Preparation and characterization of an oxide/oxide composite
Fiber-reinforced ceramic matrix composites (CMCs) are a class of materials that combine good specific mechanical properties (properties relative to their density) with resistance to high temperatures (> 1000 °C), even in oxidizing atmospheres. They are typically composed of a carbon or ceramic fiber reinforcement and a ceramic matrix (carbide or oxide.
The proposed study focuses on the development of a low-matrix oxide/oxide CMC with suitable dielectric, thermal, and mechanical properties.
This study will be conducted in collaboration with several laboratories at CEA Le Ripault.
Influence of laser bandwidth and wavelength on laser plasma instabilities
As part of the Taranis project initiated by Thales and supported by BPI France and in collaboration with numerous scientific partners such as CEA/DAM, CELIA and LULI, work on target design and definition of the laser intended to energy production in direct drive will take place. A prerequisite for this work is to understand the laser-plasma interaction mechanisms that will occur when the laser is coupled with the target. These deleterious mechanisms for the success of fusion experiments can be regulated by the use of so-called “broadband” lasers. In addition, the choice of the laser wavelength used for the target design and the laser architecture must be defined. The objective of the postdoctoral position is to study the growth and evolution of these instabilities (Brillouin, Raman) in the presence of “broadband” lasers both from an experimental and simulation point of view, and thus to be able to define the laser conditions making it possible to reduce these parametric instabilities.
Evolution of ISAAC and Xpn codes for an extension of the QRPA method to the complete processing of odd nuclei; towards a database without interpolation for odd nuclei
The treatment of odd-isospin nuclei in microscopic approaches is currently limited to the so-called «blocking» approximation. In the Hartree-Fock Bogolyubov (HFB) approach, the ground state of an odd-mass nucleus is described as a one-particle excitation (qp) on its reference vacuum. Thus, in the QRPA approach, where the basic excitations are states «with 2 quasi-particles», the blocked qp is excluded from the valence space under the Pauli exclusion principle. As a result, the chosen qp is a spectator and is not involved in the QRPA collective states. If the single nucleon should have a significant contribution some levels will not be reproduced. The development in the QRPA codes (ISAAC and Xpn) of a procedure that allows all nucleons to participate in collective states is mandatory for a microscopic description of odd nuclei. Moreover, recent Xpn developments have allowed the description of forbidden ß- first decays improving the estimation of half-life time of fission fragments. This could be extended to address ß+ and electronic captures and could be adapted to large-scale calculations useful for nuclear astrophysics.
detection of multiplets and application to turkey-Syria seismic crisis of february 2023
The correlation technique, or template matching, applied to the detection and analysis of seismic events has demonstrated its performance and usefulness in the processing chain of the CEA/DAM National Data Center. Unfortunately, this method suffers from limitations which limit its effectiveness and its use in the operational environment, linked on the one hand to the computational cost of massive data processing, and on the other hand to the rate of false detections that could generate low-level processing. The use of denoising methods upstream of processing (example: deepDenoiser, by Zhu et al., 2020), could also increase the number of erroneous detections. The first part of the research project consists of providing a methodology aimed at improving the processing time performance of the multiplets detector, in particular by using information indexing techniques developed in collaboration with LIPADE (L-MESSI method , Botao Peng, Panagiota Fatourou, Themis Palpanas. Fast Data Series Indexing for In-Memory Data. International Journal on Very Large Data Bases (VLDBJ) 2021). The second part of the project concerns the development of an auto-encoder type “filtering” tool for false detections built using machine learning. The Syria-Turkey seismic crisis of February 2023, dominated by two earthquakes of magnitude greater than 7.0, will serve as a learning database for this study.
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.
Cascade of circulicity in compressible turbulence
In this post-doctorate, we propose to study the properties of the small scales of forced compressible homogeneous turbulence. More precisely, exact statistical relations similar to the Monin-Yaglom relation will be investigated. The idea, detailed in reference [1], is to understand how the transfer of circulicity is organized in the inertial range. Circulicity is a quantity associated with angular momentum and, by extension, with vortex motions. The analysis of its inertial properties allows to complete the description of the energy cascade already highlighted in previous works [2,3].
The objective of the post-doctorate is to carry out and exploit direct simulations of compressible homogeneous turbulence with forcing, in order to highlight the inertial properties of circulicity .
To this end, the post-doctoral student will be given access to the very large computing center (TGCC) as well as a code, Triclade, solving the compressible Navier-Stokes equations [4]. This code does not have a forcing mechanism and the first task will therefore be to add this functionality. Once this task has been accomplished, simulations will be carried out by varying the nature of the forcing and in particular the ratio between its solenoidal and dilatational components. These simulations will then be exploited by analyzing the transfer terms of circulicity.
[1] Soulard and Briard. Submitted to Phys. Rev. Fluids. Preprint at arXviv:2207.03761v1
[2] Aluie. Phys. Rev. Lett. 106(17):174502, 2011.
[3] Eyink and Drivas.Phys. Rev. X 8(1):011022, 2018.
[4] Thornber et al. Phys. Fluids 29:105107, 2017.