Data assimilation for hypersonic laminar turbulent transition reconstruction
To design a hypersonic vehicle, it is necessary to accurately predict the heat flows at the wall. These flows are strongly constrained by the nature of the boundary layer (laminar/transitional/turbulent). The mechanisms behind the laminar-turbulent transition are complex and still poorly understood. What's more, transitional phenomena are highly dependent on fluctuations in the free flow around the model in the case of wind tunnel testing, or around the craft in the case of flight. These fluctuations are very difficult to measure precisely, which makes the comparison between calculation and testing very complex. To carry out a detailed analysis of flow physics during testing, we need to turn to the results of high-fidelity calculations. It is therefore crucial to be able to reproduce numerically the upstream disturbances encountered. During the course of the thesis, we will be looking to develop data assimilation methods, based on high-fidelity simulation, to invert, i.e. determine fluctuations in the light of observations. The focus will be on assembly techniques based on Bayesian inference. Emphasis will be placed on integrating a priori knowledge of fluctuations. In addition, we will try to reduce the computational cost and quantify the uncertainties on the solution obtained. In particular, the approach will be applied to a flow around the CCF12 (cone-cylinder-flare) geometry realised in the R2Ch wind tunnel at ONERA.
Implementation of covariant QRPA to describe deformed atomic nuclei
All other things being equal, what differences can be expected from the choice of a relativistic or non-relativistic interaction in the QRPA description of the excited states of the atomic nucleus? In order to answer it, the student will on one hand use numerical tools to solve non- relativistic interaction QRPA matrix equations and on the other hand use a solver of the finite amplitude method to produce QRPA response functions with relativistic interactions. These numerical tools leverage supercomputers and are widely used for nuclear data and astrophysics issues as well as to conduct academic nuclear structure studies. The relativistic extension of the matrix QRPA solver will make it possible to transfer all the expertise of nuclear data production to the case of interactions from relativistic lagrangians. Thus, an analysis of the respective merits of the two functionals will be conducted and exploited with a view to the development of new generation effective interactions.
Study of low-frequency radiation produced by particle acceleration at ultra-high laser intensity in relativistic plasmas
Today, petawatt laser sources deliver optical pulses lasting a few tens of femtoseconds with an intensity larger than 1020 W/cm2. When such a light beam interacts with a gas or a solid target, the electrons accelerated by the laser ponderomotive force become relativistic and acquire high energies, in excess of the GeV. These laser systems also produce various radiations such as hard X photons or electron-positron pairs by quantum conversion of gamma photons. As laser technology is advancing rapidly, these light sources have increasingly compact dimensions and they nowadays complement many international laboratories hosting synchrotrons or conventional particle accelerators.
If this extreme light makes it possible to generate radiation in the highest frequencies regions of the electromagnetic spectrum, it also fosters, through the production mechanisms of plasma waves and particle acceleration, conversion processes towards much lower frequencies belonging to the gigahertz and terahertz (THz) ranges.
Having high-power transmitters operating in this frequency band is attracting more and more interest in Europe, overseas and in Asia. On the one hand, the generation of intense electromagnetic pulses with GHz-THz frequencies is harmful for any electronic device close to the laser-plasma interaction zone and the diagnostics used on large-scale laser facilities like, e.g., the PETAL/LMJ laser in the Aquitaine region. It is therefore necessary to understand their nature to better circumvent them. On the other hand, the waves operating in this field not only make it possible to probe the molecular motions of complex chemical species, but they also offer new perspectives in medical imaging for cancer detection, in astrophysics for the evaluation of ages of the universe, in security as well as environmental monitoring. The processes responsible for this violent electromagnetic field emission, if properly controlled, can lead to the production of enormous magnetic fields in excess of 1000 Tesla, which presents exciting new opportunities for many applications such as particle guiding, atomic physics, magnetohydrodynamics, or modifying certain properties of condensed matter in strong field.
The objective of this thesis is to study the physics of the generation of such giant electromagnetic pulses by ultrashort laser pulses interacting with dense media, to build a model based on the different THz/GHz laser-pulse conversion mechanisms, and validate this model by using dedicated experimental data. The proposed work is mainly oriented towards an activity of analytical modeling and numerical simulation.
The doctoral student will be invited to deal with this problem theoretically and numerically using a particle code, whose Maxwell solver will be adapted to describe radiation coming from different energy groups of electron/ion populations. A module calculating online the field radiated by each particle population in the far field will be implemented. Particular attention will be given to the radiation associated with the acceleration of electrons and ions on femto- and picosecond time scales by dense relativistic plasmas and their respective roles in target charging models available in the literature. This field of physics requires a new theoretical and numerical modeling work, at the crossroads of extreme nonlinear optics and the physics of relativistic plasmas. Theory-experiment confrontations are planned within the framework of experiments carried out on site at CELIA facilities and experiments carried out in collaboration with US laboratories (LLE/Rochester). The thesis will be prepared at CELIA laboratory on the campus of Bordeaux university.
Characterizations and modeling of metal tritide aging: application to palladium tritide
Using alternative energy sources such as fusion requires the storage and use of great amount of hydrogen.
This thesis work is about the storage of hydrogen isotopes by palladium hydrides at low equilibrium pressure.
This solid state storage, which ensures both safety and compactness, is particularly interesting for tritium, the radioactive isotope of hydrogen which decay produces helium-3. Helium-3 tends to form nanobubbles which modify the physico-chemical properties of palladium tritide. This phenomenon is called aging. When helium-3 concentration reaches a critical value it is released in the gas phase which can lead to an increase in the storage facility.
In order to better understand and predict the aging phenomenon, material which are aged under tritium during several years are characterized. Studying microstructure, nanobubbles architecture, chemical composition and mechanical behavior evolutions. The acquired data are then used as inputs and outputs of the aging mechanisms modeling.
Numerical study of core collapse supernovae
Context : Numerical study of core collapse supernovae. A core collapse supernova begins with the collapse of the core. Once the density goes beyond nuclear matter density, it becomes extremely hard. The collapsing matter bounces on it and creates a shock. The shock propagates and then stalls. The situation is the following : the shock is stationary. Neutrino coming from deep in the core heat the matter close to the shock and tend to push the shock forward, onto stellar explosion. On the other hand, the rest of the star is still collapsing, and this pushes the shock inwards, which in turn tends to black hole formation. The knowledge of which progenitor (which massive star) explodes and which creates a black hole is an active research topic : there is no clear and reliable way, without doing a detailed numerical simulation, to know whether a given progenitor explodes or whether it forms a black hole.
Objectives : Acquire a good knowledge of the supernovae physics, stellar physics, and also the neutron star physics and the black hole physics. Acquire a good knowledge on the development of a numerical physics code. Acquire a good knowledge on the link between numerical physics and laser physics.
Process : The student will acquire knowledge on radiative hydrodynamics with neutrinos, in a relativistic context. The student will also acquire knowledge on general relativity. The possibility of reproducing some aspects of the supernova explosion in laboratory with laser experiments will be studied. The possible link between the progenitor (the massive star about to collapse) and the explosion (whether it explodes or it forms a black hole) will be studied in details numerically. The student will produce simplified progenitors. In these, the student will be able to vary some well chosen parameters. Finally, many possibilities exist to improve this study : implementation of other numerical methods, 3d, implementation of nucleosynthesis, etc. The student can also suggest its own way.
Modeling of radiation effects in GaN components
Radiations from space or from environments related produce failures and accelerates the lifetime of electronic components. Ionization and charges produced during irradiations disrupt the operation of system electronics in a transient or cumulative way. This produces transients leading to drifts of the characteristics of the electronic components. It is essential to evaluate the densities of charge carriers generated by radiation in the sensitive parts of the components which transform the induced charges into a transient signal ("Single Event") and the insulators in which increasing quantities of charges can be trapped, which will lead to failures over time (cumulative dose). The accurate modeling of particle transport (electrons, protons and ions) in microelectronic materials is essential to better estimate the deposition in the sensitive volumes of elementary structures of electronics. In this context, the CEA in partnership with ONERA has developed, during several PhD, the MicroElec module implemented in the Geant4 framework (international collaboration see https://geant4.web.cern.ch/) dedicated to the transport of particles in matter. This module allows to estimate the spatial distribution of the charges induced by the particle range in the active material of microelectronic transistors. Currently, the MicroElec module deals with 11 materials suitable for modern Silicon microelectronics.
Over the past few years, R&D in GaN components has made significant progress in terms of performance, reliability and cost. GaN technology is now of industrial interest even for applications to be used with radiations and a high level of reliability. However, today, some materials used in GaN technology electronics are not taken into account in MicroElec. Thus, the PhD student will contribute to the extension of the list of MicroElec materials, which will be proposed to the scientific research community of the Geant4 international collaboration where the supervisors of this thesis participate. The models developed will be compared with the results of tests carried out in the laboratory, which the candidate may attend or participate in. The TCAD (Technology Computer-Aided Design) tools of the laboratory will be used to reproduce the electronic effects of the perturbations evaluated by MicroElec in the electronic components.