# Laser-driven ion acceleration using quasi-critical-density gas jets

The proposed PhD thesis aims to study ion acceleration in gases driven by ultraintense and ultrashort laser pulses. The objective is to couple these lasers with high-density gas jets, approaching the critical density associated with the laser wavelength. These jets, produced by specially designed nozzles, may be shaped by hydrodynamic shock waves induced by low-energy auxiliary laser pulses. Compared to standard solid targets, gas jets offer several advantages: production of ion beams from any chemical element; automatic target renewal at the interaction point; low debris generation suitable for high-repetition lasers; specific acceleration processes that can give rise to relatively narrow energy distributions. Once its feasibility is demonstrated, this setup could be leveraged for studies on ion stopping power in various media and the production of medical radioisotopes.

The student will work on the preparation, realization and interpretation of experiments conducted at various laser facilities. In parallel, he/she will perform numerical (hydrodynamic and kinetic) simulations of the shaping of the gas jets and their interaction with ultraintense laser pulses.

# 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.

# Multiscale analysis of plastic strain localization under laser driven shock loading

The localization of plastic deformation in expanding metal shells has been studied for several years in CEA/DAM. In addition to explosively driven shells, the laser driven expansion of thin metal sheets yields biaxial tension conditions representative of shell pieces. This kind of set-up is being developed in CEA/DAM and generates strain rates around 10000/s on sheet parts of centimetric width. The evolution of the experimental set-up to millimetric geometries will allow to reach higher stretching rates, unexplored up to now. For all these geometries, for which the sheet thickness is low with respects to the grain size, the influence of the material microstructure is probably significant and the deformation process shall be analyzed at this scale.

The aim of this PhD work is to study plastic strain localization in a sheet of a body centered cubic (BCC) metal under laser shock loading. The phenomenon will be investigated with finite element simulations incorporating the physics at the mesoscale: plastic slip and twinning. An homogenized polycrystalline approach, using an isotropic constitutive model with mean dislocation density as an internal state variable, and a full field approach including grains, their crystal orientations and slip systems, will be compared.

# Full isotopic fission fragment distribution measurement of 241Pu using inverse kinematics at GANIL with VAMOS and PISTA

The inverse kinematics technique is used at GANIL to produce the so-called in-flight fission. The accelerated fissioning system is excited by a nuclear reaction, and in particular by a nucleon transfer reaction between the beam and the target. Fission fragments are therefore emitted at forward angles in the laboratory frame due to the kinematic boost of the reaction. The VAMOS wide-acceptance magnetic spectrometer is used to identify the mass and nuclear charge of the various fragments, while silicon telescopes are used to characterize the fissioning system by detecting the ejecta emitted by the transfer reaction.

The fission@VAMOS project involves upgrading the silicon detection system used to identify the fissioning system produced by the transfer reaction. The current device is a highly segmented silicon telescope assembly called PISTA. This improves the sensitivity and precision of the fissioning system formation conditions (mass, atomic number, excitation energy).

The subject of this thesis is therefore a detailed multi-parametric study of fission, with a focus on measuring the fission yields of the fissioning system 242Pu (n+241Pu). Finally, a large part of the work will consist of data analysis and interpretation, followed by publication.

# 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.

# Relativistic laboratory astrophysics

This PhD project is concerned with the numerical and theoretical modeling of the ultra-relativistic plasmas encountered in a variety of astrophysical environments such as gamma-ray bursts or pulsar wind nebulae, as well as in future laboratory experiments on extreme laser-plasma, beam-plasma or gamma-plasma interactions. The latter experiments are envisioned at the multi-petawatt laser facilities currently under development worldwide (e.g. the European ELI project), or at next-generation high-energy particle accelerators (e.g. the SLAC/FACET-II facility).

The plasma systems under scrutiny have in common a strong coupling between energetic particles, photons and quantum electrodynamic effects. They will be simulated numerically using a particle-in-cell (PIC) code developed at CEA/DAM over the past years. Besides the collective effects characteristic of plasmas, this code describes a number of gamma-ray photon emission and electron-positron pair creation processes. The purpose of this PhD project is to treat additional photon-particle and photon-photon interaction processes, and then to examine thoroughly their impact and interplay in various experimental and astrophysical configurations.