A proper spatial characterization of a laser beam profile is indisputably important for any laser-mater experiment as well as for protection of beamline optical elements. Method of ablation and desorption imprints provides thorough beam profile analysis applicable to a broad range of photon energies. This method, however, often requires up to thousands of shots which must be then manually analyzed. Here we present method based on deep learning image segmentation model which is able to substitute human element currently indispensable in this time-consuming ex situ post processing. It is a part of AbloCAM project – an universal device for semi-automatic beam profile analysis.
We report on ion emission from plasma produced on thick targets irradiated with nanosecond and femtosecond pulses delivered by mid-ultraviolet and soft x-ray lasers, respectively. To distinguish between different ion acceleration mechanisms, the maximum kinetic energy of ions produced under different interaction conditions is plotted versus laser fluence. The transformation of the time-of-flight detector signal into ion charge density distance-of-flight spectra makes it possible to determine the mean kinetic energy of the fastest ion groups based on the influence of the acoustic velocity of ion expansion. This allows obtaining additional characteristics of the ion production. The final energy of the group of fast ions determined using the ion sound velocity model is an order of magnitude larger in the fs-XFEL interaction than in the ns-UV one. On the contrary, the ablation yield of ions in our experiment is seven orders of magnitude greater when applying ns-UV laser pulses, not only due to higher energies of UV laser pulses, but also due to a significant difference in interaction and ion formation mechanisms.
The Spectroscopy and Coherent Scattering (SCS) instrument of the European XFEL is a soft X-ray beamline aiming to unravel electronic, spin and structural properties of materials in ultrafast processes at the nanoscale. Various experimental techniques offered at SCS have different requirements in terms of beam size at the sample. Kirkpatrick-Baez (KB) refocusing optics equipped with mechanical benders allows for independent change of the horizontal and vertical beam size. We report here on the first characterization of the SCS KB mirrors by means of a novel diffraction-based technique which images the beam profile on a 2D pixelated detector. This approach provides a quick characterization of micrometer beam sizes. Results are compared with metrology measurements obtained with a non-contact slope profiler.
For quick, efficient and accurate alignment and characterization of focused short-wavelength (i.e., extreme ultraviolet, soft x-ray, and x-ray) laser beams directly in the vacuum interaction chambers, an instrument has to be developed and implemented. AbloCAM should represent such a handy tool looking at ablation imprints of the beam in a suitable material without breaking vacuum and need for a liberation of exposed samples from the chamber to analyse them ex situ. First steps we made in this direction can be found in ref. [1] The technique of the fluence scan (F-scan method; for details see [2,3]), proven at several FEL facilities, e.g., FLASH (Free-electron LASer in Hamburg) and LCLS (Linac Coherent Light Source), makes possible to characterize the beam utilizing just an outer contour of the damage pattern. It is not necessary to measure a crater profile for the beam reconstruction. Not only lateral, but also a longitudinal distribution of irradiance can be determined in the focused beam by its imprinting (z-scan method [4]). Technically, the AbloCAM tool consists of a vacuum compatible motorized positioning system executing a series of well-defined irradiations of a chosen slab target according to algorithms fulfilling requirements of the combined F(z)-scan procedure. Damage patterns formed in that way should then be visualized in situ by means of Nomarski (DIC – Differential Interference Contrast) microscope equipped with the software which indicates and processes pattern outer contours. There is a feedback established between positioning and inspecting components and functions of the tool. The software helps to align and characterize any focused beam in the interaction chamber semi-automatically in a reasonable time.
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.
Possible dose-rate effects in a plasmid DNA exposed to pulsed extreme ultraviolet (XUV) and soft x-ray (SXR) water window radiation from two different table-top plasma-based sources was studied. Dose delivered to the target molecule was controlled by attenuating the incident photon flux with aluminum thin foils as well as varying the DNA/buffer-salt ratio in the irradiated sample. Irradiated samples were analyzed using the agarose gel electrophoresis. Some additional bands were identified in gel electrophoretograms as results of a DNA cross-linking. They were inspected by atomic force microscopy (AFM). Yields of single- and double-strand breaks (Gy-1 Da-1) were determined as a function of incident dose rate. Both yields decreased with a dose rate increasing. The ratio of single- and double-strand breaks exhibited only a slight increase at elevated dose rates. In conclusion, complex and/or clustered damages do not seem to be initiated under these irradiation conditions.
An investigation on short-wavelength ablation mechanism of poly(1,4-phenylene ether ether-sulfune) PPEESand poly (1-hexadecene-sulfone) PHDS (Figure 9-10) by EUV radiation is presented. The goal of this work is to evaluate the ablation behavior with respect to the influence of wavelength, fluence and quantum efficiency. Because there is no yet a general EUV ablation theory, data are analyzed in order to underline regularity of the process which can be used in future to detect the scaling laws of the process. The differences with longer wavelengths ablation and EUV one are pointed out and possible applications of EUV ablation are proposed.
Availability of numerical model providing reliable estimation of the parameters of ablation processes induced by extreme ultraviolet laser pulses in the range of nanosecond and sub-picosecond timescales is highly desirable for recent experimental research as well as for practical purposes. Performance of the one-dimensional thermodynamic code (XUV-ABLATOR) in predicting the relationship of ablation rate and laser fluence is investigated for three reference materials: (i) silicon, (ii) fused silica and (iii) polymethyl methacrylate. The effect of pulse duration and different material properties on the model predictions is studied in the frame of this contribution for the conditions typical for two compact laser systems operating at 46.9 nm. Software implementation of the XUV-ABLATOR code including graphical user's interface and the set of tools for sensitivity analysis was developed. Global sensitivity analysis using high dimensional model representation in combination with quasi-random sampling was applied in order to identify the most critical input data as well as to explore the uncertainty range of model results.
Single crystals of two fluorides (LiF and CaF2) and a tungstate (PbWO4) were irradiated by nanosecond pulses of 46.9-
nm radiation provided by 10-Hz capillary-discharge Ne-like Ar laser (CDL). The damage threshold was determined in
LiF using the CDL beam focused by a Sc/Si multilayer-coated spherical mirror. Irradiated samples have been
investigated by Nomarski (DIC - Differential Interference Contrast) microscopy and optical (WLI - white light
intereferometry) profiler. After an exposure by a certain number of CDL pulses, an ablation rate can be calculated from
WLI measured depth of the crater created by the XUV ablation. Potential use of XUV ablation of ionic crystals in pulsed
laser deposition (PLD) of thin layers of such a particular material, which is difficult to ablate by conventional UV-Vis-
NIR lasers, is discussed in this contribution.
The irradiation effects of multiple ultrafast shots of laser beams with estimated fluences of some tens of
mJ/cm2 on a EUV Mo/Si multilayer have been studied. Irradiation damage has been induced by multiple shots of two
different lasers (100 fs 400 nm the first, 1.5 ns 46.9 nm the second). The study has been motivated by the need of
multilayer Mo/Si optics for the delay lines of the FEL source FERMI@Elettra, where these mirrors will be used to
reflect 100 fs pulses at 13 nm with a fluence of some mJ/cm2. The analysis has been performed by means of different
techniques as EUV and soft X-ray reflectivity, XPS, and Standing wave enhanced XPS. Simulations have been carried
on by means of an indigenously developed software OPAL (Optical Properties of Anisotropic Layers) for the
calculation of the absorbed energy by the stratified medium. AFM and SEM surface images have been also acquired. In
the irradiation at 400 nm, we observed a significant change in the multilayer performance at fluences of 100 mJ/cm2 and
above with a significant reduction of reflectivity. Spectroscopic analysis allowed to correlate the decrease of reflectivity
with the degradation of the multilayer stacking, ascribed to Mo-Si intermixing at the Mo/Si interfaces of the first few
layers, close to the surface of the mirror. Preliminary tests have been also performed on the sample irradiated at 46.9
nm.
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.
Ionizing radiation induces a variety of DNA damages including single-strand breaks (SSBs), double-strand breaks
(DSBs), abasic sites, modified sugar and bases. Most theoretical and experimental studies have been focused on DNA
strand scissions, in particular production of DNA double-strand breaks. DSBs have been proven to be a key damage at a
molecular level responsible for the formation of chromosomal aberrations, leading often to cell death. The complexity of
lesions produced in DNA by ionizing radiations is thought to depend on the amount of energy deposited at the site of
each lesion. We have studied the nature of DNA damage induced directly by the pulsed 46.9 nm radiation provided by a
capillary-discharge Ne-like Ar laser (CDL). Different surface doses were delivered with a repetition rate of a few Hz and
an average pulse energy ~ 1 μJ. A simple model DNA molecule, i.e., dried closed-circular plasmid DNA (pBR322), was
irradiated. The agarose gel electrophoresis method was used for determination of both SSB and DSB yields. Results are
compared with a previous study of plasmid DNA irradiated with a single sub-nanosecond 1-keV X-ray pulse produced
by a large-scale, double-stream gas puff target, illuminated by sub-kJ, near-infrared (NIR) focused laser pulses at the
PALS facility (Prague Asterix Laser System).
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).
The desktop capillary-discharge Ne-like Ar laser (CDL) providing 10-μJ nanosecond pulses of coherent 46.9-nm
radiation with a repetition rate up to 12 Hz was developed and built at the Colorado State University in Fort Collins and
then installed in Prague. The beam of the laser was focused by a spherical mirror covered with Si/Sc multilayer coating
onto the surface of poly(methyl methacrylate) - PMMA. Interaction parameters vary by changing the distance between
sample surface and beam focus. The samples were exposed to various numbers of shots. Analysis of damaged PMMA by
atomic force (AFM) and Nomarski (DIC - differential interference contrast) microscopes allows not only to determine
the key characteristics of the focused beam (e.g. Rayleigh's parameter, focal spot diameter, tight focus position, etc.) but
also to investigate mechanisms of the radiation-induced erosion processes.
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.
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