# Kinetic description of laser-plasma interaction relevant to inertial confinement fusion

Many applications, such as inertial confinement fusion, require an understanding of the physical mechanisms involved when high-energy laser beams propagate in a plasma. In particular, in the case of fusion, the aim is to quantify the deposition of laser energy on a cryogenic deuterium-tritium target, and the efficiency with which this target can be compressed to trigger fusion reactions. However, during their propagation, laser beams create a plasma wave that grows at the expense of the incident laser energy. However, the growth of this wave is not infinite and stops when the wave breaks up. This is accompanied by the production of hot electrons, which can preheat the target and hinder its compression. The breaking of a plasma wave is a physical phenomenon of the kinetic type, which can only be correctly described by calculating the velocity distribution of the electrons in the plasma. The aim of this thesis is to study wave breaking both theoretically and numerically, using Vlasov-type kinetic codes. One of the main difficulties lies in the discontinuity of the distribution functions to be described. In addition, it is necessary to describe the surge from its linear phase to the non-linear regime, enabling the creation of hot electrons to be quantified. The ultimate goal of the thesis is to produce models that are simple enough to run on the CEA's dimensioning codes.

# Development of a neutron/gamma coincidence measurement system for the characterization of radionuclide neutron sources

This PhD work is part of sources calibration activities at the LNHB-MA and R&D activities within the SIMRI aimed at developing neutron measurement systems for the CEA and the nuclear industry. The objective of the PhD work is to develop a new measurement system using neutron/gamma coincidences to enable the characterization of the (alpha,n)-type neutron sources. These sources consists of a homogeneous mixture of an alpha particle emitter and the target substance, the nuclei of which emit neutrons via a nuclear reaction. As for example, we can cite for example: AmBe, PuBe, CmBe, or even exotic source of high emissivity and mixing several alpha radionuclides (ex. AmPuBe). For this familly of sources, the emission of neutron by reaction (alpha,n) is in simultaneous cascade with a characteristic gamma at 4.4 MeV. The detection of the neutron and the gamma in coincidence is likely to provide information of interest in the source characterization in terms of emission rate and spectral fluence. The objective is to measure precisely gamma and neutron signatures as well as gamma/neutron intensity ratios resulting from the nuclear reaction. The new measurement device must also be able to measure neutrons emitted by the spontaneous fission reaction or by (n,2n) reaction in beryllium. Others photon emission can be also provide information of interest, ex. the emission of a gamma at 2.2 MeV resulting from the capture on hydrogen. The neutron/gamma coincidence measurements can be also used to improve the evaluation of nuclear data such as cross sections of certain elements, ex. (n,gamma) reaction on oxygen or hydrogen.

# Innovative concepts for particles plasma acceleration and radiation emission in laser – overdense plasma interaction at ultra-high intensity

The present PHD work aims at exploring theoretically and numerically the generation of fast particle beams in ultra-relativistic (above 10^21 W/cm2) laser-overdense solid interaction by using properly-structured or shaped targets. Surface characteristics inducing local electromagnetic modes more intense than the laser field and where nonlinear and relativistic effects play a major role will be investigated.

On the basis of the work already carried out, the new scheme for particle acceleration will be extended in the ultra-relativistic regime of laser plasma interaction. It may lead to groundbreaking ultra-short synchronized light and electron sources with applications in probing ultrafast electronic processes. In this context, this theoretical and numerical study will allow to suggest new experimental schemes feasible on the Apollon facility and multi-PW lasers.

# 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.

# 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.