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Thesis

French

ID: <

10670/1.n9c099

>

Where these data come from
Spatial and temporal characteristics of a focal spot LMJ

Abstract

The future French Laser Megajoule (LMJ) is a multiple laser beam facility built to achieve Inertial confinement Fusion (ICF). It is mainly designed for an indirect drive scheme using X-rays conversion. In this scheme, the target in the center of the hohlraum is irradiated and then compressed by the X-rays more uniformly than what it would be in a direct drive schemes. However, a high of uniformity is still needed to reach ignition since the propagation of intense laser beams in an under-critical plasma can generate laser-plasma instabilities (LPI). The control of LPI is of crucial importance for the success of ICF. By breaking both spatial and temporal coherences, the use of optical smoothing techniques, such as smoothing by spectral dispersion (SSD), often dramatically reduces LPI and also ensures the reproducibility of laser conditions from one shot to another. An accurate description of the speckle pattern in the hohlraum is thus of great interest for ICF experiments.We focus our attention on the spatial and then temporal properties of the speckles pattern generated by multiple laser beams. At first, we establish equations for the 3D speckle size based on autocorrelation functions. Numerical simulations of the propagation of multiple laser beams in vacuum are then performed with the PARAX code in configurations where the paraxial approximation can be used. The case of speckle patterns in the LMJ configuration in the zone where all the beams overlap is eventually studied. We show that such speckles have an ellipsoidal shape. Finally, influence of the polarization of the beams on the shape, size and abundance of the speckles is also investigated. In a second part we study the important aspect of temporal smoothing techniques like the movement of the speckles. This work is also triggered by the development of a statistical model that describes the motion of hot spots in order to evaluate the contrast, the trajectory and the velocity of LMJ hot spots. We address these quantities in the case of a speckle pattern generated by multiple laser beams thanks to the autocorrelation function in intensity.

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