The nonresonant streaming instability in turbulent plasmas
The magnetic turbulence prevalent in many astrophysical systems, such as the solar wind and supernova remnants, plays a crucial role in accelerating high-energy particles, particularly within collisionless shock waves. By trapping particles near the shock front, this turbulence facilitates their energy gain through repeated crossings between the upstream and downstream regions – a process known as Fermi acceleration, believed to be the origin of cosmic rays.
It happens that the turbulence surrounding supernova remnants is likely generated by the cosmic rays themselves via plasma instabilities as they stream ahead of the shock. In the specific case of a shock wave propagating parallel to the ambient magnetic field, the dominant instability is thought to be the non-resonant streaming instability, or Bell's instability, which acts to amplify the preexisting turbulence.
The objective of this PhD is to build a comprehensive analytical model of this instability within a turbulent plasma, and to validate its predictions against advanced numerical simulations.
Measurement of the speed of sound in H2 and He, key components of gas giant interiors.
The goal of this thesis is to study hydrogen-helium mixtures in the fluid phase under high pressure and high temperature using Raman and Brillouin spectroscopy. The experiments will be conducted in a diamond anvil cell with laser heating, allowing exploration of a wide range of pressure and temperature conditions representative of the interiors of gas giant planets (1-300 GPa, 300-4000 K). Raman spectroscopy will be used to probe possible chemical changes occurring under extreme conditions, while Brillouin spectroscopy will provide access to the adiabatic sound velocity and the equations of state of these fluid mixtures. These data will be particularly useful for improving the modeling of Jupiter and Saturn’s interiors.
Description of collective phenomena in atomic nuclei beyond Time-Dependent Density Functional
Context :
Predicting the organization and dynamics of neutrons and protons within atomic nuclei is a significant
scientific challenge, crucial for designing future nuclear technologies and addressing fundamental questions
such as the origin of heavy atoms in our universe. In this context, CEA, DAM, DIF develops theoretical
approaches to simulate the dynamics of the elementary constituents of atomic nuclei. The equations of
motion, derived within the framework of quantum mechanics, are solved on our supercomputers. The 2010s
saw the rise of the time-dependent density functional theory (TDDFT) approach for tackling this problem.
While TDDFT has provided groundbreaking insights into phenomena such as giant resonances observed in
atomic nuclei and nuclear fission, this approximation has intrinsic limitations.
Objectives :
This PhD project aims to develop and explore a novel theoretical approach to describe the collective motion
of protons and neutrons within the atomic nucleus. The goal is to generalize the TDDFT framework to
improve the prediction of certain nuclear reaction properties, such as the energy distribution among the
fragments resulting from nuclear fission. Building on initial work in this direction, the PhD candidate will
derive the equations of motion for this new approach and implement them as an optimized C++ library
designed to leverage the computational power of CEA's supercomputers. The final objective will be to assess
how this new framework enhances predictions of phenomena such as the damping of giant resonances in
atomic nuclei and the formation of fragments during nuclear fission.
Reactive neural network potentials: optimization of dataset construction and application to mechanochemical reactions
The spontaneous decomposition of organic molecules during synthesis, handling, or storage causes significant safety issues in the field of energetic materials. Besides thermal activation, recent studies suggest that intramolecular deformations, such as those induced by shock waves, significantly influence chemical reactivity and may alter decomposition mechanisms.
Molecular-level studies of these phenomena present significant challenges because they require both quantum-level accuracy for bond breaking and formation and the inclusion of condensed phase effect.
To bridge this gap, we propose the development and application of machine learning-based interatomic potentials (MLIPs),
In particular, we aim to significantly advance methodologies for building reactive structural datasets, specifically tailored to complex thermal and mechanochemical reactions with multiple decomposition pathways. Leveraging these improved datasets, we will develop MLIPs to study molecular decomposition under varying temperature and pressure conditions. Besides the safety concerns inherent to energetic molecules, the tools and knowledge developed during the project are expected to be of great value to the mechanochemistry community who currently lacks a molecular-level understanding of transformations in mechanochemical systems.
Microscopic description of fission fragment properties at scission
Fission is one of the most difficult nuclear reactions to describe, reflecting the diversity of dynamic aspects of the N-body problem. During this process, the nucleus explores extreme deformation states leading to the formation of two fragments. While the number of degrees of freedom (DOF) involved is extremely large, the mean-field approximation is a good starting point that drastically reduces the DOF, with elongation and asymmetry being unavoidable. This reduction introduces discontinuities in the successive generation of states through which the nucleus transits, since continuity in energy does not ensure the continuity of states resulting from a variational principle. Recently, a new method based on constraints associated with wave function overlaps has been implemented to ensure this continuity up to and beyond the scission (Coulomb valley). This continuity is crucial for describing the dynamics of the process.
The objective of the proposed thesis is to carry out for the first time a two-dimensional implementation of this new approach in order to take into account the whole collectivity generated by elongation and asymmetry DOF. The theoretical and numerical developments will be done within the framework of the time-dependent generator coordinate method. This type of approach contains a first static step, which consists of generating potential energy surfaces (PES) obtained by constrained Hartree-Fock-Bogoliubov calculations, and a second dynamic step, which describes the dynamic propagation of a wave packet on these surfaces by solving the time-dependent Schrödinger equation. It is from this second step that the observables are generally extracted.
As part of this thesis, the PhD student will:
- as a first step, construct continuous two-dimensional PESs for the adiabatic and excited states. This will involve the three algorithms Link, drop and Deflation
- secondly, extract observables that are accessible using this type of approach: yields, the energy balance at scission, fragment deformation and the average number of emitted neutrons. In particular, we want to study the impact of intrinsic excitations on the fission observables, which are essentially manifested in the descent from the saddle point to the scission.
Finally, these results will be compared with experimental data, in actinides and pre-actinides of interest. In particular, the recent very precise measurements obtained by the SOFIA experiments for moderate to very exotic nuclei should help to test the precision and predictivity of our approaches, and guide future developments of N-body approaches and nuclear interaction in fission.
Study of radiative decay of the nucleus using a technic like Oslo-method
Radiative neutron capture is a nuclear reaction forming a compound nucleus which decays by emitting gamma-rays at excitation energy around the neutron binding energy. This well-known reaction which we known how to accurately measure its cross section at low incident neutron energies for most stable and few unstable nuclei close the stability valley, remains difficult to measure for exotic nuclei like fission fragments. Nuclear reaction models based essentially on stable nuclei, also struggle to provide reliable predictions of cross sections for these exotic nuclei. However, in the recent years, progress made related to the models and the measurements for the radiative capture show that significant improvements in including microscopic ingredients studies. These micoscopic ingredients: gamma strength function and nuclear level density, remain accesible to the experiment. These ingredients which respectively manage the way of how the gamma cascade occurs and the nuclear structure at high excitation energy can also be measured and calculated to be compared and suggest ways to improve the predictability of models. This kind of improvements have a direct impact for instance on the cross sections for these exotic nuclei which are produced in the stellar nucleosynthesis. The subject of thie thesis is to measure these quantities for a nucleus involved in the nucleosythesis using a new setup called SFyNCS.
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.