This PDF file contains the front matter associated with SPIE Proceedings Volume 11035, including the Title Page, Copyright information, Table of Contents, and Author and Conference Committee lists.

SPring-8 Angstrom Compact free-electron LAser (SACLA) at SPring-8, Japan [1,2] was inaugurated in March, 2012 as the first compact XFEL facility that combines a low-emittance electron-beam injector, a high-gradient C-band linac, and short-period in-vacuum undulators. SACLA generates intense short-wavelength XFEL radiation below 1 Å with a moderate electron beam energy of 8 GeV. To extend capabilities, state-of-the-art X-ray optic, detectors, and experimental instruments have been continu-ously developed. To further increase an intensity of XFEL pulses, we have developed a 1-um focusing optics [3] and a 50-nm focusing optics based on a two-stage scheme [4], by using state-of-the-art reflective mirrors in a collaboration with Osaka University. A significant intensity of 10^20 W/cm2 generated by the tight fo-cusing system has enabled unique opportunities for conducting non-linear X-ray sciences, for example, atomic K-alpha lasers excited with XFEL pulses [5], two-photon X-ray absorption [6], and X-ray-pump & X-ray-probe studies to investigate ultrafast damage processes induced by intense X-rays [7]. In this talk, I will review the status [8] and future perspective of the complex facilities SACLA and SPring-8. References [1] T. Ishikawa et al., Nature Photon., 6, 540 (2012). [2] M. Yabashi et al., J. Synchrotron Rad., 22, 477 (2015). [3] K. Yumoto et al., Nature Photon., 7, 43 (2013). [4] H. Mimura et al., Nature Commun., 5, 3539 (2014). [5] H. Yoneda et al., Nature, 524, 446-449 (2015). [6] K. Tamasaku et al., Nature Photon. 8, 313 (2014). [7] I. Inoue et al., Proc. Natl. Acad. Sci. USA. 113, 1492 (2016). [8] http://xfel.riken.jp/research/indexnn.html

The FERMI free electron laser at Elettra (Trieste - Italy) has reached its goal configuration in terms of machine and beamlines. In particular, both the FEL lines are routinely employed by users for their experiments, which can now be performed on all the foreseen endstations in the experimental hall. In this contribution the current performance of the machine will be presented including preliminary results from new schemes tested in the last months. Moreover, the final layout and performance of the photon beamlines now operative will be discussed, with particular attention to the optical solutions used and the diagnostics employed. A particular attention, moreover, will be devoted to contamination and damage issues on the different optical elements of the photon transport system (PADReS). This system has been operating since 2011 and is now endowed with more than 80 different optical elements including mirrors (plane, toroidal and ellipsoidal), gratings (Variable Line Spacing planes) and multilayers. Different levels of contamination can occur on them, and strategies on avoiding it as well as cleaning procedures will be discussed. At the same time, true damage of optical surfaces can happen and an example will be reported. Along PADReS, moreover, more than 60 solid state filters are employed to cut unwanted radiation, and also in this case the ways to cope with and remove contamination will be reported. Finally, the use of indentation and other techniques to compare spot size will be reported and discussed together with examples of damage issues related to samples in the endstations.

With the LCLS-II high-repetition-rate FEL, the number of pulses on the optics over ten years reaches 20 trillion. The thermal fatigue damage and lifetime of the optics, beam transport components under such large number of FEL pulses are important issues that should be addressed. For the optics, the definition of the damage should be the significant (for instance, 50%) reduction of reflectivity, which is premonitory of damage, and much more stringent than the ablation threshold. Silicon is widely used for X-ray optics both as mirror substrate and crystal monochromator. As the power absorption length of the silicon in visible optical laser wavelength (400 – 800 nm) is comparable as the one in soft X-ray FEL (300 – 1600 eV), we use optical laser of 515 nm wavelength, 200 fs pulse length and up to 0.928 MHz repetition rate for fatigue damage test on silicon. This allows remediating the limited availability or unavailability of high rep-rate FEL beam time. By monitoring the reflectivity of the sample (polished silicon wafer), we can measure the damage versus the number of pulses. Our first results show the damage threshold decreases significantly when the number of pulses increases. We have pushed the number of pulses over 100 billion, which corresponds to several days of MHz operation. The damage here can be explained as thermal mechanical fatigue damage. The outcome of this project will be a fatigue damage model predicting the lifetime of the key components, such as X-ray optics, used for high-repetition-rate X-ray FELs and also sample material under large number of laser pulses.

With the rapid development of extreme ultraviolet (EUV) light sources, such as plasma-based light source and Free Electron Laser (FEL), it provides unprecedented powerful ultra-short EUV radiations. These extremely high intense ultra-short pulses of radiation bring great challenges to the optical components utilized for steering these light beams, especially the radiation damage issues. However, more studies on the EUV damage mechanisms on optical materials are still quite desired because of limited beamtime provided by FEL facilities. In this study, we present a table-top focused EUV optical system built at the Institute of Precision Optical Engineering (IPOE) for performing EUV damage tests on optical materials. This setup consists of a laser-plasma light source, a modified Schwarzschild objective and an EUV energy attenuator. With a large numerical aperture of 0.44 and a demagnification of 11, the Schwarzschild objective is composed of two annular spherical mirrors coated with Mo/Si multilayers. By using the Zirconium filter and Mo/Si multilayers, this setup can generate the focused radiation with an energy density of 2.27 J/cm² at the wavelength of 13.5 nm on the image plane of the objective with ~8.3 ns pulse duration. The EUV energy can be changed using a gas attenuator by varying the gas pressure of Helium or Nitrogen inside the chamber. The performance and potentials of this setup are demonstrated by the single-shot or multi-shot damage tests on some samples, such as Au thin film, CaF₂ and Mo/Si multilayer mirror. The damage thresholds were determined and the possible damage mechanisms are discussed together with available experimental results.

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.

These proceedings investigate the ionization and temperature dynamics of water samples exposed to intense ultrashort X-ray free-electron laser pulses ranging from 10⁴ -10⁷ J/cm², based on simulations using a non-local thermodynamic plasma code. In comparison to earlier work combining simulations and experiments, a regime where a hybrid simulations approach should be applicable is presented.

XUV pulses at 26.2 nm wavelength were applied to induce graphitization of diamond through a non-thermal solid-to-solid phase transition. This process was observed within poly-crystalline diamond with a time-resolved experiment using ultrashort XUV pulses and cross correlated by ultrashort optical laser pulses. This scheme enabled for the first time the measurement of a phase transition on a timescale of ~150 fs. Excellent agreement between experiment and theoretical predictions was found, using a dedicated code that followed the non-equilibrium evolution of the irradiated diamond including all transient electronic and structural changes. These observations confirm that ultrashort XUV pulses can induce a non-thermal ultrafast solid-to-solid phase transition on a hundred femtosecond timescale.

When an intense ultrashort light pulse – in the visible domain- interacts with a wide band gap dielectric, a plasma can be generated by non-linear photoexcitation of carrier from the valence band to the initially empty conduction band. These carriers can be further excited in the conduction band, leading to an increase of their energy distribution, and thus of the amount of energy transferred to the material. If this deposited energy exceeds some critical threshold, permanent modification like damage or ablation may take place. The two key parameters determining the energy deposition are the density and the temperature of the plasma. In this presentation, we wish to demonstrate that a sequence of double pulse can be used to better control these two parameters, and thus to optimize energy deposition and facilitate for instance ablation of insulators and semi-conductors in the VUV domain. First, in the visible domain, using the second harmonic and the fundamental of a Ti-Sa laser, we show that time resolved double pump- and probe experiment allows to directly observe the sequence of events carrier excitation/ carrier heating, provided the parameters (energy, duration, delay) are appropriately chosen. Then, the ablation threshold (due to the first pulse) is dramatically reduced by the presence of the second pulse, while the characteristic of ablation are still determined by the first pulse [1]. Finally, new information regarding the excitation mechanisms, in particular impact ionization and avalanche are obtained [2]. In the second part, we show that this double pulse technique can be extended in the VUV domain. Using high order harmonics of a Ti-Sa laser (harmonic 25, ie. wavelength of 32 nm), whose intensity of far too low to damage any material, we could observe direct ablation of a dielectric, namely quartz, a-SiO2, when the VUV pulse if followed by an IR pulse, whereas no effect is observed with each of these pulses. [1] Guizard, S.; Klimentov S.; Mouskeftaras A. ; Fedorov N., Geoffroy G.; Vilmart G., Applied Surface Science 336, p. 206, 2015. [2] A Mouskeftaras, S Guizard, N Fedorov, S Klimentov Applied Physics A 110 (3), 709-715, 2013.

In this work investigations concerning interaction of intense, nanosecond EUV pulses with matter were performed. Various laser-produced plasma radiation sources were employed for creation of the driving EUV pulses. The sources were based on two different laser systems with pulse energies ranging from 0.8 J to 10J and pulse duration 4 ÷ 10 ns. They were equipped with the EUV collectors for focusing of the radiation. This way radiation fluence up to 0.5 J/cm2 in the interaction region was obtained. In our experiments solid material samples or gases injected into the vacuum chamber synchronously with the EUV pulses were irradiated. Irradiation of the gases resulted in ionization and excitation of atoms and molecules forming low temperature plasmas with a relatively high electron density. Emission spectra obtained from these plasmas, contained spectral lines corresponding to radiative transitions in atoms, molecules, atomic or molecular ions. For analysis of the EUV spectra numerical simulations were performed, using a collisional-radiative PrismSPECT code. For computer simulations of the molecular spectra measured in the UV/VIS range a LIFBASE and Specair codes were employed. This way ionization states together with various thermodynamic parameters were deduced. Irradiation of solid samples resulted in melting of a thin near-surface layer or, in some cases its ablation or even conversion to a low temperature plasma. It depended on physico-chemical properties of the material and its thickness. In case of organic polymers, usually ablation connected with fragmentation of the polymer molecules, took place. In case of thick samples of inorganic solids, a thin near surface layer was heated up to a high temperature exceeding melting or even boiling point. In most cases different kinds of micro- or nanostructures were created, modifying the surface morphology. Except the EUV interaction with solid materials, simultaneous EUV and the EUV induced plasma treatment was investigated. Plasmas were created in gases injected close to the exposed surface. Part of the EUV radiation was absorbed in the injected gas forming the low temperature plasma near the surface, while the other part of radiation, that was not absorbed, interacted with the surface material. This way additional atoms could be incorporated into the molecular structure of the exposed material, or reactive etching took place. Especially interesting results were obtained using molecular gases for creation the reactive plasmas.

Widely used and most reliable growing technique for fabrication of large-scale graphene grains, a thermal decomposition on silicon carbide (SiC), significantly reduces the carrier mobility partially due to crystal imperfections and partially due to substrate phonons which collide with the charge carriers. Elimination of the substrate influence is therefore essential to keep the carrier mobility at a very high level. In our experiment, samples of multi-layer epitaxial graphene grown on SiC were exposed to intense 21.2-nm radiation provided by Ne-like Zn XUV laser driven by the Prague Asterix Laser System (PALS). A sub-nanosecond pulse of energetic (58.5 eV) photons is used to break relatively weak bonds between the SiC substrate and graphene while keeping the graphene layer almost unaffected. An irradiated area was inspected by micro-Raman spectroscopy, white-light interferometry (WLI), atomic force microscopy (AFM) and scanning electron microscopy (SEM). Data show clear evidence of delamination of the multi-layer graphene which is elevated by 5 nm. Decrease of the mechanical strain and increase of number of defects in the irradiated area observed from Raman spectra is discussed.

Laser-target interaction experiments demonstrated that the return target current, j_TC(t), which neutralizes the target charge appearing when the fastest electrons escape the plasma, is one of principal characteristics of the laser-matter interaction. j_TC(t) flowing between the target and the ground is emerging just when the laser intensity exceeds the threshold intensity of the plasma formation. The experimental determination of the number of escaped electrons is primarily based on precise target current observations. We present the experimental observation of j_TC(t) neutralizing metallic and plastic targets irradiated with low intensity ranging from 10⁸ to 10¹³ W/cm² delivered by KrF and iodine lasers operated at 248-nm and 1315-nm wavelengths, respectively. Our experiments show that the charge appears on targets continuously not only during the laser-plasma interaction but also during the plasma expansion into the vacuum. The analysis of j_TC(t) allows us to determine the level of influence of the target surface pollution by chemisorbed hydrocarbons on the plasma production, which is also needed for elucidation of specific processes leading to the target charge and ion emission. Very specific feature of j_TC(t) was found for plastic targets. Our experiments also demonstrate that the accompanying phenomenon of low-intensity laser-target interaction is the generation of electromagnetic pulses that are emitted at frequencies coinciding also with the resonant frequency modes of the interaction vacuum chamber.

X-rays are most useful in diverse researches on soft matter that is currently emerging materials in fundamental and applied sciences. When hard X-ray microscopy, particularly established at synchrotron radiation sources, is applied to study soft matter such as water and polymers, many physiochemical phenomena were induced by hard X-ray irradiation. Ultrafast X-ray imaging is beneficial in studying on dynamics of soft matter because radiation damage is eliminated considerably. In this talk, we discuss examples regarding how high-dose X-ray irradiations change material properties of soft matter and how X-ray-induced changes can be reduced or utilized in experiments with nanoscale and microscale soft materials. The fundamental knowledge and experiences on X-ray-induced modifications in material properties would be important to further applications of X-rays into modern characterizations and fabrications of soft materials. [1] B. M. Weon, et al. Phys. Rev. Lett. 100, 217403 (2008) [2] B. M. Weon and J. H. Je, Appl. Phys. Lett. 93, 244105 (2008) [3] B. M. Weon and J. H. Je, Appl. Phys. Lett. 96, 194101 (2010) [4] B. M. Weon, et al. Phys. Rev. E 84, 032601 (2011) [5] B. M. Weon, et al. J. Appl. Phys. 106, 053518 (2009) [6] B. M. Weon, et al. ChemPhysChem 11, 115 (2010) [7] B. M. Weon, et al. Phys. Rev. Lett. 107, 018301 (2011)

We investigated the electrical modifications induced by hard X-ray synchrotron radiation on the YBa₂Cu₃O_7-δ high temperature superconductor. We explored two different irradiation regimes. At low X-ray doses a progressive shift of the critical temperature of the superconductor has been obtained. Conversely, increasing the X-ray dose, a transition to a nonsuperconducting high resistance state is observed. These results pave the way to the realization of electrical devices by Xray nanopatterning on this kind of oxide, extending the previous studies on Bi₂Sr₂CaCu₂O_8+δ.

Actinic irradiation (i.e. using ionizing light) of EUV pulses on a multilayer Bragg optics was investigated for characterizing the chemical effects. Photothermal effects are ruled out by the low repetition rate and low absolute energy of the EUV pulses. This would exclude the formation of melt-related particles, which have been however observed. Alternative explanations are discussed. The analysis concentrate on the structure and composition of the multilayer stack, with the aim of quantifying the kinetics.

We report results of ablation experiments of different materials through Ni grid with an intense XUV laser beam. As a source of XUV radiation (energy of about 100 J) with wavelength of 46.9 nm was used high-current capillary discharge driver. Ablated footprints were analyzed by optical microscope and by an atomic-force microscope (AFM). It was found that structure and period of diffraction pattern on PMMA sample (both in ablation and desorption area) depend on the distance from grid to the sample surface. Depth of ablation craters in a single window of PMMA for single shot was about of 80 nm, and period changes from 400 nm (on the edge) to 190 nm (in the middle) for grid further from surface, and from 400 nm (on the edge) to 10 nm (in the middle) for closer grid. Contrary to this, no diffraction patterns in ablation region and only slightly visible on the edge in the desorption region were observed on the surface of GaAs, SiC and Si samples for single shot. Depth of ablated craters in ablation region was about 100 nm for GaAs, 20 nm for Si and up to 5 nm for SiC. In desorption region depth of ablated craters is relatively shallow (up to 5 nm for GaAs and up to 2 nm for Si and SiC). In the case of irradiation samples by 5 shots ablated craters are deeper, but situation with diffraction pattern is the same as in the case of single shot for all materials.

Acquiring spectral information about the samples using near edge X-ray absorption fine structure (NEXAFS), which is a well-known and established method employed for compositional analysis of the samples, yields information about its elemental composition through the observation of the spectral features in the vicinity of the high energy side of the X-ray absorption edge. In particular, NEXAFS is often used to study the structure of intermolecular bonds of polymers by probing the electronic transitions from the core level to the unoccupied states. The NEXAFS spectra contain information which is element specific, indicating additionally the structure of the molecular bonds. This requires short wavelength sources, capable of delivering sufficient flux to achieve high signal-to-noise and of high quality spectral data in the time frame from fs to ns. These sources are synchrotrons, and FEL, but also compact sources, such as the laser-plasma source based on a double stream gas puff target. The applications of this source to recently developed compact NEXAFS spectroscopy system and an overview of some recent applications will be the main topic of this overview paper. Acquisition of static NEXAFS data for composition analysis of various materials and organic compounds, a raster scanning of the sample and acquiring spatially localized spectral data in so-called spectromicroscopy, as well as a single shot NEXAFS, allowing future timeresolved studies, will be presented with references to the original works.

We present the study of optical and spectral properties of radiation-induced stable point defects, known as color centers (CCs), in lithium fluoride (LiF) for the detection of 10 keV XFEL beam at Spring-8 Angstrom Compact free electron LAser (SACLA) in Japan. A thick LiF crystal was irradiated in four spots with 10 keV XFEL beam (pulse duration = 10 fs) with different number of accumulated shots. After irradiation the colored-LiF spots were characterized with an optical microscope in fluorescence mode and their photoluminescence intensity and spectra were analyzed.

Thin film Al filters, very popular for their high transmittance in the wavelength range 17 to 67 nm and simultaneously low transmittance in the visible and near UV region, are prone to oxidation. The amorphous Al2O3 layers on the Al surfaces have much smaller transmittance than the bulk Al material, and, therefore, they strongly influence the total transmittance of the filter. This paper gives not only the transmittance of very old Al filters, but also maps the transmittance development of Al filters in two years span since their delivery in not controlled atmosphere. Surprisingly, it turned out that while the transmission of surface layers (present on filters since the very beginning) slightly increases with time, the average attenuation rate per unit filter thickness of the bulk material (Al) with time dramatically rises.

An accurate transmission measurement of thin foils (usually made of elemental metals and/or semiconductors), which routinely are used as attenuators in soft x-ray beamlines, end-stations and instruments, represents a long standing problem over the wide experimentation field with photon beams, see for example [1-4]. Such foils are also frequently utilized for blocking long wavelength emission, i.e., UV-Vis-IR radiation, from plasma and high order harmonic sources, whilst soft x-rays emitted from the source pass through the foil with only a slight attenuation. Despite the enormous amount of data available in the literature, e.g., Henke’s tables [5], measurements made on real foils often provide surprising results. In this study, a procedure based on the ablation imprints method is utilized for determination of soft x-ray filter transmission, namely the f-scan technique [6,7]. This technique combines the GMD (Gas Monitor Detector) pulse energy measurement and attenuation of the beam by foils (made of different metallic/semiconducting elements of varying thickness) with areas of ablation imprints created on a suitable target, e.g. PMMA – Poly(methyl methacrylate). The results show only a partial overlap with transmission values found in Henke’s tables. Nevertheless, a good agreement with transmission values determined by conventional radiometry techniques at synchrotron radiation beamlines has been found. Such a difference between the experimentally obtained values and transmissions calculated for a pure element is usually explained by spontaneous formation of oxidized layers on the filter surface and in the near-surface layer and their possible alteration by intense FEL radiation. The first results obtained with Al, Nb, Zr and Si filters at FLASH/FLASH2 (Free-electron LASer in Hamburg tuned to 13.5 nm) facilities will be shown and discussed in this presentation. References 1. F. R. Powell, P. W. Vedder, J. F. Lindblom, S. F. Powell: Thin film filter performance for extreme ultraviolet and x-ray applications, Opt. Eng. 29, 614 (1990). 2. E. M. Gullikson, P. Denham, S. Mrowka, J. H. Underwood: Absolute photo absorption measurements of Mg, Al, and Si in the soft x-ray region below the L2,3 edges, Phys. Rev. B 49, 16 283 (1994). 3. R. Keenan, C. L. S. Lewis, J. S. Wark, E. Wolfrum: Measurements of the XUV transmission of aluminium with a soft x-ray laser, J. Phys. B 35, L449 (2002). 4. A. Joseph, M. H. Modi, A. Singh, R. K. Gupta, G. S. Lodha: Analysis of soft x-ray/VUV transmission characteristics of Si and Al filters, AIP Conf. Ser. 1512, 498 (2013). 5. B. L. Henke, E. Gullikson, J. C. Davis: X-ray interactions: photoabsorption, scattering, transmission, and reflection at E = 50-30000 eV, Z =1-92, At. Data Nucl. Data Tables 54, 181 (1993). 6. J. Chalupsky et al.: Spot size characterization of focused non-Gaussian X-ray laser beams, Opt. Express 18, 27836 (2010). 7. J. Chalupský, T. Burian, V. Hájková, L. Juha, T. Polcar, J. Gaudin, M. Nagasono, R. Sobierajski, M. Yabashi, J. Krzywinski: Fluence scan: an unexplored property of a laser beam, Opt. Express 21, 26363 (2013).

This contribution discusses challenges in modeling of formation of the warm dense matter (WDM) state in solids exposed to femtosecond X-ray free-electron laser pulses. It is based upon our previously reported code XTANT (X-rayinduced Thermal And Nonthermal Transition; N. Medvedev et. al., 4open 1, 3, 2018), which combines tight binding (TB) molecular dynamics for atoms with Monte Carlo modeling of high-energy electrons and core-holes, and Boltzmann collision integrals for nonadiabatic electron-ion coupling. The current version of the code, XTANT-3, includes LCAO basis sets sp³, sp³s*, and sp³d⁵, and can operate with both orthogonal and nonorthogonal Hamiltonians. It includes the TB parameterizations by Goodwin et al., a transferrable version of Vogl’s et al. TB, NRL, and DFTB. Considering that other modules of the code are applicable to any chemical element, this makes XTANT-3 capable of treating a large variety of materials. In order to extend it to the WDM regime, a few limitations that must be overcome are discussed here: shortrange repulsion potential must be sufficiently strong; basis sets must span large enough energy space within the conduction band; dependence of the electronic scattering cross sections on the electronic and atomic temperatures and structure needs to be considered. Directions at solving these issues are outlined in this proceeding.

Compact extreme ultraviolet (EUV) laser sources can be used for laboratory-scale ablation experiments at intensities of 1 × 10¹¹ Wcm⁻² and higher. The depths of ablation achieved using focused laser output at 46.9 nm to irradiate solid targets of aluminum, gold, and copper have been modeled. Two simple models are considered; an adaptation of an ultra-short pulse model, and an ablation velocity model. We show that the attenuation length of the material plays an important role in the physics of the ablation. A more detailed one-dimensional model including absorption by inverse bremsstrahlung absorption and photo-ionization, corrected to include electron degeneracy effects, is used to evaluate the opacity of the ablation plasma and subsequent ablation depths.

High intensity X-ray Free Electron Lasers (XFEL) are an ideal tool to heat materials directly and isochorically, which can cause them to be modified and damaged irreversibly. During XFEL-matter interactions, the energy of an XFEL beam will be mainly absorbed by photoionization, creating numerous high-energy photo- and Auger electrons. Modelling this process is quite complex since both atomic physics and plasma physics are involved. Atomic collisional-radiative (CR) codes such as FLYCHK are widely used to simulate such processes. However, the CR codes typically assume local thermodynamic equilibrium (LTE) and are limited to zero dimension. In order to understand the sample damaging mechanisms, we performed two-dimensional kinetic particle-in-cell simulations with radiation transport (PIC-RT) to retrieve the temporal processes of XFEL-matter interactions. The dynamics of XFEL-Matter interactions can roughly divide into three different time scales: 1) electron heating and photoionization by XFEL in ~ 10 fs ; 2) collisional heating and ionization by high-energy photo- and Auger electrons with several keV energy in tens of fs to sub-ps; and 3) collective hydrodynamic explosion driven by ~ TPa thermal pressure from ~100 fs to nanoseconds. The simulation results are compared to our recent experiment that a variety of samples were irradiated by Japanese XFEL SACLA with intensities on the order of 1020 W/cm2. The post-analysis of the irradiated samples showed that large holes with radius sizes more than one order of magnitude higher than the XFEL spot were created for metallic samples. The hole size is also much larger than the stopping range of high energy electrons. According to our PIC-RT simulations, we attribute the generation of such large holes to the micro-explosion process. Kinect simulation of the hole generation with multiple time scales is also useful and complementary to understand the change of X-Ray diffraction pattern in the experiment that infers significant material structural change on femtosecond timescales.

Modern free-electron lasers produce femtosecond X-ray pulses sufficiently intense to induce modifications of material properties. Energetic photoelectrons created upon the X-ray absorption and Auger electrons emitted after relaxation of core-hole states trigger secondary electron cascades, which increase the transient electron density. In this talk, we present and apply the XCASCADE(3D) code [1], based on classical event-by-event individual-particle Monte-Carlo approach, which is valid for relatively low absorbed fluences. The code takes into account photoabsorption, electron impact ionization, electron elastic scattering, and Auger decays of core holes. It uses atomistic cross sections and atomistic ionization potentials. Such approximation is justified for high-energy X-ray absorption and for high-energy electron-atom collisions, since they both excite electrons from the deeply lying core levels. At the same time, it makes the approach very efficient and flexible, since the corresponding cross sections can be found in the literature for many elements. In this approximation, one can treat any practical material assuming that the total cross section is the sum of cross sections for each individual atom. Elastic and inelastic anisotropic scatterings of photo- and secondary electrons on atoms are taken into account. The validity of the model is confirmed by comparing the calculated electron ranges with published data for silicon and gold. We apply the code to study the consequences of the electron cascading in ruthenium (Ru). Ru, which is a promising material for coating of X-ray mirrors [2], is simulated at different incoming photon energies, from extreme ultraviolet (XUV) to hard x-ray, in a grazing incidence geometry. The generated data then serve as an input to Two-Temperature Model, enabling analysis of the temperature evolution and damage thresholds [2]. According to the simulations, much larger electron ranges for hard x-rays case result in spread of absorbed laser energy into a larger volume compared to the XUV case and, consequently, into smaller temperature gradients. The latter is the source of thermo-induced stresses that eventually lead to material damage. As a consequence of the electron transport, one should expect higher damage thresholds with increasing energy of incident photons, even for the same photon penetration depth, which can be achieved by adjusting the incident angle. [1] V. Lipp, N. Medvedev, B. Ziaja, Proc. SPIE 10236, 102360H (2017). [2] Igor Milov, Igor A. Makhotkin, Ryszard Sobierajski, Nikita Medvedev, Vladimir Lipp et al., Opt. Express 26, 19665-19685 (2018).