Undulator design for Laser Plasma Based Free electron laser

The fourth generation of synchrotron radiation sources, commonly referred to as the Free Electron Laser (FEL), provides an intense source of brilliant X-ray beams enabling the investigation of matter at the atomic scale with unprecedented time resolution. These sources require the use of conventional linear accelerators providing high electron beam performance. The achievement of chirped pulse amplification allowing lasers to be operated at the Terawatt range, opened the way for the Laser Plasma Acceleration (LPA) technique where high energy electron bunches with high current can be produced within a very short centimeter-scale distance. Such an advanced acceleration concept is of great interest to be qualified by an FEL application for compact X-ray light sources. We explore in this paper what the LPA specificities imply on the design of the undulator, part of the gain medium. First, the LPA concept and state-of-art are presented showing the different operation regimes and what electron beam parameters are likely to be achieved. The LPA scaling laws are discussed afterwards to better understand what laser or plasma parameters have to be adjusted in order to improve electron beam quality. The FEL is secondly discussed starting with the spontaneous emission, followed by the different FEL configurations, the electron beam transport to the undulator and finally the scaling laws and correction terms in the high gain case. Then, the different types of compact undulators that can be implemented for an LPA based FEL application are analyzed. Finally, examples of relevant experiments are reported by describing the transport beamline, presenting the spontaneous emission characteristics achieved so far and the future prospects

Identification des forêts matures à partir d'images à haute et très haute résolution spatiale

Les forêts matures abritent une biodiversité spécifique. Pour cette raison, les politiques publiques encouragent la création ou le maintien de forêts matures dans la mosaïque forestière, à différentes échelles.Ce stage a pour objectif de développer une méthode qui permette au gestionnaire forestier de localiser à moindre coût les forêts matures. A partir d’images à haute et très haute résolution spatiale, en utilisant de données multi-sources et multi-temporelles, nous cherchons à construire un modèle statistique qui servira à établir une cartographie de ces forêts sur des grands territoires.Deux hypothèses sont testées : la première est que, en raison de leur structure, les forêts matures ont une signature radiométrique ou structurale différentes de celles des forêts non matures ; la seconde est que la signature radiométrique des forêts matures (non gérées) change moins au cours du temps que celle des forêts non matures (régulièrement exploitées).L’étude s’appuie sur une base de données de placettes de terrain précisément géoréférencées, constituée des Hêtraies-Sapinières des massifs préalpins des Alpes du Nord françaises. Pour chaque placette, nous avons calculé un indice de maturité (variable à prédire) puis extrait les variables radiométriques et texturales sur les images correspondantes (variables prédictives). Ensuite nous avons effectué une analyse multivariée (ACP) à partir de variables retenues afin de construire un modèle statistique qui sert à l’identification de forêts matures. Nous avons obtenu un modèle très peu significatif (R2=0,12), dans la deuxième partie de stage nous allons ajouter les variables temporales pour améliorer notre modèle

Vers un laser à électrons libres compact et avancé

X-ray Free Electron Lasers (FEL) are nowadays unique intense coherent fs light sources used for multi-disciplinary investigations of matter. A new acceleration scheme such as Laser Plasma Accelerator (LPA) is now capable of producing an accelerating gradient of few GeV/cm far superior to that of conventional RF linacs. This PhD work has been conducted in the framework of R&D programs of the LUNEX5 (free electron Laser Using a New accelerator for the Exploitation of X-ray radiation of 5th generation) project of advanced and compact Free Electron laser demonstrator with pilot user applications. It comprises a 400 MeV superconducting linac for studies of advanced FEL schemes, high repetition rate operation (10 kHz), multi-FEL lines, a Laser Wake Field Accelerator (LWFA) for its qualification by a FEL application. The FEL lines comports enables advanced seeding in the 40-4 nm spectral range using high gain harmonic generation (HGHG) and echo-enabled harmonic generation (EEHG) with compact short period high field cryogenic undulators. The study of compact devices suitable for compact FEL applications is thus examined. One first aspect concerns the reduction of the Free Electron Laser gain medium (electrons in undulator) where shortening of the period is on the expense of the magnetic field leading to an intensity reduction at high harmonics. Compact cryogenic permanent magnet based undulators (CPMUs), where the magnet performance is increased at cryogenic temperature making them suitable for compact applications, are studied. Three CPMUs of period 18 mm have been built: two are installed at SOLEIL storage ring and one at COXINEL experiment. A second part of the work is developed in the frame of the R&D programs is the COXINEL experiment with an aim at demonstrating FEL amplification using an LPA source. The line enables to manipulate the properties of the produced electron beams (as energy spread, divergence, induced dispersion due) before being used for light source applications. The electron beam generated is highly divergent and requires a good handling at an early stage with strong quadrupoles, to be installed immediately after the electron generation source. Hence, the development of the so-called QUAPEVAs, innovative permanent magnet quadrupoles with high tunable gradient, is presented. The QUAPEVAs are optimized with RADIA code and characterized with three magnetic measurements. High tunable gradient is achieved while maintaining a rather good magnetic center excursion that allowed for beam pointing alignment compensation at COXINEL, where the beam is well-focused with zero dispersion at any location along the line. The QUAPEVAs constitute original systems in the landscape of variable high gradient quadrupoles developed so far. A third part of the work concerns the observation of tunable monochromatic undulator radiation on the COXINEL line. The electron beam of energy of 170 MeV is transported and focused in a 2-m long CPMU with a period of 18 mm emitting radiation light at 200 nm. The spectral flux is characterized using a UV spectrometer and the angular flux is captured by a CCD camera. The wavelength is tuned with the undulator gap variation. The spatio-spectral moon shape type pattern of the undulator radiation provided an insight on the electron beam quality and its transport enabling the estimation of the electron beam parameters such as energy spread and divergence. The final aspect of the work is related to the comparison between the echo and high gain harmonic generation, in the frame of my participation to an experiment carried out at FERMI@ELETTRA. At FERMI, we have demonstrated a high gain lasing using EEHG at a wavelength of 5.9 nm where it showed a narrower spectra and better reproducibility compared to a two-stage HGHG. This PhD work constitutes a step forward towards advanced compact Free Electron Lasers.Les lasers à électrons libres (LEL) X sont aujourd'hui des sources lumineuses cohérentes et intenses utilisées pour des investigations multidisciplinaires de la matière. Un nouveau schéma d'accélération, l'accélérateur laser plasma (LPA), est maintenant capable de produire une accélération de quelques GeV/cm, bien supérieure à celle des linacs radiofréquence. Ce travail de thèse a été mené dans le cadre des programmes de R&D du projet LUNEX5 (laser à électrons libres utilisant un nouvel accélérateur pour l’exploitation du rayonnement X de 5e génération) de démonstrateur LEL avancé et compact avec applications utilisatrices pilotes. Il comprend un linac supraconducteur de 400 MeV de haute cadence (10 kHz) pour l’étude de schémas LEL avancés, et LPA pour sa qualification par une application LEL. La ligne LEL utilise une configuration d’injection avancée dans la plage spectrale 40-4 nm par génération d’harmoniques à gain élevé (HGHG) et schéma d’écho (EEHG) avec des onduleurs compacts cryogéniques à champ élevé de courte période courte. L'étude de solutions adaptées aux applications LEL compactes et avancées est donc examinée. Un premier aspect concerne la réduction du milieu de gain du LEL (électrons dans l'onduleur), le raccourcissement de la période se faisant au détriment du champ magnétique. Les onduleurs cryogéniques compacts à base d'aimants permanents cryogéniques (CPMU), dans lesquels les performances de l'aimant sont améliorées à la température cryogénique sont étudiés. Une deuxième partie du travail développée dans le cadre l’expérience de R&D COXINEL visant à démontrer l’amplification LEL à l’aide d’un LPA. La ligne permet de manipuler les propriétés des faisceaux d’électrons produits (dispersion en énergie, divergence, variation de pointé) avant d’être utilisées pour des applications de sources lumineuses. Le faisceau d'électrons généré est très divergent et nécessite une bonne manipulation juste après la source avec des quadrupôles forts placés immédiatement après la génération d'électrons. Ainsi, des quadrupôles innovants à aimants permanents de gradient élevé réglable appelés «QUAPEVA», sont développés. Ils sont optimisés avec le code RADIA et caractérisées avec trois mesures magnétiques. Un gradient de 200 T/m avec une variabilité de 50 % est obtenu tout en maintenant une excursion du centre magnétique réduite à ± 10 µm, qui a permis un alignement par compensation de pointé du faisceau dans COXINEL grâce au centre magnétique variable des systèmes, avec un faisceau bien focalisé sans dispersion. Les QUAPEVA constituent des systèmes originaux dans le paysage des quadrupôles à de gradient élevé et variable développés jusqu'à présent. Une troisième partie des travaux concerne l’observation du rayonnement d’onduleur monochromatique ajustable sur la ligne COXINEL. Le faisceau d'électrons d'énergie de 170 MeV est transporté et focalisé dans un CPMU de 2 m et de période de 18 mm émettant à 200 nm. Le flux spectral est caractérisé à l'aide d'un spectromètre UV et le flux angulaire mesuré par une caméra CCD. La longueur d'onde est accordée avec l’entrefer. Les distributions spatio-spectrales mesurées en forme de lune du rayonnement de l'onduleur sont bien reproduites par les simulations de rayonnement utilisant les distributions d’électrons mesurées et transportées le long de la ligne. Elles permettent aussi de renseigner sur la qualité du faisceau d’électrons, de son transport et d'en estimer les paramètres tels que la dispersion en énergie et la divergence. Le dernier aspect du travail est lié à la comparaison entre la génération des harmoniques en gain élevé et le schéma d’écho, dans le cadre de ma participation à une expérience réalisée à FERMI @ ELETTRA. Nous avons pu démontrer un LEL de type écho à 5,9 nm, avec spectres plus étroits et une meilleure reproductibilité que le schéma HGHG à deux étages. Cette thèse constitue un pas en avant vers les lasers à électrons libres compacts et avancés

Permanent Magnet-Based Quadrupoles for Plasma Acceleration Sources

The laser plasma accelerator has shown a great promise where it uses plasma wakefields achieving gradients as high as GeV/cm. With such properties, one would be able to build much more compact accelerators, compared to the conventional RF ones, that could be used for a wide range of fundamental research and applied applications. However, the electron beam properties are quite different, in particular, the high divergence, leading to a significant growth of the emittance along the transport line. It is, thus, essential to mitigate it via a strong focusing of the electron beam to enable beam transport. High-gradient quadrupoles achieving a gradient greater than 100 T/m are key components for handling laser plasma accelerator beams. Permanent magnet technology can be used to build very compact quadrupoles capable of providing a very large gradient up to 500 T/m. We present different designs, modeled with a 3D magnetostatic code, of fixed and variable systems. We also review different quadrupoles that have already been built and one design is compared to measurements

Study of an Echo-Enabled Harmonic Generation Scheme for the French FEL Project LUNEX5

International audienceIn the French LUNEX5 project (Laser à électrons libres Utilisant un Nouvel accélérateur pour l'exploitation du rayonnement X de 5ème génération), a compact advanced free-electron laser (FEL) is driven by either a superconducting linac or a laser-plasma accelerator that can deliver a 400-MeV electron beam. LUNEX5 aims to produce FEL radiation in the ultraviolet and extreme ultraviolet (EUV) range. To improve the longitudinal coherence of the FEL pulses and reduce the gain length, it will operate in Echo-Enabled Harmonic Generation (EEHG) seeding configuration. EEHG is a strongly nonlinear harmonic up-conversion process based on a two-seed laser interaction that enables to reach very high harmonics of the seed laser. Recent experimental demonstration of ECHO-75, starting from an infrared seed laser, was recently achieved at SLAC and is opened the way for EEHG scheme in the EUV and soft x-ray range. Furthermore, FELs are promising candidates for the next generation of lithography technology using EUV light. In this work, we report a preliminary study of EEHG scheme for LUNEX5 in order to reach the target wavelength of 13.5 nm, currently expected for application to lithography

An operational framework for mapping irrigated areas at plot scale using Sentinel-1 and Sentinel-2 data

International audienceIn this study, we present an operational methodology for mapping irrigated areas at plot scale, which overcomes the limitation of terrain data availability, using Sentinel-1 (S1) C-band SAR (synthetic-aperture radar) and Sentinel-2 (S2) optical time series. The method was performed over a study site located near Orléans city of north-central France for four years (2017 until 2020). First, training data of irrigated and non-irrigated plots were selected using predefined selection criteria to obtain sufficient samples of irrigated and non-irrigated plots each year. The training data selection criteria is based on two irrigation metrics; the first one is a SAR-based metric derived from the S1 time series and the second is an optical-based metric derived from the NDVI (normalized difference vegetation index) time series of the S2 data. Using the newly developed irrigation event detection model (IEDM) applied for all S1 time series in VV (Vertical-Vertical) and VH (Vertical-Horizontal) polarizations, an irrigation weight metric was calculated for each plot. Using the NDVI time series, the maximum NDVI value achieved in the crop cycle was considered as a second selection metric. By fixing threshold values for both metrics, a dataset of irrigated and non-irrigated samples was constructed each year. Later, a random forest classifier (RF) was built for each year in order to map the summer agricultural plots into irrigated/non-irrigated. The irrigation classification model uses the S1 and NDVI time series calculated over the selected training plots. Finally, the proposed irrigation classifier was validated using real in situ data collected each year. The results show that, using the proposed classification procedure, the overall accuracy for the irrigation classification reaches 84.3%, 93.0%, 81.8%, and 72.8% for the years 2020, 2019, 2018, and 2017, respectively. The comparison between our proposed classification approach and the RF classifier built directly from in situ data showed that our approach reaches an accuracy nearly similar to that obtained using in situ RF classifiers with a difference in overall accuracy not exceeding 6.2%. The analysis of the obtained classification accuracies of the proposed method with precipitation data revealed that years with higher rainfall amounts during the summer crop-growing season (irrigation period) had lower overall accuracy (72.8% for 2017) whereas years encountering a drier summer had very good accuracy (93.0% for 2019)

D’OSO à la cartographie de végétation par télédétection multi-temporelle – Exemples d’utilisation des images Sentinel-2

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Electron Beam Brightness and Undulator Radiation Brilliance for a Laser Plasma Acceleration Based Free Electron Laser

We report here on spontaneous undulator radiation and free electron laser calculations after a 10-m long transport line (COXINEL) using a Laser Plasma acceleration (LPA) source. The line enables the manipulation of the properties of the produced electron beams (energy spread, divergence, dispersion) in view of light source applications. The electron beam brightness and undulator radiation brilliance are addressed by an analytical approach enabling us to point out the influence of chromatic effects in the COXINEL case

Evaluating high resolution soil moisture maps in the framework of the ESA CCI

International audience<p>Surface Soil Moisture (SM) plays a key role in the Earth water cycle and many hydrological processes (Koster 2004), it is essential for accurate weather forecasting (Rodriguez-Fernandez 2019) and agriculture management (Guerif 2000). SM was also identified as one of the 50 “Essential Climate Variables” (ECVs) by the Global Climate Observing System (GCOS). Long time series of ECVs are crucial to monitor the Earth’s climate evolution, and developing them is the goal of initiatives such as the European Space Agency’s Climate Change Initiative (ESA CCI).</p><p>The ESA SM CCI product (Gruber 2019) provides global time series for the 1979-2021 period at 25-km resolution using scatterometers and passive microwave sensors. Based on extensive feedbacks from the user communities of SM products, a strong need for higher spatial resolutions SM data was identified (Dorigo 2018, Peng 2020). This also includes climate applications such as assessment of climate change impacts at regional level.</p><p>SM can be estimated at high spatial resolution using Synthetic Aperture Radars such as Sentinel-1 (S1). Several high resolution (HR) S1 SM data sets exist such as the products from the Copernicus Global Land Service and the one using the S²MP (Sentinel-1/2 Soil Moisture Product) algorithm (El Hajj 2017). Despite the actual short temporal coverage of such data, it is worth to evaluate them in the context of the ESA CCI as potential future HR SM long time series, and also as benchmarking references for HR SM data sets that could be obtained by the downscaling of coarser resolution sensors.</p><p>In this context, the S²MP algorithm, which was originally designed to retrieve SM at a plot level, was adapted to compute SM maps at 1-km resolution over six 100-km<sup>²</sup> regions in the Southwest and Southeast of France, Tunisia, North America, Spain and Australia. The S²MP algorithm is based on a neural network approach using backscattering coefficients and incidence angles from S1, and either NDVI from Sentinel-2 (S2) or that of Sentinel-3 (S3), as input data.</p><p>Both S1+S2 and S1+S3 1-km SM maps are compared to HR SM data from the SMAP+S1 product and the Copernicus SM and Soil Water Index (SWI) data sets. The S1+S2 and S1+S3 SM maps are in very good agreement in terms of correlation (R > 0.9), bias (< 0.05 m<sup><em>3</em></sup>.m<sup><em>-3</em></sup>) and standard deviation of the difference (STDD < 0.025 m<sup><em>3</em></sup>.m<sup><em>-3</em></sup>) over the 6 regions of study. They also are well correlated (R ~ 0.6-0.7) with the Copernicus products over homogeneous pixels containing croplands and herbaceous vegetation. However, the results are more mitigated over Tunisia and mixed land cover pixels as well as when the maps are compared to those of SMAP+S1.</p><p>The high resolution products are also evaluated against in-situ measurements along with coarse scale SM data sets (SMAP, SMOS, ESA CCI). In general, the coarse resolution SM products show better correlation than the HR ones. However, the HR products, in particular S²MP, show lower STDD and bias.</p&gt

Electron Beam Brightness and Undulator Radiation Brilliance for a Laser Plasma Acceleration Based Free Electron Laser

International audienceWe report here on spontaneous undulator radiation and free electron laser calculations after a 10-m long transport line (COXINEL) using a Laser Plasma acceleration (LPA) source. The line enables the manipulation of the properties of the produced electron beams (energy spread, divergence, dispersion) in view of light source applications. The electron beam brightness and undulator radiation brilliance are addressed by an analytical approach enabling us to point out the influence of chromatic effects in the COXINEL case