Active absorber spectrometer for Bremsstrahlung emitted during Laser-Plasma interaction
The SAABRE project involves the development of a new diagnostic tool to measure the continuous Bremsstrahlung X-ray radiation produced when a high-power laser interacts with matter.
Currently, the instrument known as the Bremsstrahlung Cannon (BC) uses a stack of X-ray-sensitive films separated by filters. Each film absorbs a portion of the incident radiation based on its energy, allowing the energy spectrum of the incident X-rays to be reconstructed. However, these films must be removed from the experimental chamber and scanned after each shot, which severely limits the use of the BC on the new generation of high-repetition-rate lasers (such as APOLLON, VEGA3, or ELI) and complicates its deployment on single-shot facilities like the LMJ.
The project therefore proposes replacing the films with millimeter-sized semiconductor crystals (Si, CdTe, CZT, GaAs, and perovskites). These sensors will enable instantaneous measurement of the radiation dose, conversion of the signal into digitized electrical data analyzed in real time, and adaptation of the instrument to different types of experiments by changing the configuration of the filters used (inertial confinement fusion or laser-plasma acceleration).
The postdoctoral fellow hired will lead the entire development of this new detector, from the study of the proposed semiconductors to the creation of a prototype simulated with GEANT4, which will then be calibrated and experimentally tested on facilities producing high-energy gamma rays. The device will be used in particular for the EUROPA project, dedicated to the production of innovative medical radioisotopes, and will lead to scientific publications and conference presentations.
Influence of laser bandwidth and wavelength on laser plasma instabilities
As part of the Taranis project initiated by Thales and supported by BPI France and in collaboration with numerous scientific partners such as CEA/DAM, CELIA and LULI, work on target design and definition of the laser intended to energy production in direct drive will take place. A prerequisite for this work is to understand the laser-plasma interaction mechanisms that will occur when the laser is coupled with the target. These deleterious mechanisms for the success of fusion experiments can be regulated by the use of so-called “broadband” lasers. In addition, the choice of the laser wavelength used for the target design and the laser architecture must be defined. The objective of the postdoctoral position is to study the growth and evolution of these instabilities (Brillouin, Raman) in the presence of “broadband” lasers both from an experimental and simulation point of view, and thus to be able to define the laser conditions making it possible to reduce these parametric instabilities.
Numerical studies of laser plasma interaction in intermediate field on Laser Megajoule
In the Inertial Confinement Fusion experiments (ICF), intense laser beams cross a gas filled hohlraum. The gas is fully ionized and laser beams then propagate into a sub-critical plasma where laser plasma instabilites can develop. Optical smoothing techniques enable to break both spatial and temporal coherences so that both spatial and temporal scales of the beam become smaller than those required for the development of the instabilites. The breaking of spatial coherence is done thanks to the use of a phase plate which spreads the laser energy in a multitude of light grains called speckles. The breaking of temporal coherence is done by using a phase modulator which widens the spectrum and by dispersing each frequency with a grating. It is essential to know the statistical properties of speckles (width, lenght, contrast, coherence time, velocities ...) to be able to predict the instabilities levels which can depend on time and on the distance of propagation of the beam. .
For the sake of simplicity, the laser plasma instabilities are very often studied at the best focus of the beam. However, in the FCI experiments, laser beams are focused near the laser entrance hole of the hohlraum whose length is about 1 cm. The development of instabilities can then occur before the best focus (outside the hohlraum) and mainly beyond the best focus (far inside the hohlraum). The goal of this post-doctoral contract is to study the development of instabilities when it occurs in the intermediate field (far from the best focus of the beam) and to assess the efficiency of different smoothing options on Lase MagaJoule (LMJ) to limit these instabilities. We will especially study propagation instabilities (self-focusing, forward stimulated Brillouin scattering) and stimulated Brillouin backscattering. This work will be done thanks to numerous existing numerical codes and diagnostic tolls.
Minimizing the laser imprint through machine learning within the frameword of inertial confinement fusion
The postdoc will be based at the CELIA laboratory which develops studies on different patterns of inertial fusion by laser. In order to optimize the implosion of the target, the laser pulse is shaped spatially and temporally, in particular by a pre-pulse of a hundred picoseconds and intensity of a few hundred TW /cm2. However, the latter introduces spatial inhomogeneities to the surface and volume of the target, amplified by the initial solid behavior of matter. These fingerprints generated by the pre-pulse will degrade the symmetry of the target during its implosion, and therefore decrease the effectiveness of inertial confinement. At present, most models assume a plasma state from the beginning of the interaction, and are thus unable to account for certain experimental observations. To overcome this lack, we have just developed an original multi-physics simulation tool that includes the phase transition of a homogeneous material induced by the laser. In order to mitigate the laser imprint effect, a polystyrene foam (heterogeneous material) can be deposited on the surface of the target. The multiple optical reflections in the foam smooth the spatial profile of laser intensity, thus reducing absorption inhomogeneities. In order to reduce the influence of the laser fingerprint, the post-doctoral fellowship will aim to develop a microscopic model describing the evolution of the optical response of a foam during the solid-to-plasma transition. The first step of the work will be to couple the Helmholtz equation (describing laser propagation) to a solid transition model-plasma, and to study the influence of parameters. The second step will be to use an artificial intelligence algorithm (neural network) to optimize the optical response of the foam.