X-ray Free Electron Laser (XFEL) radiation may transform diamond into graphite. Two X-ray pulses were used; the first as pump to trigger the phase transition and the second as probe performing X-ray diffraction. The experiment was performed at the SACLA XFEL facility at the beamline 3 experimental hutch 5. The samples were polycrystalline diamond. The pump and probe photon energies were 7 and 10.5 keV, respectively, and the delay between the X-ray pulses was varied from 0 to 286 fs. To provide a range of energy densities, the X-ray focus was adjusted between 150 nm and 1 um. The (111), (220) and (311) diffraction peaks were observed. The intensity of each diffraction peak decreased with time indicating a disordering of the crystal lattice. From a Debye-Waller analysis, the root-mean-square (rms) atomic displacement perpendicular to particular lattice planes are calculated. At higher fluences, the rms atomic displacement perpendicular to the (111) planes is significantly larger than that perpendicular to the (220) or (311) planes. By accepting two successive XFEL pulses at a time delay of 33 ms, graphite (002) diffraction was observed beginning at a threshold dose of 1.7 eV/atom. The experimental results will be compared with calculations using a hybrid model based on tight-binding molecular dynamics.
For LCLS-II, we have developed fluorescence intensity monitors and power meters as intensity monitors. The Fluorescence Intensity Monitor (FIM) provides the non-invasive, pulse-by-pulse normalization of experiments. For the LCLS-II instruments, the diagnostic was constructed with an array of four microchannel plate assemblies and four avalanche photodiodes. The diagnostics are being installed in each Kirkpatrick Baez mirror chamber. The noise of the diagnostic will be evaluated against a goal of 1 %.
The X-ray power meter delivers average power values. For the LCLS-II instruments, a power meter was selected compatible with high average power. In the LCLS-II instruments, power meters are being installed with each profile monitor in order to evaluate the transmission along the X-ray transport. A calibration of a set of power meters was carried out against a gas monitor detector at FLASH. In addition for all the power meters, a relative calibration was performed with a visible light source. At the endstations, a power meter will determined the pulse energy at the sample.
For the LCLS-II X-ray instruments, we have developed laser power meters as compact X-ray power monitors. A calibration of the responsivity of the power meters was carried out against a silicon photodiode with synchrotron radiation and a gas monitor detector with FEL X-rays. A manipulator with two power meters was installed in various locations at the LCLS. In the LCLS front end, the power meters were compared with the gas detectors, which are calibrated by the electron energy loss method. The agreement between the power meters and the gas detectors was better than 20% at 1500 eV with the pulse energy measured by the gas detectors higher than that from the power meters. In the AMO instrument, the power meters evaluated the improvement in beamline transmission caused by the oxygen plasma cleaning of the Kirkpatrick-Baez mirrors. Measurements were also conducted one and two years later to observe the effect of further contamination of the optical surfaces. Finally at the SXR instrument, the power meters determined the pulse energy at the sample for a beamtime, where the X-ray intensity was an important parameter.
The Icarus camera system, combining a sensor developed by Sandia and readout electronics by LLNL, provides 0.5 Hz bursts of four frames with 3 ns separation. The sensor has 1024×512 25 μm pixels and is 25 μm thick. The system was developed for single line-of-sight measurements at the ns time scale for electrons and X-rays at facilities such as NIF. We report on initial tests of the Icarus system with hard X-rays pulse pairs with nanosecond time spacings at the LCLS, a newly available beam mode. We describe noise, gate profiles, gain, cross-talk, persistence, linearity, and quantum efficiency for the first version of the sensor. We present evidence of the suitability of the system for science measurements at a free electron laser with an X-ray pump X-ray probe experiment. We expect further developments of the technology to allow use of 350 ps bunch separation from the LCLS accelerator and, with a pulsed delay tube like DIXIE, to eventually reach sub-25 ps time-resolved X-ray imaging of processes such as plasma evolution.
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: Power meters, X-rays, Free electron lasers, Sensors, Laser development, Reflectivity, Mirrors, Temperature metrology, Liquid crystal lasers, Stanford Linear Collider
For the LCLS-II instruments we are developing laser power meters as compact intensity monitors that can operate at soft and tender X-ray photon energies. There is a need to monitor the relative X-ray intensity at various locations along an X-ray FEL beamline in order to observe a possible decrease in the reflectivity of X-ray mirrors. In addition for experiments, it is valuable to know the absolute intensity at the sample. There are two types of laser power meters based on thermopile and pyroelectric sensors. The thermopile power meters measure an average temperature and are compatible with the high repetition rates of LCLS-II. Pyroelectric power meters provide a pulse-by-pulse response. Ultra-high vacuum compatibility is being tested by residual gas analysis. An in-house development beamtime is being conducted at the LCLS SXR instrument. Measurements using both thermopile and pyroelectric power meters will be conducted at a set of photon energies in the soft X-ray range. The detectors’ response will be compared with the gas monitor detector installed at the SXR instrument.
The number of proposals for LCLS science has rapidly increased as all six LCLS x-ray instruments have come online. It
created rising demand on beam time. Statistics shows that only about 25 % of LCLS proposals can be allocated beam
time. One way to increase access is to share the x-ray beam between the different instruments. The purpose of this study
is to quickly switch the x-ray beam between the Matter in Extreme Conditions (MEC) Instrument and the Coherent X-ray
Imaging (CXI) or X-ray Correlation Spectroscopy (XCS) Instruments, in order that two of the instruments can
perform experiments simultaneously. In the most common operational mode, the MEC Instrument uses one x-ray pulse
every 10 minutes, limited by the repetition rate of the high power nanosecond laser system. The MEC M3H mirror steers
the x-ray beam to the MEC Instrument from the XCS or CXI Instruments. If the M3H mirror could switch the x-ray
beam to MEC within a fraction of the 10 minutes waiting time, multiplexing of the x-ray beam would be achieved. The
M3H mirror system has two motion stages for translation and rotation. The long path, 230 m, from the mirror to MEC
hutch makes the pointing resolution 0f 100 microns and stability requirements challenging. The present study
investigates such capabilities by measuring the correlation between the translation speed and the beam pointing
reproducibility. We show that mirror translation can multiplex the LCLS x-ray beam.
Daniele Cocco, Rafael Abela, John Amann, Ken Chow, Paul Emma, Yiping Feng, Georg Gassner, Jerome Hastings, Philip Heimann, Zhirong Huang, Henrik Loos, Paul Montanez, Daniel Morton, Heinz-Dieter Nuhn, Daniel Ratner, Larry Rodes, Uwe Flechsig, James Welch, Juhao Wu
After the successful demonstration of the hard X-ray self-seeding at LCLS, an effort to build a system for working in the soft X-ray region is ongoing. The idea for self-seeding in the soft X-ray region by using a grating monochromator was first proposed by Feldhauset. al. The concept places a grating monochromator in middle of the undulators and selects a narrow bandwidth “seed” from the SASE beam produced by the upstream section of undulators, which is then amplified to saturation in the downstream section of the undulators. The seeded FEL beam will have a narrower bandwidth approaching the transform limit. The challenge is to accommodate a monochromator and refocusing system as well as the electron beam magnetic chicane into a very limited space. The Soft X-raySelf Seeding system replaces only a single undulator section of ~ 4 m. Theoverall project and the expected FEL performances are described elsewhere. Here we present the detailed optical design solution, consisting of a fixed incidence angle toroidal blazed grating with variable groove density, a rotating plane mirror (the only required motion for tuning the energy) to redirect the selected monochromatic beam onto an exit slit, and two more mirrors, one sphere and one flat, to focus and overlap the ‘seed’ onto the electron beam in the downstream undulators.
During the last years, scanning coherent x-ray microscopy, also called ptychography, has revolutionized nanobeamcharacterization at third generation x-ray sources. The method yields the complete information on the complex valued, nanofocused wave field with high spatial resolution. In an experiment carried out at the Matter in Extreme Conditions (MEC) instrument at the Linac Coherent Light Source (LCLS) we successfully applied the method to an attenuated nanofocused XFEL beam with a size of 180(h) × 150(v)nm2 (FWHM) in horizontal (h) and vertical direction (v), respectively. It was created by a set of 20 beryllium compound refractive lenses (Be-CRLs). By using a fast detector (CSPAD) to record the diffraction patterns and a fast implementation of the phase retrieval code running on a graphics processing unit (GPU), the applicability of the method as a real-time XFEL nanobeam diagnostic is highlighted.
This manuscript presents an overview of recent work performed on x-ray optics development, metrology and calibration for the Soft X-ray Research (SXR) and the Coherent X-ray Imaging (CXI) instruments at the Linac Coherent Light Source (LCLS) free-electron laser. We also present results on the first LCLS exposures of boron carbide (B4C)-coated samples at photon energies near the carbon K edge and discuss relevant analysis and implications for future experiments.
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.
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 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).
John Corlett, William Barletta, Stefano DeSantis, Larry Doolittle, William Fawley, Philip Heimann, Stephen Leone, Steven Lidia, Derun Li, Gregory Penn, Alex Ratti, Matheus Reinsch, Robert Schoenlein, John Staples, Gregory Stover, Steve Virostek, Weishi Wan, Russell Wells, Russell Wilcox, Andy Wolski, Jonathan Wurtele, Alexander Zholents
We describe the design concepts for a potential future source of femtosecond x-ray pulses based on synchrotron radiation production in a recirculating electron linac. Using harmonic cascade free-electron lasers (FEL's) and spontaneous emission in short-period, narrow-gap insertion devices, a broad range of photon energies are available with tunability from EUV to hard x-ray regimes. Photon pulse durations are controllable and range from 10 fs to 200 fs, with fluxes 107-1012 photons per pulse. Full spatial and temporal coherence is obtained for EUV and soft X-rays. A fiber laser master oscillator and stabilized timing distribution scheme are proposed to synchronize accelerator rf systems and multiple lasers throughout the facility, allowing timing synchronization between sample excitation and X-ray probe of approximately 20-50 fs.
The Linac-based Ultrafast X-ray source (LUX) is a proposed recirculating linear accelerator for the purpose of producing intense, tunable, high repetition rate ultrafast x-ray pulses. An angle-time or position-time correlation is induced in the electron bunches by a dipole-mode RF cavity. Undulators and wigglers are sources of synchrotron radiation. Asymmetrically-cut crystals are used as optical elements of an x-ray pulse compression scheme. X-ray pulse durations of 50-100 fs are obtained over a range of photon energies from 2 to 12 keV. An undulator beamline consists of a collimating mirror, two asymmetric crystals and Kirkpatrick-Baez mirrors and provides compressed, monochromatic and focused x-rays for time-resolved experiments.
The performance of CsI photocathodes has been characterized for use with grazing incidence soft x-rays. The total electron yield and pulsed quantum efficiency from a CsI photocathode has been measured in a reflection geometry as a function of photon energy (100 eV to 1 keV), angle of incidence and the electric field between the anode and photocathode. The total electron yield and pulsed quantum efficiency increase as the x-ray penetration depth approaches the secondary electron escape depth. Unit quantum efficiency in a grazing incidence geometry is demonstrated. A weak electric field dependence is observed for the total yield measurements; whilst no significant dependence is found for the pulsed quantum efficiency. Theoretical predictions agree accurately with experiment.
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.
The toroidal, silicon mirror on microdiffraction beamline 7.3.3 at the Advanced Light Source provides a 1:1 focus of the bend magnet source. The mirror is bent by two lead springs that are bolted to it through a pari of adhesive bonded end blocks. Because of the high loads that these adhesive joints must carry, three specific features of the bonds were tested: bondline geometry of the mating end blocks, surface preparation of the adherends, and strength of the adhesive. Bond strengths were evaluated by loading small test mirrors to failure using two epoxies under two different conditions of surface preparation - acid etching and simple UHV cleaning. In addition, the mirror's temperature distribution and figure errors were calculated with an Ansys Finite Element Model. The model's predictions were correlated to long trace profilometry as well as x-ray focus measurements.
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.
At the Advanced Light Source an undulator beamline, with an energy range from 6 to 30 eV, has been constructed for chemical dynamics experiments. The higher harmonics of the undulator are suppressed by a novel, windowless gas filter. In one branchline high flux, 2% bandwidth radiation is directed toward an end station for photodissociation and crossed molecular beam experiments. A photon flux of 1016 photon/sec has been measured at this end station. In a second branchline a 6.65 m off-plane Eagle monochromator delivers narrow bandwidth radiation to an end station for photoionization studies. At this second end station a peak flux of 3 X 1011 was observed for 25,000 resolving power. This monochromator has achieved a resolving power of 70,000 using a 4800 grooves/mm grating, one of the highest resolving powers obtained by a VUV monochromator.
A step-and-scan type scanning scheme has been investigated for the purpose of simplifying the high-precision scanning mechanism adopted in the 6.65-m off-plane Eagle type monochromator on an undulator beamline of the Advanced Light Source at Lawrence Berkeley National Laboratory. In this scheme it is proposed to scan over a wavelength range of 80 - 180 nm by covering a range of 0.99 (lambda) t - 1.01 (lambda) t at a time by simply rotating the grating fixed at the position for a wavelength (lambda) t to which the undulator is tuned. When the undulator is tuned to another (lambda) t, the grating is translated to a new fixed position and scanning is made by simple grating rotation. A ruled grating with varied line spacing and straight grooves and a holographic grating recorded with spherical wave- fronts were designed to match the proposed scanning scheme and to meet the required energy resolution of approximately 1.0 cm-1 over the entire scanning range. The results of ray tracing show that the designed gratings with 2400 grooves/mm and 6.1-m radius of curvature would provide a resolution of -1, a good correction of astigmatism, and a grating travel distance of only 11 or 14 mm over the scanning range of 80 - 180 nm.
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