T. Whitcher

Soft x-ray free electron laser microfocus for exploring matter under extreme conditions

We have focused a beam (BL3) of FLASH (Free-electron LASer in Hamburg: lambda = 13.5 nm, pulse length 15 fs, pulse energy 10-40 microJ, 5 Hz) using a fine polished off-axis parabola having a focal length of 270 mm and coated with a Mo/Si multilayer with an initial reflectivity of 67% at 13.5 nm. The OAP was mounted and aligned with a picomotor controlled six-axis gimbal. Beam imprints on poly(methyl methacrylate) - PMMA were used to measure focus and the focused beam was used to create isochoric heating of various slab targets. Results show the focal spot has a diameter of < or =1 microm. Observations were correlated with simulations of best focus to provide further relevant information.

Probing near-solid density plasmas using soft X-ray scattering

X-ray scattering using highly brilliant x-ray free-electron laser (FEL) radiation provides new access to probe free-electron density, temperature and ionization in near-solid density plasmas. First experiments at the soft x-ray FEL FLASH at DESY, Hamburg, show the capabilities of this technique. The ultrashort FEL pulses in particular can probe equilibration phenomena occurring after excitation of the plasma using ultrashort optical laser pumping. We have investigated liquid hydrogen and find that the interaction of very intense soft x-ray FEL radiation alone heats the sample volume. As the plasma establishes, photons from the same pulse undergo scattering, thus probing the transient, warm dense matter state. We find a free-electron density of (2.6 ± 0.2) × 1020 cm−3 and an electron temperature of 14 ± 3.5 eV. In pump–probe experiments, using intense optical laser pulses to generate more extreme states of matter, this interaction of the probe pulse has to be considered in the interpretation of scattering data. In this paper, we present details of the experimental setup at FLASH and the diagnostic methods used to quantitatively analyse the data.

X-ray laser-induced ablation of lead compounds

The recent commissioning of a X-ray free-electron laser triggered an extensive research in the area of X-ray ablation of high-Z, high-density materials. Such compounds should be used to shorten an effective attenuation length for obtaining clean ablation imprints required for the focused beam analysis. Compounds of lead (Z=82) represent the materials of first choice. In this contribution, single-shot ablation thresholds are reported for PbWO4 and PbI2 exposed to ultra-short pulses of extreme ultraviolet radiation and X-rays at FLASH and LCLS facilities, respectively. Interestingly, the threshold reaches only 0.11 mJ/cm2 at 1.55 nm in lead tungstate although a value of 0.4 J/cm2 is expected according to the wavelength dependence of an attenuation length and the threshold value determined in the XUV spectral region, i.e., 79 mJ/cm2 at a FEL wavelength of 13.5 nm. Mechanisms of ablation processes are discussed to explain this discrepancy. Lead iodide shows at 1.55 nm significantly lower ablation threshold than tungstate although an attenuation length of the radiation is in both materials quite the same. Lower thermal and radiation stability of PbI2 is responsible for this finding.

Optical emission spectroscopy of various materials irradiated by soft x-ray free-electron laser

The beam of Free-Electron Laser in Hamburg (FLASH) tuned at either 32.5 nm or 13.7 nm was focused by a grazing incidence elliptical mirror and an off-axis parabolic mirror coated by Si/Mo multilayer on 20-micron and 1-micron spot, respectively. The grazing incidence and normal incidence focusing of ~10-fs pulses carrying an energy of 10 μJ lead at the surface of various solids (Si, Al, Ti, Ta, Si3N4, BN, a-C/Si, Ni/Si, Cr/Si, Rh/Si, Ce:YAG, poly(methyl methacrylate) - PMMA, stainless steel, etc.) to an irradiance of 1013 W/cm2 and 1016 W/cm2, respectively. The optical emission of the plasmas produced under these conditions was registered by grating (1200 lines/mm and/or 150 lines/mm) spectrometer MS257 (Oriel) equipped with iCCD head (iStar 720, Andor). Surprisingly, only lines belonging to the neutral atoms were observed at intensities around 1013 W/cm2. No lines of atomic ions have been identified in UV-vis spectra emitted from the plasmas formed by the FLASH beam focused in a 20-micron spot. At intensities around 1016 W/cm2, the OE spectra are again dominated by the atomic lines. However, a weak emission of Al+ and Al2+ was registered as well. The abundance ratio of Al/Al+ should be at least 100. The plasma is really cold, an excitation temperature equivalent to 0.8 eV was found by a computer simulation of the aluminum plasma OE spectrum. A broadband emission was also registered, both from the plasmas (typical is for carbon; there were no spectral lines) and the scintillators (on Ce:YAG crystal, both the luminescence bands and the line plasma emission were recorded by the spectrometer).

Emission Spectroscopy from an XUV Laser Irradiated Solid Target

We have used the XUV FLASH laser at DESY to irradiate solid targets with intense XUV pulse at 13.5nm and ~1016 Wcm−2. XUV emission spectroscopy using a grating spectrometer has been used to observe both continuum radiation and line emission from Al IV. We present some preliminary results that indicate time integrated temperatures of below 20eV while simulation indicates higher initial temperatures.

IN‐SITU PROBING OF LATTICE RESPONSE IN SHOCK COMPRESSED MATERIALS USING X‐RAY DIFFRACTION

Lattice level measurements of material response under extreme conditions are required to build a phenomenological understanding of the shock response of solids. We have successfully used laser produced plasma x‐ray sources coincident with laser driven shock waves to make in‐situ measurements of the lattice response during shock compression for both single crystal and polycrystalline materials. Using a detailed analysis of shocked single crystal iron which has undergone the α‐e phase transition we can constrain the transition mechanism to be consistent with a compression and shuffle of alternate lattice planes.

Non-thermal damage to lead tungstate induced by intense short-wavelength laser radiation (Conference Presentation)

Interaction of short-wavelength free-electron laser (FEL) beams with matter is undoubtedly a subject to extensive investigation in last decade. During the interaction various exotic states of matter, such as warm dense matter, may exist for a split second. Prior to irreversible damage or ablative removal of the target material, complicated electronic processes at the atomic level occur. As energetic photons impact the target, electrons from inner atomic shells are almost instantly photo-ionized, which may, in some special cases, cause bond weakening, even breaking of the covalent bonds, subsequently result to so-called non-thermal melting. The subject of our research is ablative damage to lead tungstate (PbWO4) induced by focused short-wavelength FEL pulses at different photon energies. Post-mortem analysis of complex damage patterns using the Raman spectroscopy, atomic-force (AFM) and Nomarski (DIC) microscopy confirms an existence of non-thermal melting induced by high-energy photons in the ionic monocrystalline target. Results obtained at Linac Coherent Light Source (LCLS), Free-electron in Hamburg (FLASH), and SPring-8 Compact SASE Source (SCSS) are presented in this Paper.

Probing dynamic material strength using in situ x-ray diffraction

The lattice level strain measured using in situ x-ray diffraction during shock compression of rolled iron foils is used along with the pressure dependent elastic constants to estimate the dynamic strength of 1±1 GPa at 15 GPa. We examine these results in the context of the constant stress (Voigt) and constant strain (Ruess) limit of grain interaction, discussing the implications at the lattice level.

PROSPECTS FOR USING X‐RAY FREE‐ELECTRON LASERS TO INVESTIGATE SHOCK‐COMPRESSED MATTER

Within the next few years hard X‐ray Free Electron Lasers will come on line. Such systems will have spectral brightnesses ten orders of magnitude greater than any extant synchrotron, with pulse lengths as short as a few femtoseconds. It is anticipated that large‐scale optical lasers capable of shock‐compressing matter to multi‐megabar pressures will be sited alongside the X‐ray source. We discuss how such systems can further our knowledge of shocked and isochorically heated matter, in particular investigating the potential to perform polycrystalline diffraction and the creation of warm dense matter.

A GEOMETRY FOR SUB‐NANOSECOND X‐RAY DIFFRACTION FROM LASER‐SHOCKED POLYCRYSTALLINE FOILS

In situ picosecond X‐ray diffraction has proved to be a useful tool in furthering our understanding of the response of shocked crystals at the lattice level. To date the vast majority of this work has used single crystals as the shocked samples, owing to their diffraction efficiency, although the study of the response of polycrystalline samples is clearly of interest for many applications. We present here the results of experiments to develop sub‐nanosecond powder/polycrystalline diffraction using a cylindrical pinhole camera. By allowing the incident X‐ray beam to impinge on the sample at non‐normal angles, the response of grains making a variety of angles to the shock propagation direction can potentially be interrogated.