Spectrocopie de plasma induit par laser pour l'analyse des composants face au plasma de tokamaks (étude paramétrique et mesures autocalibrées)

Lors du fonctionnement d'un réacteur de fusion nucléaire par confinement magnétique comme ITER, une fraction de tritium est piégée par les composants face au plasma et doit être mesurée pour des raisons de sureté nucléaire. La spectroscopie de plasma induit par laser est proposée pour effectuer cette mesure. Le plasma laser produit sur des tuiles de Tore Supra en composite à fibre de carbone est analysé à l'aide d'une étude paramétrique : il doit avoir une température supérieure à 10000 K et une densité électronique supérieure à 10^17 cm^-3 pour optimiser l'application. Une méthode "autocalibrée" prenant en compte l'auto-absorption des raies est utilisée pour déterminer la concentration relative d'hydrogène à partir des spectres expérimentaux. La caractérisation spatio-temporelle du panache d'ablation révèle la présence d'un gradient de température dirigé du centre vers la périphérie du plasma. La prise en compte de ce gradient permet de déduire le rapport des concentrations H/C. L'incertitude de la mesure est évaluée et discutée. La mesure du rapport isotopique D/H sous pression réduite d'argon met en évidence un effet de ségrégation qui doit être pris en compte afin d'éviter des erreurs de mesure de l'ordre de 50%. Les matériaux à base de tungstène sont analysés et les difficultés associées aux données spectroscopiques sont abordées. Enfin, la faisabilité de l'analyse LIBS résolue en profondeur est validée pour des échantillons métalliques multicouches préalablement étalonnés.During the operation of a nuclear fusion device like the future reactor ITER, a fraction of tritium is trapped in the plasma facing components and has to be measured in order to fulfill nuclear safety requirements. Laser-induced breakdown spectroscopy is proposed to achieve this measurement. The laser plasma produced on carbon fibre composite tiles from the Tore Supra reactor is analyzed via a parametric study : it has to have a temperature over 10000 K and an electron density over 10^17 cm^-3 to optimize the application. A calibration-free procedure that takes into account self-absorption is proposed to determine the relative concentration of hydrogen from the experimental spectra. The time- and space-resolved spectral emission of the plasma plume is investigated and reveals the presence of a temperature gradient from the core towards the periphery. This gradient is taken into account and the H/C concentration ratio is deduced. The accuracy of the results is evaluated and discussed. The study of the D/H isotopic ratio under low pressure argon reveals the presence of plume segregation that leads to an error of about 50%, error that can partially be reduced. Tungsten materials are investigated and difficulties related to spectroscopic databases are discussed. Finally, the feasibility of LIBS analysis with depth resolution is validated for multilayered metallic samples.AIX-MARSEILLE2-Bib.electronique (130559901) / SudocSudocFranceF

Transient non-collinear magnetic state for all-optical magnetization switching

Resonant absorption of a photon by bound electrons in a solid can promote an electron to another orbital state or transfer it to a neighboring atomic site. Such a transition in a magnetically ordered material could affect the magnetic order. While this process is an obvious road map for optical control of magnetization, experimental demonstration of such a process remains challenging. Exciting a significant fraction of magnetic ions requires a very intense incoming light beam, as orbital resonances are often weak compared to above-band-gap excitations. In the latter case, a sizeable reduction of the magnetization occurs as the absorbed energy increases the spin temperature, masking the non-thermal optical effects. Here, using ultrafast x-ray spectroscopy, we were able to resolve changes in the magnetization state induced by resonant absorption of infrared photons in Co-doped yttrium iron garnet, with negligible thermal effects. We found that the optical excitation of the Co ions affects the two distinct magnetic Fe sublattices differently, resulting in a transient non-collinear magnetic state. The present results indicate that the all-optical magnetization switching most likely occurs due to the creation of a transient, non-collinear magnetic state followed by coherent spin rotations of the Fe moments

Real-time spatial characterization of micrometer-sized X-ray free-electron laser beams focused by bendable mirrors

A real-time and accurate characterization of the X-ray beam size is essential to enable a large variety of different experiments at free-electron laser facilities. Typically, ablative imprints are employed to determine shape and size of μm-focused X-ray beams. The high accuracy of this state-of-the-art method comes at the expense of the time required to perform an ex-situ image analysis. In contrast, diffraction at a curved grating with suitably varying period and orientation forms a magnified image of the X-ray beam, which can be recorded by a 2D pixelated detector providing beam size and pointing jitter in real time. In this manuscript, we compare results obtained with both techniques, address their advantages and limitations, and demonstrate their excellent agreement. We present an extensive characterization of the FEL beam focused to ≈1 μm by two Kirkpatrick-Baez (KB) mirrors, along with optical metrology slope profiles demonstrating their exceptionally high quality. This work provides a systematic and comprehensive study of the accuracy provided by curved gratings in real-time imaging of X-ray beams at a free-electron laser facility. It is applied here to soft X-rays and can be extended to the hard X-ray range. Furthermore, curved gratings, in combination with a suitable detector, can provide spatial properties of μm-focused X-ray beams at MHz repetition rate

CAMP@FLASH: an end-station for imaging, electron- and ion-spectroscopy, and pump–probe experiments at the FLASH free-electron laser

The non-monochromatic beamline BL1 at the FLASH free-electron laser facility at DESY was upgraded with new transport and focusing optics, and a new permanent end-station, CAMP, was installed. This multi-purpose instrument is optimized for electron- and ion-spectroscopy, imaging and pump–probe experiments at free-electron lasers. It can be equipped with various electron- and ion-spectrometers, along with large-area single-photon-counting pnCCD X-ray detectors, thus enabling a wide range of experiments from atomic, molecular, and cluster physics to material and energy science, chemistry and biology. Here, an overview of the layout, the beam transport and focusing capabilities, and the experimental possibilities of this new end-station are presented, as well as results from its commissioning

Electron population dynamics in resonant non-linear x-ray absorption in nickel at a free-electron laser

Free-electron lasers provide bright, ultrashort, and monochromatic x-ray pulses, enabling novel spectroscopic measurements not only with femtosecond temporal resolution: The high fluence of their x-ray pulses can also easily enter the regime of the non-linear x-ray–matter interaction. Entering this regime necessitates a rigorous analysis and reliable prediction of the relevant non-linear processes for future experiment designs. Here, we show non-linear changes in the L3-edge absorption of metallic nickel thin films, measured with fluences up to 60 J/cm2. We present a simple but predictive rate model that quantitatively describes spectral changes based on the evolution of electronic populations within the pulse duration. Despite its simplicity, the model reaches good agreement with experimental results over more than three orders of magnitude in fluence, while providing a straightforward understanding of the interplay of physical processes driving the non-linear changes. Our findings provide important insights for the design and evaluation of future high-fluence free-electron laser experiments and contribute to the understanding of non-linear electron dynamics in x-ray absorption processes in solids at the femtosecond timescale

Photon shot-noise limited transient absorption soft X-ray spectroscopy at the European XFEL

Femtosecond transient soft X-ray Absorption Spectroscopy (XAS) is a very promising technique that can be employed at X-ray Free Electron Lasers (FELs) to investigate out-of-equilibrium dynamics for material and energy research. Here we present a dedicated setup for soft X-rays available at the Spectroscopy & Coherent Scattering (SCS) instrument at the European X-ray Free Electron Laser (EuXFEL). It consists of a beam-splitting off-axis zone plate (BOZ) used in transmission to create three copies of the incoming beam, which are used to measure the transmitted intensity through the excited and unexcited sample, as well as to monitor the incoming intensity. Since these three intensity signals are detected shot-by-shot and simultaneously, this setup allows normalized shot-by-shot analysis of the transmission. For photon detection, the DSSC imaging detector, which is capable of recording up to 800 images at 4.5 MHz frame rate during the FEL burst, is employed and allows approaching the photon shot-noise limit. We review the setup and its capabilities, as well as the online and offline analysis tools provided to users.Comment: 11 figure

Symmetry-dependent ultrafast manipulation of nanoscale magnetic domains

Femtosecond optical pumping of magnetic materials has been used to achieve ultrafast switching and recently to nucleate symmetry-broken magnetic states. However, when the magnetic order parameter already presents a broken-symmetry state, such as a domain pattern, the dynamics are poorly understood and consensus remains elusive. Here, we resolve the controversies in the literature by studying the ultrafast response of magnetic domain patterns with varying degrees of translation symmetry with ultrafast x-ray resonant scattering. A data analysis technique is introduced to disentangle the isotropic and anisotropic components of the x-ray scattering. We find that the scattered intensity exhibits a radial shift restricted to the isotropic component, indicating that the far-from-equilibrium magnetization dynamics are intrinsically related to the spatial features of the domain pattern. Our results suggest alternative pathways for the spatiotemporal manipulation of magnetism via far-from-equilibrium dynamics and by carefully tuning the ground-state magnetic textures

Nonequilibrium sub–10 nm spin-wave soliton formation in FePt nanoparticles

Magnetic nanoparticles such as FePt in the L1 0 phase are the bedrock of our current data storage technology. As the grains become smaller to keep up with technological demands, the superparamagnetic limit calls for materials with higher magnetocrystalline anisotropy. This, in turn, reduces the magnetic exchange length to just a few nanometers, enabling magnetic structures to be induced within the nanoparticles. Here, we describe the existence of spin-wave solitons, dynamic localized bound states of spin-wave excitations, in FePt nanoparticles. We show with time-resolved x-ray diffraction and micromagnetic modeling that spin-wave solitons of sub–10 nm sizes form out of the demagnetized state following femtosecond laser excitation. The measured soliton spin precession frequency of 0.1 THz positions this system as a platform to develop novel miniature devices

Spectrocopie de plasma induit par laser pour l'analyse des composants face au plasma de tokamaks : étude paramétrique et mesures autocalibrées

Lors du fonctionnement d'un réacteur de fusion nucléaire par confinement magnétique comme ITER, une fraction de tritium est piégée par les composants face au plasma et doit être mesurée pour des raisons de sureté nucléaire. La spectroscopie de plasma induit par laser est proposée pour effectuer cette mesure. Le plasma laser produit sur des tuiles de Tore Supra en composite à fibre de carbone est analysé à l'aide d'une étude paramétrique : il doit avoir une température supérieure à 10000 K et une densité électronique supérieure à 10^17 cm^-3 pour optimiser l'application. Une méthode "autocalibrée" prenant en compte l'auto-absorption des raies est utilisée pour déterminer la concentration relative d'hydrogène à partir des spectres expérimentaux. La caractérisation spatio-temporelle du panache d'ablation révèle la présence d'un gradient de température dirigé du centre vers la périphérie du plasma. La prise en compte de ce gradient permet de déduire le rapport des concentrations H/C. L'incertitude de la mesure est évaluée et discutée. La mesure du rapport isotopique D/H sous pression réduite d'argon met en évidence un effet de ségrégation qui doit être pris en compte afin d'éviter des erreurs de mesure de l'ordre de 50%. Les matériaux à base de tungstène sont analysés et les difficultés associées aux données spectroscopiques sont abordées. Enfin, la faisabilité de l'analyse LIBS résolue en profondeur est validée pour des échantillons métalliques multicouches préalablement étalonnés.During the operation of a nuclear fusion device like the future reactor ITER, a fraction of tritium is trapped in the plasma facing components and has to be measured in order to fulfill nuclear safety requirements. Laser-induced breakdown spectroscopy is proposed to achieve this measurement. The laser plasma produced on carbon fibre composite tiles from the Tore Supra reactor is analyzed via a parametric study : it has to have a temperature over 10000 K and an electron density over 10^17 cm^-3 to optimize the application. A calibration-free procedure that takes into account self-absorption is proposed to determine the relative concentration of hydrogen from the experimental spectra. The time- and space-resolved spectral emission of the plasma plume is investigated and reveals the presence of a temperature gradient from the core towards the periphery. This gradient is taken into account and the H/C concentration ratio is deduced. The accuracy of the results is evaluated and discussed. The study of the D/H isotopic ratio under low pressure argon reveals the presence of plume segregation that leads to an error of about 50%, error that can partially be reduced. Tungsten materials are investigated and difficulties related to spectroscopic databases are discussed. Finally, the feasibility of LIBS analysis with depth resolution is validated for multilayered metallic samples

Simulation of Plasma Emission Spectra for Quantitative Elemental Analysis via Laser-Induced Breakdown Spectroscopy

International audienceThe calculation of the plasma emission spectrum with the aid of an appropriate model presents a promising approach for material analysis via laser-induced breakdown spectroscopy (LIBS) as it may lead to an increase of the versatility of the method. Indeed, the properties of laser-produced plasmas strongly depend on the material's elemental compositionelemental composition of the material, and the calibration of LIBS measurements is often difficult. Therefore, the analyses remain qualitative or semi-quantitative in many cases. This is in particular the case for organic materials as the mechanisms of laser energy deposition and plasma formation critically depend on the content of minor elements with low ionization potential. The development of the appropriate plasma model is a particularly difficult task, as laser-produced plasmas are characterized by a complex expansion process into the surrounding atmosphere. Several experimental investigations show that the plasma's temperature and density profiles are spatially non-uniform. The gradients remain up to time delays typically applied in LIBS experiments. The nonuniformity may be ignored when the analysis only concerns plasma species of similar excitation and ionization energies. Contrarily, the gradients have to be considered if species of significantly different excitation energies are involved as these species are located in zones of different temperature [1]. In the present paper, we discuss the possibilities of performing LIBS analysis via plasma modelling by considering the nonuniform character of the laser-produced plasma. In particular, we present an approach based on the calculation of the plasma's spectral radiance that takes advantage of the analysis of the spectral line shapes as a feedback in the LIBS measurement procedure [2]. [1] E. Tognoni, G. Cristoforetti, S. Legnaioli, V. Palleschi, Spectrochim. Acta. Part B (2010) 65, 1-14. [2] L. Mercadier, J. Hermann, C. Grisolia, A. Semerok, J. Anal. At. Spectrom. (2013) to be publishe