KEYWORDS: Physics, Stanford Linear Collider, Free electron lasers, Lead, Photons, States of matter, Materials processing, Electrons, Analytical research, Raman spectroscopy
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.
We report a recent experiment where the first hard x-ray beam line, X-ray Pump Probe (XPP) instrument using the
SLAC National Accelerator Laboratory's Linac Coherent Light Source (LCLS) free electron laser, was used to heat thin
foils to high energy densities ~ 107 J/cm3. An intense 9 keV, 60 fs (FWHM) duration beam with energy of 2 - 4 mJ at
the XPP beam line was focused using beryllium lenses to an irradiance approaching 1016 Wcm-2. Targets of 0.5 - 3.5 μm
thick foils of Ag and Cu were studied using a suite of diagnostics including Fourier Domain Interferometry, energy
calorimetry and grating and crystal spectrometers. The experimental details and spectroscopic results from the campaign
will be described. Preliminary results indicate that the target is heated relatively uniformly to a temperature lower than
20 eV.
We report on the x-ray absorption of Warm Dense Matter experiment at the FLASH Free Electron Laser (FEL) facility at DESY. The FEL beam is used to produce Warm Dense Matter with soft x-ray absorption as the probe of electronic structure. A multilayer-coated parabolic mirror focuses the FEL radiation, to spot sizes as small as 0.3μm in a ~15fs pulse of containing >1012 photons at 13.5 nm wavelength, onto a thin sample. Silicon photodiodes measure the transmitted and reflected beams, while spectroscopy provides detailed measurement of the temperature of the sample. The goal is to measure over a range of intensities approaching 1018 W/cm2. Experimental results will be presented along with theoretical calculations. A brief report on future FEL efforts will be given.
A multilayer-coated 27-cm focal length parabola, optimized to reflect 13.5 nm wavelength at normal incidence,
was used in multiple FLASH experiments and focused the beam to a sub-micron beam size. The intensity of the beam
was measured indirectly from the depths of craters left by the FLASH beam on PMMA-coated substrates. Comparing
simulated and experimental shapes of the craters we found the best match for a wavefront error of 0.45 nm, or λ/30. We
further estimated that the FWHM of the focal spot was 350 nm and that the intensity in the focus was 1018 W/cm2. The
sub-micron FLASH beam provided extreme intensity conditions essential for warm dense matter experiments. The same
optic was used in multiple experiments and survived the beam. However, after the first measurements, which took place
over several days, the optical surface was contaminated. This contamination reduced the mirror reflectivity, which was
partially recovered by oxygen plasma cleaning. However, even the partially cleaned multilayer-coated optic is still
diffraction limited and can focus the beam in future experiments to a sub-micron beam size.
The transient evolution of X-ray emission from high density plasmas usually terminates with a recombination
regime at low density. For matter irradiated by short pulses (< 1 ps) the accumulation of x-ray emission at low density
cannot be suppressed by streak camera techniques as current time resolution is limited (> 0.5 ps). We propose
intrinsically fast x-ray signals realized by the radiation emission from hollow ion states K0Ln to temporally isolate high
density information from the low density radiative recombination regime. Simulations carried out for short pulse (100
fs) intense x-ray free electron laser radiation (XFEL) interacting with dense plasmas demonstrate that the hollow ion x-ray
emission "switch" has a time scale faster than 50 fs. Moreover, the time of the hollow-ion emission signal is related
to the interaction time and is thus directly related only to the highest density regimes even if the usual time integrated
spectroscopic techniques are employed.
Short pulse (< 100 fs) tunable X-ray and VUV laser sources, based on the free electron laser (FEL) concept, will be a
watershed for high energy density research in several areas. These new 4th generation light sources will have extremely
high fields and short wavelength (~0.1 nm) with peak spectral brightness -photons/(s/mrad2/mm2/0.1% bandwidth- 1010
greater than 3rd generation light sources. We briefly discuss several applications: the creation of warm dense matter
(WDM), probing of near solid density plasmas, and laser-plasma spectroscopy of ions in plasmas. The study of dense
plasmas has been severely hampered by the fact that laser-based probes that can directly access the matter in this regime
have been unavailable and these new 4th generation sources will remove these restrictions. Finally, we present the plans
for a user-oriented set of facilities that will incorporate high-energy, intense short-pulse, and x-ray lasers at the first x-ray
FEL, the LCLS to be opened at SLAC in 2009.
Hard x-ray spectra were recorded by the High Energy Electronic X-Ray (HENEX) spectrometer from a variety of targets irradiated by the Omega laser at the Laboratory for Laser Energetics. The HENEX spectrometer utilizes four reflection crystals covering the 1 keV to 20 keV energy range and one quartz(10-11) transmission crystal (Laue geometry) covering the 11 keV to 40 keV range. The time-integrated spectral images were recorded on five CMOS x-ray detectors. Spectra were recorded from gold and other metal targets and from krypton-filled gasbags and hohlraums. In the spectra from the krypton-filled targets, the helium-like K-shell transitions n=1-2, 1-3, and 1-4 appeared in the 13 keV to 17 keV energy range. A number of additional spectral features were observed at energies lower than the helium-like n=1-3 and n=1-4 transitions. Based on computational simulations of the spectra using the FLYCHW/FLYSPEC codes, which included opacity effects, these additional features are identified to be inner-shell transitions from the Li- like through N-like krypton charge states. The comparisons of the calculated and observed spectra indicate that these transitions are characteristic of the plasma conditions immediately after the laser pulse when the krypton density is 2x1018 cm-3 and the electron temperature is in the range 2.8 keV to 3.2 keV. These spectral features represent a new diagnostic for determining the charge state distribution, the density and electron temperature, and the plasma opacity. The intense 13 keV krypton K-shell emission should be useful for backlighters and radiography of dense plasmas. Laboratory experiments indicate that it is feasible to record K-shell spectra from gold and higher Z targets in the > 60 keV energy range using a Ge(220) transmission crystal.
Laser-based plasma spectroscopic techniques have been used with great success to determine the line shapes of atomic transitions in plasmas, study the population kinetics of atomic systems embedded in plasmas, and look at the redistribution of radiation. However, the possibilities for optical lasers end for plasmas with ne > 1022 cm-3 as light propagation is severely altered by the plasma. The construction of the Tesla Test Facility (TTF) at DESY (Deutsche Elektronen-Synchrotron), a short pulse tunable free electron laser in the vacuum-ultraviolet and soft X-ray regime (VUV FEL), based on the SASE (self amplified spontaneous emission) process, will provide a major advance in the capability for dense plasma-related research. This source will provide 1013 photons in a 200 fs duration pulse that is tunable from ~6 nm to 100 nm. Since an VUV FEL will not have the limitation associated with optical lasers the entire field of high density plasmas kinetics in laser produced plasma will then be available to study with the tunable source. Thus, one will be able to use this and other FEL x-ray sources to pump individual transitions creating enhanced population in the excited states that can be easily monitored. We show two case studies illuminating different aspects of plasma spectroscopy.
The temporal coherence properties of the X-ray pulse from a Free Electron Laser (FEL) will be altered during the process of dynamical diffraction from a perfect crystal. We present simulations of this process based on time-dependent dynamical diffraction theory. In addition, we present simulations of the diffraction of chirped X-ray pulses, demonstrating methods of pulse recompression by use of strained crystals.
We describe how the powerful technique of spectrally resolved Thomson scattering can be extended to the x-ray regime, for direct measurements of the ionization state, density, temperature, and the microscopic behavior of dense cool plasmas. Such a direct measurement of microscopic parameters of solid density plasmas could eventually be used to properly interpret laboratory measurements of material properties such as thermal and electrical conductivity, EOS and opacity. In addition, x-ray Thomson scattering will provide new information on the characteristics of hitherto difficult to diagnose Fermi degenerate and strongly coupled plasmas.
Although the realisation of femtosecond X-ray free electron laser (FEL) X-ray pulses is still some time away, X-ray diffraction experiments within the sub-picosecond domain are already being performed using both synchrotron and laser- plasma based X-ray sources. Within this paper we summarise the current status of some of these experiments which, to date, have mainly concentrated on observing non-thermal melt and coherent phonons in laser-irradiated semiconductors. Furthermore, with the advent of FEL sources, X-ray pulse lengths may soon be sufficiently short that the finite response time of monochromators may themselves place fundamental limits on achievable temporal resolution. A brief review of time-dependent X-ray diffraction relevant to such effects is presented.
Time-dependent x-ray diffraction has been measured from laser-irradiated semiconductor crystals. Laser pulses with 100 fs duration and 800 nm wavelength excite the sample inducing phase transitions. 5 keV x-rays from the Advanced Light Source are diffracted by a sagittally-focusing Si (111) crystal and then by the sample crystal, InSb (111), onto an avalanche photodiode. By detecting individual pulses of synchrotron radiation, which have a duration of 70 ps, the diffracted intensity is observed to decrease because of photoabsorption in a disordered surfaced layer. Rocking curves measured after the laser irradiation show a tail, which results from a strained region caused by expansion of the crystal lattice.
The emission of ultrashort thermal x-ray radiation is shown to be controlled by two factors; peak temperature and the population of `cold' electrons. Two experiments are used to illustrate these mechanisms. The first measures the time history of 50 angstroms - 130 angstroms x-rays from ultrashort pulse laser heated solid targets. This experiment is used to illustrate the effect of the peak temperature on the full width at half maximum of the emission. The second experiment measures the dependency of decay time of the 1s2(S0) - 1s2p(1P1) transition (He(alpha )) emission on target thickness. This is used to infer the effects of unheated material or `cold' electrons on the time history of the x-ray emission.
A magnetically focused x-ray streak camera was designed and tested using sub-200 fs soft x-ray pulses generated by high harmonic emission in a gas. The temporal resolution of the camera was demonstrated to be under 0.54 ps for the ultraviolet and 0.88 ps in the soft-x-ray wavelength region. Our streak camera represents the fastest x-ray detector developed to date, and should allow sub-picosecond time resolution experiments to be performed using either synchrotron or laser-plasma-based x-ray sources.
The 33.8 angstroms emission from laser-irradiated targets was studied using a concave mirror with a W/B4C multilayer coating. The mirror had peak normal-incidence reflectance of 1.8% at a wavelength of 33.8 angstroms. Imaged were radiatively heated, low-density plastic and silica foams, x-ray laser targets, and a gas-filled enclosure.
We report preliminary results from the analysis of streaked soft x-ray neon spectra obtained from the interaction of a picosecond Nd:glass laser with a gas jet target. In these experiments streaked spectra show prompt harmonic emission followed by longer time duration soft x-ray line emission. The majority of the line emission observed was found to originate from Li- and Be-like Ne and the major transitions in the observed spectra have been identified. Li-like emission lines were observed to decay faster in time than Be-like transitions, suggesting that recombination is taking place. Line ratios of n equals 4 - 2 and n equals 3 - 2 transitions supported the view that these lines were optically thin and thick, respectively. The time history of Li-like Ne 2p-4d and 2p-3d lines is in good agreement with a simple adiabatic expansion model coupled to a time dependent collisional-radiative code. Further x-ray spectroscopic analysis is underway which is aimed at diagnosing plasma conditions and assessing the potential of this recombining neon plasma as a quasi-steady-state recombination x-ray laser medium.
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