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.
KEYWORDS: Physics, Current controlled current source, Laser optics, Free electron lasers, X-ray optics, X-rays, Femtosecond phenomena, X-ray lasers, X-ray sources, High power lasers
The combination of powerful optical lasers and an x-ray free-electron laser (XFEL) provides unique capabilities to study the transient behavior of matter in extreme conditions. The high energy density science instrument (HED instrument) at the European XFEL will provide the experimental platform on which an unique x-ray source can be combined with various types of high-power optical lasers. In this paper, we highlight selected scientific examples together with the associated x-ray techniques, with particular emphasis on femtosecond (fs)-timescale pump–probe experiments. Subsequently, we present the current design status of the HED instrument, outlining how the experiments could be performed. First user experiments will start at the beginning of 2018, after which various optical lasers will be commissioned and made available to the international scientific community.
For the High Energy Density (HED) experiment [1] at the European XFEL [2] an x-ray split- and delay-unit (SDU) is
built covering photon energies from 5 keV up to 20 keV [3]. This SDU will enable time-resolved x-ray pump / x-ray
probe experiments [4,5] as well as sequential diffractive imaging [6] on a femtosecond to picosecond time scale.
Further, direct measurements of the temporal coherence properties will be possible by making use of a linear
autocorrelation [7,8]. The set-up is based on geometric wavefront beam splitting, which has successfully been
implemented at an autocorrelator at FLASH [9]. The x-ray FEL pulses are split by a sharp edge of a silicon mirror
coated with multilayers. Both partial beams will then pass variable delay lines. For different photon energies the angle
of incidence onto the multilayer mirrors will be adjusted in order to match the Bragg condition. For a photon energy of
hν = 20 keV a grazing angle of θ = 0.57° has to be set, which results in a footprint of the beam (6σ) on the mirror of
l = 98 mm. At this photon energy the reflectance of a Mo/B4C multi layer coating with a multilayer period of d = 3.2 nm
and N = 200 layers amounts to R = 0.92. In order to enhance the maximum transmission for photon energies of hν = 8
keV and below, a Ni/B4C multilayer coating can be applied beside the Mo/B4C coating for this spectral region. Because
of the different incidence angles, the path lengths of the beams will differ as a function of wavelength. Hence, maximum
delays between +/- 2.5 ps at hν = 20 keV and up to +/- 23 ps at hν = 5 keV will be possible.
With the development of hard X-ray free electron lasers, there is a pressing need to experimentally determine the single shot damage limits of presently used and potential future optical coating materials. To this end we present damage results, and analysis of fluence threshold limits, from grazing incidence geometry experiments conducted at the Spring-8 Angstrom Compact free electron LAser (SACLA) on Carbon coatings at 7 and 12 keV photon energies.
At XFEL sources, coherent and time-resolved experiments will strongly depend on the properties of the incoming
radiation passed through beamline optical elements to experimental stations. We investigate analytically and make
numerical modeling of SASE pulse propagation through optical transport systems of hard X-ray FEL beamlines. The
results on evolution of SASE XFEL pulses and its statistical properties during propagation through a double crystal
monochromator in Bragg and Laue diffraction geometry are presented.
R. Fäustlin, S. Toleikis, Th. Bornath, T. Döppner, S. Düsterer, E. Förster, C. Fortmann, S. Glenzer, S. Göde, G. Gregori, R. Irsig, T. Laarmann, H. Lee, B. Li, K.-H. Meiwes-Broer, J. Mithen, A. Przystawik, H. Redlin, R. Redmer, H. Reinholz, G. Röpke, F. Tavella, R. Thiele, J. Tiggesbäumker, I. Uschmann, U. Zastrau, Th. Tschentscher
We present collective Thomson scattering with soft x-ray free electron laser radiation as a method to track the evolution
of warm dense matter plasmas with ~200 fs time resolution. In a pump-probe scheme an 800 nm laser heats a 20 μm
hydrogen droplet to the plasma state. After a variable time delay in the order of ps the plasma is probed by an x-ray ultra
violet (XUV) pulse which scatters from the target and is recorded spectrally. Alternatively, in a self-Thomson scattering
experiment, a single XUV pulse heats the target while a portion of its photons are being scattered probing the target.
From such inelastic x-ray scattering spectra free electron temperature and density can be inferred giving insight on
relaxation time scales in plasmas as well as the equation of state. We prove the feasibility of this method in the XUV
range utilizing the free electron laser facility in Hamburg, FLASH. We recorded Thomson scattering spectra for
hydrogen plasma, both in the self-scattering and in the pump-probe mode using optical laser heating.
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.
The European X-Ray Free Electron Laser will deliver high intensity ultrashort pulses of x-rays. The results of
the x-ray interaction with matter in such a regime are not yet fully understood and the energy threshold for surface
modifications remains unknown. The behavior of optical components under irradiation is a major issue for the
European XFEL project. In fact some experiments rely on the coherence and high quality wave front of the beam and
any degradation, even on the nanometer scale, of the x-ray optical components will affect the performance of these
experiments.
Hence investigation of radiation effects on materials is needed. We will describe the on-going program at the
European XFEL which aims at developing new approaches for beamline design specific to FEL light source. Different
tools are used in order to simulate the beam propagation and interaction with optical elements.
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.
Ultra-fast soft x-ray lasers have opened a new area of laser-matter interactions which in most cases differ from the well
understood interaction of UV-vis radiation with solid targets. The photon energy >30eV essentially exceeds the width of
band gap in any known material and excites the electrons from the deep atomic and valence levels directly to the
conduction band. Both thermal and non-thermal phenomena can occur in such a material being caused by electron
thermalization and bond breaking, respectively. We report the first observation of non-thermal single-shot soft x-ray
laser induced desorption occurring below the ablation threshold in a thin layer of poly (methyl methacrylate) - PMMA.
Irradiated by the focused beam from the Free-electron LASer in Hamburg (FLASH) at 21.7nm, the samples have been
investigated by an atomic-force microscope (AFM) enabling the visualization of mild surface modifications caused by
the desorption. A model describing non-thermal desorption and ablation has been developed and used to analyze singleshot
imprints in PMMA. An intermediate regime of materials removal has been found, confirming the model predictions.
We also report below-threshold multiple-shot desorption of PMMA induced by high-order harmonics (HOH) at 32nm as
a proof of an efficient material removal in the desorption regime.
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).
Analytical and numerical simulations were carried out for both, surface profiles measured on a real ultra precise mirror
by use of the BESSY-NOM slope measuring profiler as well as for model local surface distortions. The effect of mirror
imperfections could be properly handled in the frame of the wave optics approach. In spite of the large distances, for
hard X-rays one still needs to carry out full-scale calculations surpassing the far field approximation. It is shown that the
slope errors corresponding to medium spatial frequency components are of a special importance for the properties of
coherent beam reflection from ultra smooth mirrors. The typical height errors for this component should not exceed 1-2
nm. Calculations show that reflection on such a mirror surface still imposes substantial wave field distortions at distances
of several hundred meters from the mirror relevant for European XFEL beamlines. Requirements and trade-off for high
precision mirrors and demands to coherent beams propagations are discussed.
X-ray free-electron lasers generate ultrashort and very intense x-ray radiation in the wavelength domain reaching from
the VUV (100 nm and shorter) all the way to the hard x-ray domain (typically 0.1 nm). FEL radiation features extreme
brilliance, ultrashort pulse duration, and high peak power. Superconducting accelerators provide furthermore the
possibility to accelerate a large number of electron bunches during a single radio-frequency pulse. Likewise the total
number of x-ray pulses available for the experiments is increased leading to a significantly higher average brilliance.
FEL light sources, and those based on super-conducting accelerator technology, are therefore considered to provide a
new quality of short wavelength radiation if compared to existing x-ray sources. The high intensity and the high
repetition rate lead to new requirements for x-ray optics in terms of peak and average power. Values for peak and
average power are presented in relation to the proposed realization of the photon beamlines at the European XFEL
facility.
TESLA is the project to build a superconducting linear collider for particle physics and an X-ray free-electron laser (XFEL) laboratory. The XFEL operates in the high-gain self-amplified spontaneous emission mode and peak brilliances of the order 1035,photons/(smm2mrad20.1%bw) corresponding to 1012 photons in a 100 femtosecond X-ray pulse have been calculated. The covered wavelength range extends from 25 to 0.85A. To obtain SASE FEL radiation at these wavelengths the electron accelerator must fulfill extreme requirements with respect to emittance and bandwidth. The paper describes the technical realization of the XFEL laboratory, introduces the properties of XFEL photon beams and gives an overview of the scientific applications of XFEL radiation.
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