Study of Laser-induced damage of LMJ diffraction gratings

Studies of laser induced damage of UV optics of the LMJ facility lead for the past few years have allowed the improvement of the optical component manufacturing especially the polished side of the 3w diffraction gratings. Yet, laser induced damage of the etched surface is ruled by other mechanisms that needs to be studied in order to improve the global Laser-induced damage threshold of the gratings.
This post-doctoral research project consist in :
- characterizing the Laser induced damage sites observed in order to understand the underlying mechanisms and the influences parameters (for example the electric field intensity within the grating profile)
- determining the laser induced damage threshold (LIDT) of samples representative of LMJ gratings
- evaluating the LIDT regarding the LMJ environment and assessing the impact on the latter on the optical performance and time of use the components
- suggesting improvements on the manufacturing process in clothe collaboration with the manufacturer and other laboratories expert in the field (CEA or else)

Development, metrological validation and outdoor testing of a multitrack Raman/FO measurement unit dedicated to the safety of future cryogenic liquid hydrogen dispensing stations

Context: The domestic and industrial use of liquid hydrogen as the fuel of the future requires the definition of a suitable safety code. At present, tank separation criteria have been defined by anticipation using a conservative approach. It is therefore necessary to carry out full-scale experiments ("pool spreading") in order to provide input for calculation codes and build relevant standards. These experiments require the implementation of instrumentation adapted to the measurement of all gases present in free space (O2, N2, H2O, H2) in order to establish a measurement of partial pressures during each test, correlated with the other means of measurement in place (thermometry, catharometry, PIV, BOS, etc.).

Mission: In the context of an ANR-PEPR project (ESKHYMO) managed by CEA Liten, a Raman/FO Multitrack spectrometric measurement unit will be developed jointly by CEA List and CEA DES on the basis of an existing device. Raman measurement is multi-elemental, multi-track (a single measurement unit for several probes), non-explosive, and delivers a self-standardized measurement to a reference species (usually nitrogen at atmospheric pressure). The Raman/FO measurement unit comprises a laser, a spectrometer associated with a scientific CCD camera, and a fiber-optic circuit for remote measurement. The design of the Raman/FO probes will also be based on an existing CEA product, which will be miniaturized for deployment in field conditions. Four Raman/FO probes will be produced and then calibrated in air (climatic chamber) and hydrogen (shock tube or vacuum chamber) at CEA DES DM2S in Saclay. Finally, the final device will be deployed on the test site for multi-gas measurements during spraying experiments, in partnership with Air Liquide and accrediting bodies (INERIS).

Skills: Optics, laser, fiber optics, spectrometry

Design of a high-energy phase contrast radiography chain

As part of hydrodynamic experiments carried out at CEA-DAM, the laboratory is seeking, using pulsed X-ray imaging, to radiograph thick objects (several tens of mm), made of low-density materials (around 1 g/cm3), inside which shock waves propagate at very high speeds (several thousand m/s). For this type of application, it is necessary to use energetic X-ray sources (beyond 100 keV). Conventional X-ray imaging, which provides contrast due to variations in absorption cross sections, proves insufficient to capture the small density variations expected during the passage of the shock wave. A theoretical study recently carried out in the laboratory showed that the complementary exploitation of the information contained in the X-ray phase should enable better detectability. The aim of the post-doctorate is to provide experimental proof of concept for this theoretical study. For greater ease of implementation, the work will mainly focus on the dimensioning of a static X-ray chain, where the target is stationary and the source emits continuous X-ray radiation. Firstly, the candidate will have to characterize in detail the spectrum of the selected X-ray source as well as the response of the associated detector. In a second step, he (she) will design and have manufactured interference gratings adapted to high-energy phase measurements, as well as a representative model of the future moving objects to be characterized. Finally, the student will carry out radiographic measurements and compare them with predictive simulations. The student should have a good knowledge of radiation-matter interaction and/or physical and geometric optics. Proficiency in object-oriented programming and/or the Python and C++ languages would be a plus.

Simulation of the interaction of a high energy pulsed X-ray beam with a scintillator

In the context of hydrodynamic experiments, the CEA-DAM uses pulse radiography facilities which generate, in a few tens of nanoseconds, a very high dose of energetic X-ray photons, up to 20 MeV. After crossing the studied object, the X photons interact with a detector, composed of a scintillator crystal converting the X photons into visible photons, which are then detected by a CCD camera. The objective of this post-doctorate is to set up a complete simulation chain of the detector, including the emission of visible photons by the scintillator and their transport by the optical chain to the CCD camera. Initially, the candidate will have to model the different mechanisms involved in the detection chain and identify the most relevant simulation tools to reproduce them. In a second step, he (she) will be required to compare the simulation results with experimental characterization campaigns, carried out using a pulsed X source. Finally, the candidate will be able to propose, using the chosen simulation chain, possible developments for future detection chains. This work may lead to publications.

Design of a photonic Doppler velocimetry diagnostic in the mid-infrared for high velocities

This post-doctorate aims to design, using innovative technological éléments, a photonic Doppler velocimetry diagnostic operating in the mid-infrared (between 3 µm and 5 µm) to probe clouds of dense particles moving at high speeds (up to 5000 m/s), in shock physics. Schematically, two laser waves slightly offset in frequency are caused to interfere on a photodetector connected to a digitizer, one serves as a reference and the other carries the speed information of the targeted object, by Doppler effect. The development of new optical components and advanced technologies in this range of wavelengths is currently in full swing, for applications in Defense, gas detection, etc... In a first design phase, the candidate will therefore have to identify and choose the most relevant photonic components for our needs. To do this, he or she will have to optimize the overall performance of the measurement chain, using commercial simulation tools or tools developed at CEA-DAM. In a second step, he (she) will constitute the measurement chain with the selected optical elements. He (she) may also be required to participate in the design and manufacture of precision mechanical elements to ensure the interface between the elements. Depending on the state of progress, the system thus designed may be deployed on dedicated experiments. This work may lead to publications.

Study and modeling of fiber Bragg grating acoustic receivers

CEA List has been working for several years on the development of advanced monitoring solutions using fibre optic acoustic receivers called Fiber Bragg Gratings. These optical sensors have a great potential for structural health monitoring, both because of their ability to be integrated into materials (concrete, organic composites, metal) and because of their ability to be deployed in severe environments (embedded, radiative, high temperature).
A post-doctoral work is proposed to carry out modelling of these Fiber Bragg Grating transducers in order to refine the understanding of their sensitivity to ultrasonic guided elastic waves and to help in the design of an associated control system thanks to an intelligent placement of the sensors. Ultimately, the aim is to be able to simulate their response within the Civa non-destructive testing software developed by CEA List, and more particularly via its module dedicated to Structural Health Monitoring (SHM). Such work would strongly contribute to the adoption and exploitation of this technology for Structural Health Monitoring applications.

Highgly reflective materials laser microwelding

In the frame of the Simulation Program, CEA/DAM conducts experiments on high powerful lasers involving complex targets. Intensive research is therefore conducted to study and manufacture a large panel of targets - with ambitious scientific and technological challenges ahead. In particular, CEA wants to extend its laser microwelding capabilities–at a sub-mm scale. The challenge is to weld both high-reflective and thin materials (aluminum, copper, gold …) with an accurate mastering of heat deposition and penetration depth. The goal is to implement, optimize and qualify a process based on the latest source generation (UV or green laser source), and to get an innovative set of experimental data. A phenomenological model might also be proposed.

The latest generation of laser source emitting in visible wavelengths (green, blue) will be exploited. He/she will participate in the design and qualification testing of the laser station associated with this new source. Once validated, he/she will carry out the study of the operational and metallurgical weldability of the sub-elements. He/she will compare his/her results with the use of a pulse infrared laser. He/she will appraise the joints obtained using different approaches and optimize the design of the welded joints. Its experimental study will go as far as carrying out functional tests on prototypes. External collaborations will be set up to compare the results obtained with simulations in order to deduce a phenomenological model.

Etudes et développement d’un système laser dans l’UV pour la démonstration à l’échelle laboratoire de l’épuration isotopique du palladium (naturel).

Le palladium est un métal rare dont la demande mondiale est en forte augmentation. Or, il est présent en tant que produit de fission dans les combustibles nucléaires usés qui sont retraités en France. Il serait donc intéressant de recycler ce métal. Pour cela, il est nécessaire de procéder à une épuration isotopique, afin de supprimer un des isotopes du palladium, le 107, qui est un radionucléide artificiel à vie longue émetteur béta. Dans le cadre d'un nouveau projet sur 4 ans construit en réponse à l'appel d'offre du Plan d'Investissement et d'Avenir de l’État, le Service d’Etude des Procédés d’Enrichissement propose un contrat post-doctoral portant sur le développement d’un système laser dans l’UV pour le procédé de séparation isotopique du palladium par Lasers actuellement en cours de développement. L’objectif principal du projet est la démonstration finale de la faisabilité de séparation de palladium naturel (et non radioactif) pour la phase suivante de développement d’un premier pilote.
Le post-doctorant devra développer des lasers prototypes de procédé à haute cadence en partant du visible (système lasers colorant) jusqu’à l'UV. Le passage dans l’UV se fait par doublage de fréquence avec des objectifs élevés en terme de performance. Il s’agit d’utiliser un cristal doubleur de fréquence de type BBO, LBO, KDP ou autre. Pour ce faire, le post-doctorat participera à la définition de ce cristal, mais aussi au développement de l’environnement du cristal doubleur (comportement, performances attendues et la tenue au flux des différents matériels). Des échanges seront mis en place sur ce sujet spécifique avec des spécialistes reconnus au sein de la Direction de la Recherche Fondamentale du CEA. La programmation (en Python et/ou sous Labview) de ces outils ou asservissements est à développer également. Une attention particulière sera portée sur les publications à réaliser essentiellement dans le cadre du doublage de fréquence, sujet complexe très étudié mondialement.

Numerical studies of laser plasma interaction in intermediate field on Laser Megajoule

In the Inertial Confinement Fusion experiments (ICF), intense laser beams cross a gas filled hohlraum. The gas is fully ionized and laser beams then propagate into a sub-critical plasma where laser plasma instabilites can develop. Optical smoothing techniques enable to break both spatial and temporal coherences so that both spatial and temporal scales of the beam become smaller than those required for the development of the instabilites. The breaking of spatial coherence is done thanks to the use of a phase plate which spreads the laser energy in a multitude of light grains called speckles. The breaking of temporal coherence is done by using a phase modulator which widens the spectrum and by dispersing each frequency with a grating. It is essential to know the statistical properties of speckles (width, lenght, contrast, coherence time, velocities ...) to be able to predict the instabilities levels which can depend on time and on the distance of propagation of the beam. .
For the sake of simplicity, the laser plasma instabilities are very often studied at the best focus of the beam. However, in the FCI experiments, laser beams are focused near the laser entrance hole of the hohlraum whose length is about 1 cm. The development of instabilities can then occur before the best focus (outside the hohlraum) and mainly beyond the best focus (far inside the hohlraum). The goal of this post-doctoral contract is to study the development of instabilities when it occurs in the intermediate field (far from the best focus of the beam) and to assess the efficiency of different smoothing options on Lase MagaJoule (LMJ) to limit these instabilities. We will especially study propagation instabilities (self-focusing, forward stimulated Brillouin scattering) and stimulated Brillouin backscattering. This work will be done thanks to numerous existing numerical codes and diagnostic tolls.

Theoretical and experimental studies of the polarized light's propagation into OLED structure

In collaboration with chemists from CEA Saclay and the University of Rennes, Leti's LCEM laboratory is interested in new chiral molecules for OLED (Organic Light Emitting Device) sources able to emit circularly polarized light (CP). The interest of these CPOLED sources is multiple and encompasses both micro-screens and healthcare applications. While the state of the art is quite extensive on the chemical part, few studies have looked at the generation and transport of light in CPOLEDs components.Likewise, the conditions for measuring the polarity of the light emitted are not very detailed in the existing literature.
At the LCEM laboratory, where these chiral molecules are integrated into CPOLED devices, the goal is to design OLED architectures that can better preserve the polarization of light. To do this, it is essential to understand the propagation of light in OLED stacks from a theoretical and experimental point of view. This work is part of a larger collaboration set up in the ANR "i-chiralight" project.
In this context, we are proposing a study which will take place in two phases.
- Study of simple emitting materials: The materials to be studied will be thin layers deposited under vacuum using evaporation's system of thin layers available in the laboratory. The organic materials used will be supplied by our chemical partners in Saclay or Rennes. Optical characterizations such as ellipsometry,photoluminescence, etc. will be carried out in order to assess the performance of molecules in terms of emission efficiency but also in terms of the rotational power of light. For this last point, a model able to calculate all the terms of the Müller matrices is under development and the validation of this one will be a work to be carried out by the post-doctoral fellow.
- Study of complete OLED components: In the second phase of this work, we will focus on the complete OLED system by studying the propagation of optical modes in the stack of the different layers const

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