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.
Development of an uncertainty propagation method of function-typed input data applied to the decay heat calculation
Characterising the energy released by the disintegration of the radionuclides present in spent nuclear fuel is essential for the design, safety and analysis of storage, transport and disposal systems. Few measurements of this decay heat are available today. In addition, the available experimental values do not cover the wide spectrum of possible combinations between parameters such as discharge burn-up rate, 235U enrichment, cooling time, fuel design parameters, or operating conditions. The estimation of decay heat is therefore mainly based on calculation codes.
The evaluation of the uncertainty associated with the estimation of decay heat is important to achieve reliable predictions. Many efforts have been made to properly evaluate biases and uncertainties coming from nuclear data such as cross sections. The number of studies concerning uncertainties of an epistemic nature (uncertainty in the manufacture of some components, error in reading or adjusting mobile structures, etc…) is comparatively small. Among the latter, while the treatment of complex dependencies of scalar input parameters is well taken into account today, functional-type dependencies, i.e., expressed in the form of a function, are very little explored.
While uncertainties arising from the processing of fixed input parameters, such as fuel manufacturing parameters, independent of time, are quite well covered, the uncertainties coming from the processing of variable (or functional) parameters, such as operating history, evolving during reactor operations, are not. Irradiation history actually brings together several inter-correlated quantities (operating power, absorber movements, core evolution …), subject to modifications over time and influencing the value of numerous observables of interest, including decay heat. The models used today in industrial simulation tools do not make it possible to estimate this impact and to infer a validated uncertainty.
This research work will investigate the impact on decay heat of the uncertainties associated with input parameters having functional dependencies. We will particularly focus on the irradiation history of the reactors (PWR type). A first part of the work will be dedicated to the development of a substitution model for decay heat estimation and quantification of uncertainties of a functional nature. The second part will be devoted to the development of a sensitivity analysis method. Finally, a third part will concern the development of an inverse method for quantifying the uncertainties coming from irradiation modelling.
The doctoral student will be hosted in a reactor physics research unit of the CEA IRESNE located in Cadarache where he will collaborate with other doctoral students and specialists in the field.
Spin-photon coupling and quantum electrodynamics in hybrid semiconducting architectures
Recent years have witnessed a tremendous progress in the development of quantum technologies able to probe and harness quantum degrees of freedom in solid state systems. In this context, the CEA of Grenoble has recently pioneered the demonstration of a hybrid CMOS architecture where a single photon trapped in a superconducting resonator is strongly coupled to the spin of a single hole confined in a double quantum dot [1,2]. This experiment opens important perspectives for the development of novel hybrid circuit Quantum Electrodynamics architectures where the photons can probe, entangle and control the quantum state of distant spins.
The actual potential of such platforms for quantum technologies remains to be assessed from the theoretical side, in particular for applications to quantum computation and simulation. Differently from purely superconducting transmon or flux qubits, the mechanism underpinning strong spin-photon coupling relies on the presence of a significant spin orbit interaction in the valence bands of silicon.
This PhD thesis will reinforce the theoretical activity of the CEA on this topic and will investigate how to optimize readout and manipulation protocols for architectures based on silicon and germanium. Particular effort will be devoted to the quantitative modeling of spin-photon coupling and of the mechanisms limiting the performances of such devices (noise effects). We will also explore the many-body effects emerging when coupling several spins through single or multiple resonators.
[1] Strong coupling between a photon and a hole spin in silicon, Cécile X. Yu, Simon Zihlmann, José C. Abadillo-Uriel, Vincent P. Michal, Nils Rambal, Heimanu Niebojewski, Thomas Bedecarrats, Maud Vinet, Étienne Dumur, Michele Filippone, Benoit Bertrand, Silvano De Franceschi, Yann-Michel Niquet and Romain Maurand, Nature Nanotechnology 18, 741 (2023)
[2] Tunable hole spin-photon interaction based on g-matrix modulation, V. P. Michal, J. C. Abadillo-Uriel, S. Zihlmann, R. Maurand, Y.-M. Niquet, and M. Filippone, Phys. Rev. B 107, L041303 (2023)
Quantum fragmented states in frustrated magnets
The last few decades of condensed matter research have seen the emergence of a rich new physics, based on the notion of "spin liquids". Interest in these new states of matter stems from the fact that they exhibit large-scale quantum entanglement, a property that is fundamental to quantum computation. By directly exploiting this notion of entanglement, a quantum computer would enable revolutionary approaches to certain classes of problems, compared with conventional computers.
The study of spin liquids is therefore a key technological issue, and the aim of this thesis project is to contribute to this fundamental research effort.
Theoretical description of odd and odd-odd nuclei fission within the TDGCM method
The fission process, for which a heavy nucleus splits into two – or three – fragments, is the nuclear phenomenon used in nuclear reactors. Nuclear fission data are therefore of great importance for the study and development of reactors. Whereas nuclei built with an odd number of protons and/or neutrons represent three-quarters of the nuclides chart, there is no microscopic, consistent, fully quantum mechanical model to describe their fission. We propose to develop such a model on the basis of the Time Dependent Generator Coordinate Method [1,2,3] for odd and odd-odd nuclei. The goal is the building of a microscopic and quantum mechanical framework to calculate the primary fission yields and the sharing of the energy available at scission for all type of fissioning nuclei, including those that are experimentally out of reach. The PhD student’s work will consist of developing formal and numerical tools aimed at generating fission potential energy surfaces and studying the nucleus dynamics on such surfaces. The PhD student will develop skills in theoretical nuclear physics, analytical derivations, numerical implementation, high performance computing and data analysis. There is the possibility of a 6-month pre-thesis internship in the host lab.
[1] D. Régnier et al, Phys. Rev. C 93, 054611 (2016)
[2] D. Régnier et al, Computer Physics Communications 225 (2018) 180–191
[3] L. M. Robledo et al 2019 J. Phys. G: Nucl. Part. Phys. 46 013001
MACHINE LEARNING FOR INVERSE PROBLEMS IN HADRON STRUCTURE
Characterizing the multidimensional structure of hadrons in terms of quarks and gluons is one of the major objectives of hadronic physics today. This is not only the central theme of many experimental facilities worldwide, but also one of the main reasons for the construction of future colliders in the USA and China. It is also one of the key areas of research for intensive numerical simulations of the strong interaction. However, in both cases, the connection between measured and simulated data on the one hand, and the multidimensional structure of hadrons on the other, is not direct. The data are linked to the hadron structure via mathematically ill-posed multidimensional inverse problems. It has been shown that these inverse problems lead to a significant increase in uncertainties, to the point of becoming the dominant source of uncertainty in some cases. The aim of this thesis is to use machine learning tools to assess, reduce and correctly propagate uncertainties from experimental or simulation data to the multidimensional structure of hadrons. The strategy for achieving this is to develop an original neural network architecture capable of taking into account the full range of theoretical properties arising from quantum chromodynamics, and then to adapt it to inverse problems linking experimental and simulation data to the 3D structure of hadrons.
Spin, Symmetries, Topology and Altermagnetism
The central topic of the thesis is a recently proposed form of matter called altermagnetism. In common with simple antiferromagnets these are magnetic materials supporting long-range magnetic order with no net moment. In simple antiferromagnets the up and down spin electronic bands are degenerate. But in altermagnets they are not. One way of thinking about these materials is that they are nonmagnetic in real space but magnetic in momentum space thus combining features of ferromagnets and antiferromagnets. These materials have generated a great deal of interest in the spintronics community. Roughly speaking this community has, for a long time, been interested in antiferromagnets that support spin currents because antiferromagnets are insensitive to stray fields and can support faster device switching than in typical ferromagnets. Altermagnets have the potential to realize the dreams of antiferromagnetic spintronics. At the same time, altermagnets are of fundamental interest in condensed matter physics. It turns out that altermagnetism is grounded in a peculiar type of symmetry breaking described by the theory of spin groups.
The goal of this thesis project is to extend our understanding of spin groups in condensed matter especially in the direction of altermagnetism and topological materials.
Ab-initio nuclear cross section modeling combining R-matrix theory and self-consistent Green functions
Microscopic nuclear physics aims at describing structure and reaction properties of atomic nuclei starting from nucleonic scales and using the elementary interaction between nucleons as the basic input. Ab-initio methods use systematic interactions derived via effective theories of underlying Quantum Chromo-Dynamics and adjusted on properties of light systems. Most methods describing N interacting quantum bodies rely on the estimation of a wave function that is the solution of an N-body Schrödinger-type equation. Self-consistent Green’s function theory works differently, as it recasts the N-body problem by substituting suitably chosen Green’s functions for the wave function. An interesting aspect of Green’s function theory is that it involves a systematically improvable field – the self-energy – describing the interaction “felt” by the nucleon. This field, used in structure calculations, can thus also be used for describing nuclear reactions.
The PhD student will first study the formalism and learn how to use related tools, namely the spherical HFB code sPAN, which provides first-order contributions to the Green’s functions. The student will then implement second-order descriptions. Finally, the obtained self-energy will be used for nuclear reaction calculations, namely in the context of R-matrix theory. The latter is a convenient tool to describe (unbound) nucleons in the continuum, treating both the direct and exchange parts of the nucleon-nuclei interaction.