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.

Definition of an asynchronous on-the-fly data compression model on accelerators for HPC

This thesis is related to high-performance computing for numerical simulation of complex physical phenomena.
The CEA provides hardware and software resources to achieve the required computing power.
We have witnessed the advent of accelerators, leading to new problems. In particular, memory management becomes critical for achieving exascale performance as the memory ratio per number of computing units is reducing.
This problem affects all areas requiring a large volume of data. Thus, many aspects of this thesis will be general and of global interest.

This thesis will aim to propose an asynchronous model for making data available through compression/decompression techniques. It should be efficient enough to be used "on the fly" (during computations without slowing them down), allowing memory constraints to be relaxed.
Targeted codes are iterative and sequence different phases. Ideally, all computations will be performed on accelerators, leaving CPU resources unoccupied. The proposed model should take advantage of these specificities. The final goal will be to integrate the work into a representative code to evaluate gains in an industrial context.

Multi-architecture Adaptive Mesh Refinement for multi-material compressible hydrodynamics simulation

CEA DAM is actively developing scientific software in computational fluid mechanics (CFD) for the numerical simulation of compressible and multi-material flows. Such numerical tools requires the use of parallel programming models designed for efficient use of large supercomputers. From the algorithmic point view, the fluid dynamics equations must be discretized and solved using the adaptive mesh refinement (AMR) strategy which allows to reduce the computational cost of such simulations, in particular the number of cells (therefore the memory footprint) and to concentrate the computational work load on the areas of interest (discontinuities, shocks, multi-fluid interfaces, etc. ).

Over the past fifteen years, with the appearance of graphics processors (GPUs), the hardware architectures used in the field of high-performance computing (HPC) have evolved profoundly. This PhD thesis is about designing a parallel implementation of the AMR techniques for the case of multi-material flows with the aim of using as efficiently as possible a GPU-based supercomputer. After required numerical verification and validation process, the developed code will be used to perform numerical simulation of a blast wave and its interaction with surrounding structures.

Study of the influence of the ferrite additive manufacturing process on mechanical and magnetic properties

Traditional methods of manufacturing ceramic parts include costly processes such as slip casting, pressing or injection molding; they require specific equipment and expertise. When small quantities of ceramic parts, or prototypes, with specific properties are required, manufacturers are still forced to make costly investments. In addition, traditional manufacturing processes restrict design freedom and make it difficult to create internal channels, overhangs or lattice structures, for example. 3D printing opens up new prospects for innovation in the field of technical ceramics, by offering low-cost machines and opening up the field of possibilities for the design of complex parts impossible to obtain with molding methods.
This is the background to the subject of this thesis, on a ceramic material of interest to the CEA: (Ni-ZnFe2O4) ferrite. The CEA masters the manufacture of this material by traditional methods of powder pressing followed by sintering, but would like to extend its skills by producing parts with more complex geometries, with a reduced time between the design stage and the manufacture of a first prototype.
The work will involve optimizing the microstructure of ferrite implemented using Fused Deposition Modeling (FDM) 3D printing technology, then measuring mechanical and magnetic properties, as well as magneto-elastic effects. An analysis will be carried out to correlate the relationship between microstructure and material properties. The results will be compared with the conventionally developed material. This will highlight the influence of the manufacturing process on properties. Finally, a part with a complex geometry will be developed with the aim of understanding the difficulties associated with the change of scale. This stage will be accompanied by an initial assessment of the robustness of the process.

Damage and laser cleaning of optics on power laser systems

Design and characterisation of a power amplifier using GaN technology

Synthesis and post-synthesis treatments of ultra-light weight mesoporous metals obtained by plasma electrolysis for laser targets fabrication

For fundamental physics experiments conducted on the Megajoule Laser, the CEA must develop mesoporous metal materials with very low apparent density. Based on the discovery by CEA researchers of a new reactive mechanism between plasma and liquid, CEA has developed a unique electrolytic plasma synthesis process in the world. This technology converts thousands of flashes into as many metallic nano-filaments in seconds to form metals in the form of a nano-structured, ultra-light sponge.
The understanding of the physico-chemical mechanisms that govern the synthesis of these foams is crucial to optimize the properties of synthetic raw materials. A first part of the thesis will consist in continuing the studies already carried out and completing the innovative phenomenological model in the field of electrolytic plasmas.
In a second step, the influence of a heat treatment on the crystallization of these materials and their mechanical resistance will be conducted in order to optimize their subsequent shaping by laser or ultra-precision mechanical machining.

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.

Modeling of Particles Capture by Aqueous Foams

Behavioral function based modelisation on power supplies submitted to high level electrical pulses