a spatial cross-section which is remarkably more uniform and of significantly smaller size if compared with that of the protons emitted from uncoated foils. The experimental results are discussed in terms of the differences between the propagation of the fast electron current inside the back-coated and uncoated targets.

The development activity of a new experimental technique for the study of the fast electron transport in high density matter is reported. This new diagnostic tool enables the X-ray 2D imaging of ultrahigh intensity laser plasmas with simultaneous spectral resolution in a very large energy range to be obtained. Results from recent experiments are discussed, in which the electron propagation in multilayer targets was studied by using the Kα. In particular, results highlighting the role of anisotropic Bremsstrahlung are reported, for the sake of the explanation of the capabilities of the new diagnostics. A discussion of a test experiment conceived to extend the technique to a single-shot operation is finally given.

The aim of the reported research is to contribute to investigation of new processes and methods interlinking nuclear and laser-plasma physics. With respect to requirements of nuclear experiments at medium-size high-power lasers, the selection of proper candidates for studying the excitation and decay of low-lying nuclear states is reviewed. An experimental approach to the identification of low-energy nuclear transitions is discussed, simple estimates of the 181Ta excitation yield in the laser-generated plasma provide a theoretical basis for planning future work. First tests and results of the experiments at the laser facility PALS are presented.

We describe recent measurements in which a novel imaging technique was used to investigate the transport of high energy electrons produced by the interaction of a femtosecond laser pulse with a three-layer target at an intensity of 5 × 1019 W cm−2. The imaging system was configured to work in a single-photon detection regime to identify the energy of the x-ray photons and to discriminate among Kα photons generated in each target layer. Electrons emerging from the rear side after propagation through all the target layers were also detected using a custom developed detector. The results on fast electron propagation are combined with the information obtained from electron diagnostics and are modelled using analytical and numerical codes to obtain a detailed description of electron propagation dynamics.

Laser plasma interaction experiments have been performed using an fs Titanium Sapphire laser. Plasmas have been generated from planar PMMA targets using single laser pulses with 3.3 mJ pulse energy, 50 fs pulse duration at 800 nm wavelength. The electron density distributions of the plasmas in different delay times have been characterized by means of Nomarski Interferometry. Experimental data were compared with hydrodynamic simulation. First results to characterize the plasma density and temperature as a function of space and time are obtained. This work aims to generate plasmas in the warm dense matter (WDM) regime at near solid-density in an ultra-fast laser target interaction process. Plasmas under these conditions can serve as targets to develop X-ray Thomson scattering as a plasma diagnostic tool, e.g., using the Vacuum ultraviolet (VUV) free-electron laser (FLASH) at Dentsches Elektronen-Synchrotron (DESY) Hamburg.

We report on studies of plasmas, formed by the interaction of 80 fs, 1 J Ti:Sapphire laser pulses with solid matter. The laser intensity was 5 × 1019 W/cm2. The Kα1, 2 X-ray emission spectra at ≈4.5 keV were measured using a toroidally bent GaAs (400) crystal spectrometer with the spectral and spatial resolutions of 0.3 eV and 4 μm, respectively. A Si (220) polarizer was used for determining the polarization properties of the recorded emission. A strong polarization dependence of the Kα satellite emission was observed, which may indicate an anisotropy of the relativistic electron velocity distribution. Also, electrons with energies in the range of 500 keV–13 MeV were detected using an electron spectrometer.

Acoustical phonons in InSb were induced with femtosecond light pulses and probed by diffraction of ultrashort X-ray pulses in the crystal lattice. The time dependent transient X-ray diffraction signal due to elastic lattice deformation was measured with subpicosecond resolution. The elastic lattice deformation depends on the temporal evolution of the energy transfer from excited electrons in the semiconductor into the lattice. As already shown in previous investigation a conventional thermoelastic model is not sufficient to describe this coupling process. Here a complex simulation including a two temperature model of the electron and lattice as well as the microscopic behavior of the electron plasma is applied to explain important effects like thermal carrier diffusion and band gap deformation found in the lattice deformation experiments. When this model is used, with realistic values for both pump laser fluences and bulk material constants, excellent agreement between the experimentally observed time dependent lattice deformation and calculated values is obtained throughout the observation period. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The direct and real-time study of structural processes has been very successful in revealing timescales of phase transitions, non-thermal melting, acoustic waves, optical phonons and charge transfer in solids. Time-resolved X-ray diffraction (TRXD) is a rapidly growing research area of much current interest. The most important requirement for such a challenging task is to provide a source of hard X-rays with sub-picosecond pulse duration. In this chapter the development and application of a table-top laser system to generate hard X-rays with sub-picosecond pulse durations for time-resolved X-ray measurements are described.

We investigate the properties of a laser-plasma accelerator as a source of x-ray radiation and combine the x-ray and electron data and demonstrates the use of the betatron radiation as a diagnostic tool for the injection and acceleration mechanism.

To extend ultra-fast time resolved X-ray diffraction measurements to a wider class of phenomena, this chapter presents investigations of solids at cryogenic temperatures. The setup is described, and results of static measurements of the temperature dependency of SrTiO3 lattice constants, as well as results on the transient behaviour of a system consisting of a SrTiO3 single crystal covered by a thin film of the high temperature superconductor YBa2Cu3O7–δ are presented.

Laser-plasma particle accelerators could provide more compact sources of high-energy radiation than conventional accelerators. Moreover, because they deliver radiation in femtosecond pulses, they could improve the time resolution of X-ray absorption techniques. Here we show that we can measure and control the polarization of ultra-short, broad-band keV photon pulses emitted from a laser-plasma-based betatron source. The electron trajectories and hence the polarization of the emitted X-rays are experimentally controlled by the pulse-front tilt of the driving laser pulses. Particle-in-cell simulations show that an asymmetric plasma wave can be driven by a tilted pulse front and a non-symmetric intensity distribution of the focal spot. Both lead to a notable off-axis electron injection followed by collective electron–betatron oscillations. We expect that our method for an all-optical steering is not only useful for plasma-based X-ray sources but also has significance for future laser-based particle accelerators., Radiation sources driven by laser-plasma accelerators have the potential to produce shorter bursts of radiation at lower cost than those based on conventional accelerators. Schnell et al. demonstrate the ability to control the polarization of the bursts of hard X-rays produced by such a source.

The polarization purity of 6.457- and 12.914-keV x rays has been improved to the level of $2.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ and $5.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$. The polarizers are channel-cut silicon crystals using six 90\ifmmode^\circ\else\textdegree\fi{} reflections. Their performance and possible applications are demonstrated in the measurement of the optical activity of a sucrose solution.

We explore the InSb-semiconductor lattice dynamics after excitation of high density electron-hole plasma with an ultrashort and intense laser pulse. By using time resolved x-ray diffraction, a sub-mÅ and sub-ps resolution was achieved. Thus, a strain of 4% was measured in a 3 nm thin surface layer 2 ps after excitation. The lattice strain was observed for the first 5 ps as exponentially decaying, changing rapidly by time and by depth. The observed phenomena can only be understood assuming nonlinear time dependent laser absorption where the absorption depth decreases by a factor of twenty compared to linear absorption.

We developed a detection scheme, capable of measuring X-ray line shape of tracer ions in μm thick layers at the rear side of a target foil irradiated by ultra intense laser pulses. We performed simulations of the effect of strong electric fields on the K-shell emission of silicon and developed a spectrometer dedicated to record this emission. The combination of a cylindrically bent crystal in von Hamos geometry and a CCD camera with its single photon counting capability allows for a high dynamic range of the instrument and background free spectra. This approach will be used in future experiments to study electric fields of the order of TV/m at high density plasmas close to solid density.

We study warm dense matter formed by subpicosecond laser irradiation at several ${10}^{19}\text{ }\text{W}/{\text{cm}}^{2}$ of thin Ti foils using x-ray spectroscopy with high spectral $(E/\ensuremath{\Delta}E\ensuremath{\sim}15,000)$ and one-dimensional spatial $(\ensuremath{\Delta}x=13.5\text{ }\ensuremath{\mu}\text{m})$ resolutions. $\text{Ti}\text{ }K\ensuremath{\alpha}$ doublets modeled by line-shape calculations are compared with Abel-inverted single-pulse experimental spectra and provide radial distributions of the bulk-electron temperature and the absolute-photon number $K\ensuremath{\alpha}$ yield in the target interiors. A core with $\ensuremath{\sim}40\text{ }\text{eV}$ extends homogeneously up to ten times the laser-focus size. The spatial distributions of the bulk-electron temperature and $K\ensuremath{\alpha}$ yield are strongly correlated.

We study warm dense matter formed by subpicosecond laser irradiation at several 10(19) W/cm(2) of thin Ti foils using x-ray spectroscopy with high spectral (E/DeltaE approximately 15,000) and one-dimensional spatial (Deltax=13.5 microm) resolutions. Ti Kalpha doublets modeled by line-shape calculations are compared with Abel-inverted single-pulse experimental spectra and provide radial distributions of the bulk-electron temperature and the absolute-photon number Kalpha yield in the target interiors. A core with approximately 40 eV extends homogeneously up to ten times the laser-focus size. The spatial distributions of the bulk-electron temperature and Kalpha yield are strongly correlated.

Front and rear side x-ray emission from thin titanium foils irradiated by ultraintense laser pulses at intensities up to $\ensuremath{\approx}5\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$ was measured using a high-resolution imaging system. Significant differences in intensity, dimension, and spectrum between front and rear side emission intensity in the 3--12 keV photon energy range was found even for $5\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ thin Ti foils. Simulations and analysis of space-resolved spectra explain this behavior in terms of directional bremsstrahlung emission from fast electrons generated during the interaction process.

Spectrally resolved two-dimensional imaging of ultrashort laser-produced plasmas is described, obtained by means of an advanced technique. The technique has been tested with microplasmas produced by ultrashort relativistic laser pulses. The technique is based on the use of a pinhole camera equipped with a charge coupled device detector operating in the single-photon regime. The spectral resolution is about 150 eV in the 4-10 keV range, and images in any selected photon energy range have a spatial resolution of 5 microm. The potential of the technique to study fast electron propagation in ultraintense laser interaction with multilayer targets is discussed and some preliminary results are shown.

In this paper we describe recent studies on X-ray emission from ultra-fast laser interactions with solids. We describe the dedicated equipment including a powerful femtosecond, Titanium-Sapphire laser system and custom developed diagnostics for the characterization of both the laser performance and the X-ray emission. We show the experimental results obtained from irradiation of Aluminium and Titanium targets including X-ray yield and spectra obtained using single-photon counting and spectroscopy. We discuss correlation of X-ray emission with the measured properties of hot electrons emerging from the target rear side. In particular, forward accelerated fast electrons propagating through a Ti foil are found to be emitted in a cone perpendicular to the target. A comparison of the experimental findings with the results of a PIC simulation is also reported, aimed at identifying the physical processes responsible for the production of this forward propagating population of fast electrons. Finally, we show results of simple optical spectroscopy measurements of scattered light and we discuss the use of these results in view of optimization and control of this kind of X-ray sources.

We have measured the polarization purity of undulator radiation at 12.9 keV, with hitherto unachievable precision. We could measure a polarization purity of 1.8 × 10−4 by using a silicon channel-cut crystal with six Bragg reflections at 45° as analyzer.

Hot electron generation plays an important role in the fast ignition approach to inertial confinement fusion (ICF) and other applications with ultra-intense lasers. Hot electrons of temperature up to 10–20 MeV have been produced by high contrast picosecond duration laser pulses focussed to intensities of ∼1020 W cm−2 with a deliberate pre-pulse on solid targets using the Vulcan Petawatt Laser facility. We present measurements of the number and temperature of hot electrons obtained using an electron spectrometer. The results are correlated to the density scale length of the plasma produced by a controlled pre-pulse measured using an optical probe diagnostic. 1D simulations predict electron temperature variations with plasma density scale length in agreement with the experiment at shorter plasma scale lengths (

We demonstrate that an ultrashort-pulse laser-driven x-ray diode can be used for time-resolved experiments on a picosecond timescale. Hence, acoustical phonons in germanium are observed after ultrashort laser-excitation and the results are compared with calculations according to a microphysical model. We also show the advantages of this kind of picosecond x-ray source compared to other sources on the basis of its properties.

In this Note we report on the characterization of a “deep depletion” charge-coupled device (CCD) in the keV range. Measurement of quantum efficiency for different energies from 4.5 until 27keV is given. A measurement of the capability of the CCD to detect single photons is also reported together with a determination of the spatial and spectral resolution. A Monte Carlo simulation was used to give a quantitative comparison with the measured spatial resolution.