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Ion beam analysis techniques are among the most powerful tools for advanced material characterization. Despite their growing relevance in a widening number of fields, most ion beam analysis facilities still rely on the oldest accelerator technologies, with severe limitations in terms of portability and flexibility. In this work we thoroughly address the potential of superintense laser-driven proton sources for this application. We develop a complete analytical and numerical framework suitable to describe laser-driven ion beam analysis, exemplifying the approach for Proton Induced X-ray/Gamma-ray emission, a technique of widespread interest. This allows us to propose a realistic design for a compact, versatile ion beam analysis facility based on this novel concept. These results can pave the way for ground-breaking developments in the field of hadron-based advanced material characterization., Comment: 12 pages, 5 figures. Submitted to Scientific Reports on 26th April 18. First decision received on 27th September 18. Revised version submitted on 18th October 18

In this talk the potential of laser-driven radiation sources for the analysis of materials is discussed. Non-destructive analytical techniques based on charged particle-induced X-ray emission spectroscopy (EDX, PIXE), neutrons (NAA) and photons (PAA) are well recognized as major tools of materials science. Despite their exceptional capabilities, the further developments of these materials characterization techniques have been prevented for several decades by the limits of the adopted particle sources. Because of their potential compactness and versatility, laser-driven radiation sources may offer a major breakthrough with respect to conventional accelerators. A unique feature is represented by the possibility of generating different radiation sources with minor variations in the setup. As a consequence, laser-driven particles could play a pivotal role in materials science applications [1], as shown by results on the possibility to exploit laser-driven ion sources for these purposes [2]. Here we present a complete experimental and theoretical investigation aimed at performing a combined laser-driven Energy Dispersive X-ray spectroscopy and quantitative laser-driven Particle Induced X-ray Emission analysis [3]. We also show the results of theoretical investigations focused on other applications like the Neutron Activation Analysis and the Photon Activation Analysis techniques. The experiments have been carried out at the Centro de Laseres Pulsados (CLPU) in Salamanca with the 200 TW VEGA-2 laser. We consider a case study of particular interest, i.e. the analysis of a non-homogeneous sample, constituted by a micrometric surface layer and a thick substrate. We perform sample irradiation either with both electrons and protons (fig. 1A) or only with protons (fig. 1B). We show that the X-ray production is dominated by the electrons in the first configuration and it can be exploited to easily identify the elements, leading to the demonstration of a laser-driven EDX characterization. Then, we exploit the second configuration to estimate the layer thickness via laser-driven PIXE quantitative analysis. We also show the results from a theoretical investigation aimed at assessing the applicability of laser-driven sources to PAA and NAA studies. By coupling analytical modelling, Particle-In-Cell and Monte Carlo simulations, we show that the use of advanced double-layer targets and suitable converters can allow to exploit the laser-driven protons and electrons for the generation of neutrons and photons required by such applications. These results provide new perspectives toward the development of versatile particle acceleration systems for multiple materials science studies. [1] M. Passoni, et al., Advanced laser-driven ion sources and their applications in materials and nuclear science, Plasma Phys. Control. Fusion, 14022, (2019), 62 [2] M. Barberio, et al., Laser-Accelerated Proton Beams as Diagnostics for Cultural Heritage, Sci. Rep., 40415, (2017), 7; M. Passoni, et al., Superintense laser-driven ion beam analysis, Sci. Rep., 9.1, (2019), 1 [3] F. Mirani, et al., Integrated quantitative PIXE analysis and EDX spectroscopy using a laser-driven particle source. Sci. Adv. 7, eabc8660 (2021). Co-Authors list: F. Mirani1, A. Maffini1, F. Casamichiela1, A. Pazzaglia1, A. Formenti1, D. Dellasega1, V. Russo1, D. Vavassori1, D. Bortot1, M. Huault2,3, G. Zeraouli2,3, V. Ospina2,3,4, S. Malko2,3, J. I. Apinaniz2, J. A. Perez-Hernandez2, D. De Luis2, G. Gatti2, L. Volpe2,4, A. Pola1 and D. Calzolari1 1Politecnico di Milano, Via Ponzio 34/3, I-20133 Milan, Italy. 2Centro de Laseres Pulsados, Edicio M5. Parque Cientifico, Salamanca, Spain. 3Universidad de Salamanca, Patio de Escuelas 1, 37008 Salamanca, Spain 4Universite de Bordeaux, CNRS, CEA-DAM/DIF, CELIA, UMR 5107, Talence, France

Laser-driven radiation sources are attracting increasing attention for several materials science applications. While laser-driven ions, electrons and neutrons have already been considered to carry out the elemental characterization of materials, the possibility to exploit high-energy photons remains unexplored. Indeed, the electrons generated by the interaction of an ultra-intense laser pulse with a near-critical material can be turned into high-energy photons via bremsstrahlung emission when shot into a high-Z converter. These photons could be effectively exploited to perform Photon Activation Analysis (PAA). In the present work, laser-driven PAA is proposed and investigated. We develop a theoretical approach to identify the optimal experimental conditions for laser-driven PAA in a wide range of laser intensities. Lastly, exploiting the Monte Carlo and Particle-In-Cell tools, we successfully simulate PAA experiments performed with both conventional accelerators and laser-driven sources. Under high repetition rate operation (i.e. 1−10 Hz) conditions, the ultra-intense lasers can allow performing PAA with performances comparable with those achieved with conventional accelerators. Moreover, laser-driven PAA could be exploited jointly with complementary laser-driven materials characterization techniques under investigation in existing laser facilities. Photon activation analysis is a non-destructive technique for material characterization that require high photon energies. Here, laser-driven accelerator is considered as a high-energy photon source and its optimization for photon activation analysis explored theoretically.

Plasma–material interaction (PMI) in tokamaks determines the life-time of first-wall (FW) components. Due to PMI, FW materials are eroded and transported within the device. Erosion is strongly influenced by the original morphology of the component, due to particle redeposition on near surface structures and to the changing of impact angle distributions, which results in an alteration of the sputtering effects. The Monte-Carlo impurity transport code ERO2.0 is capable of modelling the erosion of non-trivial surface morphologies due to plasma irradiation. The surface morphology module was validated against experimental data with satisfactory agreement. In this work, we further progress in the validation of the ERO2.0 capabilities by modelling both numerically generated surfaces as well as real surfaces, generated using atomic force microscopy (AFM) measurements of reference tungsten samples. The former are used to validate ERO2.0 against one of the morphology evolution models present in literature, in order to outline the conditions for reliable code solutions. Modifications induced in AFM-generated surfaces after argon and helium plasma irradiation are compared, showing a similar post-exposure morphology, mostly dominated by surface smoothing. Finally, the ERO2.0 morphology retrieved after He plasma exposure is compared to experimentally-available scanning electron microscopy and AFM measurements of the same surface morphology exposed in the linear plasma device GyM, showing the need for further improvements of the code, while a good agreement between experimental and simulated erosion rate is observed.

We discuss the applicability of laser induced breakdown spectroscopy (LIBS) for deuterium retention analysis in compact and porous tungsten-oxide (W-O) coatings. Deuterium loading was performed by exposing the coatings to deuterium plasma in Magnum-PSI linear plasma device. The deuterium signals obtained by ex-situ LIBS had sufficiently good signal-to-noise ratio for reliable separation of essentially broadened hydrogen and deuterium lines as well as for comparison of the lateral and depth distributions of deuterium in the coatings. Strong deuterium signal was obtained for the first laser shot which corresponded to the surface layer of the W-O coatings whereas deeper in the coating the signal decreased to noise level. In addition, the deuterium signal was highest in the central region of compact W-O coating. For both coatings, depth profiles of elements obtained by LIBS match with those recorded by secondary ion mass spectroscopy (SIMS) in the lateral direction along the sample surface. The results of LIBS and SIMS results were supported by Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD) data which showed that the exposure to deuterium plasma resulted in remarkable changes in the surface morphology along the sample surface. The study demonstrates the LIBS potential in deuterium retention measurements in plasma facing components.

In this work the influence of surface roughness on the sputter yield of Mo under keV D ion bombardment was investigated for different impact angles. For this purpose, thin films of Mo (~ 120 nm) were deposited by pulsed laser deposition onto graphite substrates with varying surface roughness (Ra ranging from 5 nm to 2–3 µm). The as-deposited samples were irradiated at room temperature by 3 keV D3+ ions originating from an electron-cyclotron-resonance ion gun. Samples were exposed to D ions at angles between 0° and 70° and fluences in range of 1023 D/m2. The areal densities of the Mo marker layers were determined with Rutherford backscattering spectroscopy. For all the surfaces we observed a strong angular dependence of the sputter yield. For smooth and intermediate surface roughnesses, up to Ra ~ 280 nm, we obtained an increase of the sputter yield with the angle up to a factor of five compared to 0°. In contrast, at the highest surface roughness in the 2–3 µm range the sputtering yield decreases with increasing impact angle. The obtained data were compared to SDTrimSP-3D simulations. We obtained good agreement between the simulated and experimental sputter yield for surfaces for which we could provide high resolution atomic force microscopy (AFM) surface representations. As high-resolution surface mapping was not possible for surface roughness of 2–3 μm, we found large deviation between the calculation and the measured data. The combination of measured and simulated data represent important input for predicting the erosion rates of surfaces at inner walls of thermonuclear fusion devices, which are expected to change surface roughness over time by sustained plasma exposure.

Linear plasma devices represent an essential tool for nuclear fusion research, whereby understanding crucial aspects related to plasma-wall interactions or edge plasma behaviour. Simplified models are of great importance to complement and integrate experimental and simulation results of complex systems such as plasmas in linear machines, because they are fast and simple to employ. In this work, we present a global volume-averaged (0D) model for plasma investigation in linear machines. The 0D model equations are based on the space integration of the state of the art edge plasma model implemented in the SOLPS-ITER code. Comparisons between helium plasmas described with 2D simulations performed with SOLPS-ITER and with the 0D model highlight that contributions often neglected in tokamak edge models, e.g. electron-neutral excitation, may be relevant when describing weakly ionized plasmas in linear devices. The model is used to perform sensitivity studies with respect to several parameters and to analyse the time evolution of the system, leading to the identification of two relevant time scales governing the system. Lastly, a comparison of 0D results with experimental data from the linear device GyM is performed, showing satisfactory agreement. Our methods and results provide crucial interpretative keys in the investigation of the physics of edge plasmas.

The roughness of metallic surfaces has a vital impact on the erosion of plasma-facing materials. Roughness determines the effective sputtering yield Yeff of the facing material. The angular/energy distribution of sputtered particles, and the spatial erosion and deposition distribution. The model for simulation the effect of the surface roughness was earlier implemented into the 3D Monte-Carlo code ERO2.0 and validated using results of ion beam experiments and experiments in the linear plasma device PSI-2. In the present study the developed ERO2.0 surface morphology model was applied to the JET-ILW tungsten (W) divertor consisting of smooth bulk W and W-coated CFC components. Influence of the surface roughness on the W erosion as well as on the transport of sputtered material in conditions of inclined magnetic field was investigated. Simulation results are in a good agreement with existing experimental findings.

The purpose of this study is to investigate the applicability of in-situ laser induced breakdown spectroscopy (LIBS) for deuterium retention measurements in tungsten coatings with different morphology and oxygen content. These were exposed to a Gaussian beam of deuterium plasma in the Magnum-PSI linear plasma device. The deuterium line intensities determined by LIBS were compared with the deuterium content measured by nuclear reaction analysis (NRA). Both LIBS and NRA results showed that higher deuterium retention was achieved in the coating region corresponding to the periphery of the plasma beam. This decreasing deuterium retention in the central region can be attributed to higher surface temperature. At the same time, the deuterium retention in different coating types assessed by LIBS D intensity was markedly different from the retention determined by NRA. Porous W-O coating with high oxygen content had the highest deuterium retention according to NRA while D intensity obtained by LIBS was an order of magnitude smaller when compared with other coatings. The deuterium retention in compact W coating and thick W coating was almost the same and LIBS D intensities were also comparable for these coatings. The results demonstrate the LIBS applicability and its limits in different coating types.

The interaction between intense 30 fs laser pulses and foam-coated 1.5 μm-thick Al foils in the relativistic regime (up to 5 × 1020 W cm−2) is studied to optimize the laser energy conversion into laser-accelerated protons. A significant enhancement is observed for foam targets in terms of proton cut-off energy (18.5 MeV) and number of protons above 4.7 MeV (4 × 109 protons/shot) with respect to uncoated foils (9.5 MeV, 1 × 109 protons/shot), together with a sixfold increase in the bremsstrahlung yield. This enhancement is attributed to increased laser absorption and electron generation in the foam meso- and nanostructure.

Laser-driven ion sources are interesting for many potential applications, from nuclear medicine to material science. A promising strategy to enhance both ion energy and number is given by Double-Layer Targets (DLTs), i.e. micrometric foils coated by a near-critical density layer. Optimization of DLT parameters for a given laser setup requires a deep and thorough understanding of the physics at play. In this work, we investigate the acceleration process with DLTs by combining analytical modeling of pulse propagation and hot electron generation together with Particle-In-Cell (PIC) simulations in two and three dimensions. Model results and predictions are confirmed by PIC simulations—which also provide numerical values to the free model parameters—and compared to experimental findings from the literature. Finally, we analytically find the optimal values for near-critical layer thickness and density as a function of laser parameters; this result should provide useful insights for the design of experiments involving DLTs. Laser-driven ion acceleration is an important topic for next-generation compact accelerators and material characterisation. The authors present a theoretical study on ion acceleration with near-critical double-layer targets that supports the experimental realisation of these targets and the interpretation of experiments of laser-ion acceleration.

The surface morphology of plasma-facing components (PFCs) and its evolution during plasma irradiation has been shown to have a significant effect on the erosion and subsequent transport of sputtered particles in plasma. This in turn can influence the resulting lifetime of PFCs. A model for treatment of the effect of surface roughness on the erosion of PFCs has recently been incorporated into the three-dimensional Monte Carlo code ERO2.0. First simulations have confirmed a significant influence of the assumed surface roughness (for both regular and stochastic numerically constructed samples) on both the effective sputtering yields Y-eff and the effective angular distributions of sputtered particles. In this study, a series of experiments at the linear plasma device PSI-2 are conducted to test the effect of surface roughness on the sputtering parameters. Graphite samples prepared with a 100 nm molybdenum layer with various surface roughness characteristic sizes (R-a = 110 nm, 280 nm, 600 nm and R-a < 20 nm) were exposed to a helium plasma in the PSI-2 linear plasma device at a magnetic field B.=.0.1 T. These PSI-2 experiments were simulated using ERO2.0 with a surface morphology model. Simulations are able to reproduce the experimentally observed significant suppression of erosion for higher R-a values.

Edge plasma codes, such as SOLPS, are widely used to study plasma transport in tokamaks scrape-off layers (SOL). The possibility to apply these codes to non hydrogenic plasmas and to linear plasma devices is gaining the interests of the fusion community. These facilities play a pivotal role in plasma-material interaction studies for future fusion devices and may allow to test the code capabilities both in terms of geometry and simulated plasma species. In this contribution, we apply the SOLPS-ITER code for the simulations of Argon plasmas in the medium-flux linear machine GyM. A sensitivity scan over the pumping efficiency, transport coefficients and absorbed electron power was done, performing B2.5-EIRENE coupled simulations. A quantitative analysis of the different flux contributions is provided through the use of a two-point modelling. Comparison with experiments shows a promising qualitative and quantitative agreement, with the sole exception of the simulated neutrals pressure. Dedicated EIRENE standalone simulations were performed to investigate this issue, also highlighting the role of neutral-neutral elastic collisions at high values of the puffing intensity.

Laser-driven ion sources are approaching the requirements for several applications in materials and nuclear science. Relying on compact, table-top, femtosecond laser systems is pivotal to enable most of these applications. However, the moderate intensity of these systems (I ≲ 1019 W cm−2) could lead to insufficient energy and total charge of the accelerated ions. The use of solid foils coated with a nanostructured near-critical layer is emerging as a promising targeted solution to enhance the energy and the total charge of the accelerated ions. For an appropriate theoretical understanding of this acceleration scheme, a realistic description of the nanostructure is essential, also to precisely assess its role in the physical processes at play. Here, by means of 3D particle-in-cell simulations, we investigate ion acceleration in this scenario, assessing the role of different realistic nanostructure morphologies, such as fractal-like foams and nanowire forests. With respect to a simple flat foil, the presence of a nanostructure allows for up to a × 3 increase of the maximum ion energy and for a significant increase of the conversion efficiency of laser energy into ion kinetic energy. Simulations show also that the details of the nanostructure morphology affect both the maximum energy of the ions and their angular distribution. Furthermore, combined 3D particle-in-cell and Monte Carlo simulations show that if accelerated ions are used for neutron generation with a beryllium converter, double-layer nanostructured targets allow to greatly enhance the neutron yield. These results suggest that nanostructured double-layer targets could be an essential component to enable applications of hadron sources driven by compact, table-top lasers.

Titanium (Ti), due to its excellent properties, is widely exploited in thin film technology that usually leads to the production of {\alpha}-phase (hcp) Ti films. In this work, we investigate the phase evolution of Ti films deposited by varying type and energy of the film-forming species. To investigate different plasma species environments, films with different thicknesses are grown by using conventional Direct Current Magnetron Sputtering (DCMS) and High Power Impulse Magnetron Sputtering (HiPIMS). Furthermore, HiPIMS depositions with different substrate bias voltage US (0 V, -300 V and -500 V) are performed to investigate different ion energy ranges. Microstructure, morphology and residual stress of the deposited films, as well as the DCMS and HiPIMS plasma composition, are analysed with different characterization techniques. The DCMS samples exhibit the Ti {\alpha}-phase only and show a tensile residual stress decreasing with thickness. As far as HiPIMS samples are concerned, a compressive-tensile-compressive (CTC) behavior is observed for residual stresses as thickness increases. Specifically, films deposited in low energy ion conditions (US =0 V) show the presence of the Ti fcc phase up to a maximum thickness of about 370 nm. Differently, films deposited under high energy conditions (US = -300 V and -500 V) show the nucleation of the Ti {\omega}-phase for thicknesses greater than 260 and 330 nm, respectively. The formation of these unusual Ti phases is discussed considering the different deposition conditions., Comment: This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (ENSURE grant agreement No. 647554)

Plasma sheaths characterized by electrons with relativistic energies and far from thermodynamic equilibrium are governed by a rich and largely unexplored physics. A reliable kinetic description of relativistic non-equilibrium plasma sheaths—besides its interest from a fundamental point of view—is crucial to many application, from controlled nuclear fusion to laser-driven particle acceleration. Sheath models proposed in the literature adopt either relativistic equilibrium distribution functions or non-relativistic non-equilibrium distribution functions, making it impossible to properly capture the physics involved when both relativistic and non-equilibrium effects are important. Here we tackle this issue by solving the electrostatic Vlasov–Poisson equations with a new class of fully-relativistic distribution functions that can describe non-equilibrium features via a real scalar parameter. After having discussed the general properties of the distribution functions and the resulting plasma sheath model, we establish an approach to investigate the effect of non-equilibrium solely. Then, we apply our approach to describe laser–plasma ion acceleration in the target normal sheath acceleration scheme. Results show how different degrees of non-equilibrium lead to the formation of sheaths with significantly different features, thereby having a relevant impact on the ion acceleration process. We believe that this approach can offer a deeper understanding of relativistic plasma sheaths, opening new perspectives in view of their applications.

Nitrogen (N) seeding is routinely applied in tokamaks with tungsten (W) walls to control the power exhaust toward the divertor. Open questions, concerning the interaction of N with W, are the influence of ion energy and W temperature on retention of implanted N and the erosion by deuterium (D) of the tungsten nitride being formed. Moreover, the extremely high particle fluxes in ITER and DEMO will erode the W tiles and the sputtered atoms will re-deposit forming W-based layers with a different behaviour toward the interaction with N seeded D plasmas. In this work, W films with different morphology and structure were exposed to the N seeded D plasma of the linear device GyM, in order to address all these issues. The experiments were performed at the fixed N 2 /D 2 partial pressure ratio of ∼ 4% keeping the total pressure constant at 5.3 × 1 0 − 4 mbar. The exposure conditions were: (i) sample temperature of ∼ 850 K, (ii) particle fluxes of 2 − 2 . 2 × 1 0 20 ions ⋅ m − 2 ⋅ s−1 and (iii) particle energies up to ∼ 320 eV. W columnar films (c-W) with properties close to those of virgin W coatings deposited on the tiles of JET Iter-Like Wall and ASDEX Upgrade and W amorphous films (a-W) resembling nanostructured W-based deposits found in present-day tokamaks and expected in ITER and DEMO, were considered. W columnar and amorphous coatings were produced by means of magnetron sputtering and pulsed laser deposition, respectively. The specimens were characterised by profilometry, X-ray depth-profiling photoelectron spectroscopy, optical microscopy, scanning electron microscopy, atomic force microscopy and X-ray diffraction. The main evidence is that the behaviour of the W films upon D+N plasma exposure in GyM strictly depends on their morphology and nanostructure. For all the films, a surface N-enriched layer, which is thermally stable and does not decompose at least up to ∼ 850 K, is observed. Moreover, blisters are not present on the surface of the samples. The c-W coatings erode faster than the a-W ones and have a higher nitrogen retention and diffusivity. The mechanisms behind these results are here discussed together with their possible implications from the point of view of the topic of plasma–wall interaction in tokamaks.

Among the existing elemental characterization techniques, Particle Induced X-ray Emission (PIXE) and Energy Dispersive X-ray (EDX) spectroscopy are two of the most widely used in different scientific and technological fields. Here we present the first quantitative laser-driven PIXE and laser-driven EDX experimental investigation performed at the Centro de L\'aseres Pulsados in Salamanca. Thanks to their potential for compactness and portability, laser-driven particle sources are very appealing for materials science applications, especially for materials analysis techniques. We demonstrate the possibility to exploit the X-ray signal produced by the co-irradiation with both electrons and protons to identify the elements in the sample. We show that, using the proton beam only, we can successfully obtain quantitative information about the sample structure through laser-driven PIXE analysis. These results pave the way towards the development of a compact and multi-functional apparatus for the elemental analysis of materials based on a laser-driven particle source., Comment: This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (ENSURE grant agreement No. 647554). Submitted to Science Advances on 20th May 20

At least the far scrape-off layer of magnetically confined fusion plasmas transport is intermittent and non-diffusive as observed by the appearance of plasma filaments. Transport codes using effective diffusion coefficients are still the main workhorse investigating the scrape-off layer and divertor regions. An effective perpendicular diffusion coefficient for intermittent filamentary dominated perpendicular transport in the scrape-off layer is motivated by the telegraph equation, describing an exponentially decaying correlated random walk. On short time scales, the telegraph equation describes the ballistic transport of filamentary structures with a typical velocity ub and correlation time τ. In stationary conditions, the corresponding diffusion coefficient is given by [Formula: see text]. Since ub and τ can be determined experimentally, it is proposed to use [Formula: see text] as an input for modeling or for interpretation of perpendicular transport in the far scrape-off layer.

Micron-thick boron films have been deposited by Pulsed Laser Deposition in vacuum on several substrates at room temperature. The use of high energy pulses (>700 mJ) results in the deposition of smooth coatings with low oxygen uptake even at base pressures of 10$^{-4}$ - 10$^{-3}$ Pa. A detailed structural analysis, by X-Ray Diffraction and Raman, allowed to assess the amorphous nature of the deposited films as well as to determine the base pressure that prevents boron oxide formation. In addition the crystallization dynamics has been characterized showing that film crystallinity already improves at relatively low temperatures (800 {\deg}C). Elastic properties of the boron films have been determined by Brillouin spectroscopy. Finally, micro-hardness tests have been used to explore cohesion and hardness of B films deposited on aluminum, silicon and alumina. The reported deposition strategy allows the growth of reliable boron coatings paving the way for their use in many technology fields., Comment: The research leading to these results has also received funding from the European Research Council Consolidator Grant ENSURE (ERC-2014-CoG No. 647554)

Metallic amorphous tungsten-oxygen and amorphous tungsten-oxide films, deposited by Pulsed Laser Deposition, are characterized. The correlation is investigated between morphology, composition, and structure, measured by various techniques, and the mechanical properties, characterized by Brillouin Spectroscopy and the substrate curvature method. The stiffness of the films is correlated to the oxygen content and the mass density. The elastic moduli decrease as the mass density decreases and the oxygen-tungsten ratio increases. A plateau region is observed around the transition between the metal-like (conductive and opaque) films and the oxide ones (non-conductive and transparent). The compressive residual stresses, moderate stiffness and high local ductility of compact amorphous tungsten-oxide films are interesting for applications involving thermal or mechanical loads. The coefficient of thermal expansion is quite high (8.9 ⋅ 10−6 K−1), being strictly correlated to the amorphous structure and stoichiometry of the films. Upon thermal treatments the coatings show a quite low relaxation temperature of 450 K. Starting from 670 K, they crystallize into the γ monoclinic phase of WO3, the stiffness increasing by about 70%. The measured thermomechanical properties provide a guidance for the design of devices which include a tungsten based layer, in order to assure their mechanical integrity. Keywords: Tungsten oxide coatings, Thermomechanical properties, Residual stresses, Thermal expansion coefficient, Thermal stability

In this paper we report the development of a new method for the evaluation of thin films mass thickness and composition based on the Energy Dispersive X-Ray Spectroscopy (EDS). The method exploits the theoretical calculation of the in-depth characteristic X-ray generation distribution function, $\phi$/($\rho$ z), in multilayer samples, obtained by the numerical solution of the electron transport equation, to achieve reliable measurements without the need of a reference sample and multiple voltages acquisitions. The electron transport model is derived from the Boltzmann transport equation and it exploits the most updated and reliable physical parameters in order to obtain an accurate description of the phenomenon. The method for the calculation of film mass thickness and composition is validated with benchmarks from standard techniques. In addition, a model uncertainty and sensitivity analysis is carried out and it indicates that the mass thickness accuracy is in the order of 10 $\mu$g/cm$^2$, which is comparable to the nuclear standard techniques resolution. We show the technique peculiarities in one example measurement: two-dimensional mass thickness and composition profiles are obtained for a ultra-low density, high roughness, nanostructured film., Comment: This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (ENSURE grant agreement No. 647554)

Publisher Copyright: © Max-Planck-Institut fuer Plasmaphysik. A desirable scenario for future fusion devices is one in which dissipative processes in the scrape-off layer (SOL) are maximized, aiming to detach the divertor plasma. The access to such a regime in current devices is thought to be correlated to the increase of the perpendicular particle transport in the SOL. In this work we investigated numerically how increasing perpendicular transport globally affects the SOL plasma through the SOLPS-ITER code package. For this we modelled one L-mode discharge, performed at the ASDEX Upgrade tokamak, trying to obtain the most accurate fit to the experimental data at the outer midplane. Studying the plasma solutions and analyzing the resulting momentum and power balances in the SOL allowed to characterize how enhancing perpendicular SOL transport leads to the experimentally observed phenomena, i.e. the formation of a density shoulder at the midplane and the partial detachment of the divertor plasma. The results suggest that strong momentum losses caused by the increase of transport are able to explain the qualitatively observed detachment in the modelled discharge. The concurrent enhanced ionization of neutrals resulting from divertor recycling, triggered by an increase of radial energy transport in the SOL, can be invoked as a cause for the shoulder formation.

Phosphoric acid is an inorganic acid used for producing graphene sheets by delaminating graphite in (electro-)chemical baths. The observed phenomenology during the electrochemical treatment in phosphoric acid solution is partially different from other acidic solutions, such as sulfuric and perchloric acid solutions, where the graphite surface mainly forms blisters. In fact, the graphite surface is covered by a thin layer of modified (oxidized) material that can be observed when an electrochemical potential is swept in the anodic current regime. We characterize this particular surface evolution by means of a combined electrochemical, atomic force microscopy and Raman spectroscopy investigation.

In view of large-scale applications, electrochemical exfoliation of graphite for the production of graphene sheets must follow chemical processes that ensure high quality of the products—wide-size graphene foils, single- or few-layer thickness, and low level of defectivity—in order to guarantee high electrical transport and good mechanical properties. Understanding the exfoliation process of graphite at the atomic scale, that is, the intercalation of graphene layers in the electrolyte solution, is fundamental to really be able to control and optimize such processes. This can be obtained, for instance, by investigation of the exfoliated graphite—the surface of the original crystal left behind in the chemical solution—and by real-time monitoring of graphite surface morphological and structural modifications during the exfoliation process. Here, we monitor graphite surface changes as a function of the electrochemical potential by both electrochemical (EC) atomic force microscopy and EC scanning tunneling mic...

In this work, we discuss the exposures of polycrystalline tungsten (W) and nanostructured W coatings to high energy ( ~ 30 ÷ 300 eV ) medium-flux D ( ~ 10 20 m - 2 s - 1 ) deuterium plasmas of the linear machine GyM at an overall fluence of 10 24 m - 2 . The energy and particle flux are tailored to that of charge-exchange neutrals at the vessel main chamber of present-day tokamaks and expected in ITER. Scanning Electron Microscopy measurements show the formation of surface nanostructures that are found to strongly depend on the W morphology and crystallinity, as well as on the energy of the ion species. The obtained nanostructures are few nanometres deep and similar to the one obtained after exposure of bulk W to divertor-like conditions. Dedicated annealing experiments are performed on selected samples, showing that the nanostructures developed on W coatings are thermally stable up to their recrystallisation temperature. The development of micrometric-sized blisters is also observed and found to strongly depend on the features of the W coatings, the substrate and the applied bias voltage.

Raman microscopy is one of the methods that could be used for future post-mortem analyses of samples extracted from ITER plasma facing. This study shows that this technique is useful for studying tungsten-based materials containing impurities including oxides and nitrides. Here, we apply pulsed laser deposition and DC argon glow discharges to produce tungsten-containing synthetic films (compact, porous) and nanoparticles and investigate the influence of their morphology on the measured Raman spectra. The amounts of oxygen and/or nitrogen in the films are also investigated. Comparative data are obtained by X-ray Photoelectrons Spectroscopy, Atomic Force Microscopy, Electron Microscopies (Scanning and Transmission), Energy Dispersive X-ray spectroscopy, Time-of-Flight Elastic Recoil Detection Analysis. The power density of the laser beam used to perform Raman microscopy is varied by up to 4 orders of magnitude (0.01-20 mW/$\mu$m 2) to investigate thermal stability of films and nanoparticles. As a first result, we give evidence that Raman microscopy is sensitive enough to detect surface native oxides. Secondly, more tungsten oxides are detected in porous materials and nanoparticles than in compact films, and the intensities of the Raman band correlate to their oxygen content. Thirdly, thermal stability of these films (i.e. structural and chemical modification under laser heating) is poor when compact films contain a sufficiently large amount of nitrogen. This finding suggests that nitrogen can be substituted by oxygen during Raman laser induced heating occurring in ambient air. Finally, our methodology can be used to rapidly characterize morphology and chemistry of the samples analyzed, and also to create oxides at the micrometer scale. keywords: PLD, Raman spectroscopy, tungsten oxide, tungsten nitride, plasma wall interaction, laser heating, post-mortem analysis 2, Comment: Nuclear Fusion, IOP Publishing, 2020

A systematic and attentive comparison of the deuterium (D) retention in tungsten (W) coatings with different nano-crystalline structures after the plasma exposure in comparison with polycrystalline tungsten (PCW) is presented. While a wide database is available for PCW, only a few data about the D retention in coatings with different structures exist. The D retention in W coatings produced by three different deposition techniques on different substrates was studied with respect to the influence of (a) coating crystallite size, (b) coating thickness, (c) specimen temperature during D plasma exposure, (d) presence of argon (Ar) used as working gas during the coating deposition and (e) substrate material. It is shown that the variation of the processing parameters, such as temperature, deposition rate, Ar implantation, etc. even within one deposition method results in different grain size distributions and structure of coating and has a significant effect on the D retention. It is revealed that the substrate material and the presence of Ar in a coating play a minor role in the D retention in the coating. It is shown that both the D concentration and the D retention in coatings drastically increase with decreasing the grain size. Consequently, in the case of using of W coating as a protective layer of a structural material, a compromise in the development of nanostructured tungsten films is necessary to keep the hydrogen isotope concentration at an acceptable level.

[object Object]Tungsten-on-n+ silicon ohmic contacts were obtained by depositing 100 nm-thick W coatings on silicon substrates using pulsed laser deposition at room temperature, without native oxide removal. The high energy of the impinging species (about 10-100 eV per atom) led to sputtering phenomena and to the implantation of Watoms through the Si oxide. W coatings were characterized, as-deposited and after performing rapid thermal annealing steps. The morphology and crystallinity were characterized by scanning electron microscopy, X-ray diffraction and reflectometry. The interdiffusion of W and Si was shown by scanning Auger microscopy. The ohmic character of the contacts and contact resistance were investigated by the transfer length method. Fast annealing at moderate temperatures (450 °C) remarkably improved contact performance without significant variation of the features of either W film or Si substrates. The ohmic character of the contact was preserved even after annealing at high temperature (1000 °C) at which a complete interdiffusion of Si into the W film takes place.

In this work we produced tungsten (W) and W oxide (WO x ) films by pulsed laser deposition (PLD) with the aim of the addressing modifications of structure and morphology that occur after annealing treatments and high-flux deuterium plasma. Thanks to the high flexibility of PLD we produced nanostructured W containing non-bounded oxygen, different types of WO x and multilayered films. W coatings are dense, non-porous and exhibit a nanocrystalline structure, resembling the coatings used as first wall in tokamaks. The oxide films are nearly stoichiometric amorphous WO x ( x = 3) with different morphology from compact to porous. Depending on annealing temperature, nucleation of different crystalline phases (e.g. WO 3 , W 18 O 49 ) occurs. Exposure of films to high-flux (∼10 24 m −2 s −1 ) deuterium plasmas in Magnum-PSI at different surface temperatures ( T max = 580 K) determines material modifications at the nanoscale (e.g. nanometric defects) but no delamination. In addition preliminary deuterium retention results are reported.

International audience; Structured targets offer great control over ultra-intense laser-plasma interaction, allowing the optimization of laser-target coupling for specific applications. By means of particle-in-cell simulations we investigated three applications in particular: high-order harmonic generation (HHG) with grating targets, enhanced target coupling with multilayer targets and the generation of intense laser-driven terahertz (THz) pulses with structured targets. The irradiation of a solid grating target at the resonance angle for surface plasmon excitation enhances the HHG with respect to flat targets. Multilayer targets consisting of solid foils coated with a very low-density near-critical layer lead to a strong laser absorption and hot electron production that can improve laser-driven ion acceleration. We also explored the generation of THz radiation showing how using either gratings or multilayer targets the emission can be strongly enhanced with respect to simple flat targets.

International audience; Driven by the perspective of large-scale, high-quality graphene production via chemical routes, the investigation of electrochemical anion intercalation between the basal graphite planes has seen a renewed interest among the scientific community. At relatively high electrochemical potentials, when oxidation occurs, graphite electrodes undergo significant anion intercalation processes. The latter swell the uppermost graphite layers (i.e., graphene sheets), reduce the interplane interaction and favor the graphite delamination in liquid. Different intercalation stages are observed in a perchloric acid electrolyte, which are usually interpreted in terms of different perchlorate penetration depths. Nonetheless, the understanding of the morphological changes occurring at the electrode surface during the different intercalation stages is still not completely clear. We combine different microscopy techniques including optical, scanning electron and electrochemical atomic force microscopies to analyze the morphological evolution of the graphite surface at different length scales as a function of the applied electrochemical potential. Whereas both carbon dissolution and blisters affect the surface on the micrometer scale as soon as intercalation starts, we find that the graphite surface is cracked on the sub-millimeter scale only when intercalation at a higher potential is reached, inducing a significant aging of the electrode surface.

International audience; A number of laser facilities coming online all over the world promise the capability of high-power laser experiments with shot repetition rates between 1 and 10 Hz. Target availability and technical issues related to the interaction environment could become a bottleneck for the exploitation of such facilities. In this paper, we report on target needs for three different classes of experiments: dynamic compression physics, electron transport and isochoric heating, and laser-driven particle and radiation sources. We also review some of the most challenging issues in target fabrication and high repetition rate operation. Finally, we discuss current target supply strategies and future perspectives to establish a sustainable target provision infrastructure for advanced laser facilities.

In this work, nanocrystalline diamond was deposited with a direct-current micro-plasma device on substrates with different carbon diffusivity. No substrate pre-treatment was performed and the same deposition conditions were adopted for all substrates, with the intention to investigate the first stages of growth. Samples were grown with increasing deposition time. Scanning electron microscopy and visible Raman spectroscopy were used to derive morphological and structural information. The growth dynamics was found to be the same on all substrates, with the deposition of a graphite layer prior to diamond growth. This layer was extensively characterized and a link between the structure and morphology of this layer and the subsequent features of the diamond grown on it was found. Moreover, diamond tens of micrometers thick was deposited in few hours, opening the possibility of producing diamond samples of practical interest. Particularly, the growth of nanocrystalline diamond on iron without substrate pre-treatment or interlayer deposition is reported, showing the feasibility of depositing diamond even on ferrous materials.

In this work, the growth dynamics of nanocrystalline diamond, deposited on molybdenum and silicon substrates with a direct-current micro-plasma device, is experimentally investigated. No substrate pre-treatment was performed and comparable deposition conditions were adopted for both substrates, addressing both morphological and structural properties for various deposition times. An extensive characterization of the resulting different stages of growth was achieved, exploiting scanning electron microscopy, X-ray diffraction and Raman spectroscopy at two wavelenghts (both visible and UV). The deposition of graphite upon formation of a carbide layer prior to diamond nucleation has been found to be the triggering step towards diamond growth on both substrates, but with a different growth kinetics. Besides showing the potential of the adopted deposition method and characterization strategy, our results also show the possibility of producing nanocrystalline diamond with reduced deposition times and without any substrate pre-treatment. (C) 2017 Elsevier B.V. All rights reserved.

Laser interaction with uniform and nanostructured near-critical plasmas has been investigated by means of 2D particle-in-cell simulations. The effect of a nanostructure (modeled as a collection of solid-density nanospheres) on energy absorption and radiative losses has been assessed in a wide range of laser intensities and average densities of the target. The nanostructure was found to affect mainly the conversion efficiency of laser energy into ion kinetic energy and radiative losses for the highest simulated intensities., The research leading to these results has received funding from the European Research Council Consolidator Grant ENSURE (ERC-2014-CoG No. 647554)