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Theses

Endommagement surfacique de la silice avec des faisceaux laser type LMJ

Abstract : Each of the 176 beams of the Laser MégaJoule (LMJ) facility will deliver an energy of 8 kJ at 351 nm, in the nanosecond regime, on a millimeter scale target. This energy is distributed evenly on a 400400mm2 square aperture, prior to the beam focusing. This energy density is likely to be absorbed by sub-surface defects induced by the polishing processes. This absorption eventually leads to laser-induced damage sites. These initiated damage sites absorb the laser energy during the subsequent shots and are likely to grow in size. The thick (34mm) optical windows situated after the frequency conversion module are exposed to the highest laser energies at 351nm and are the most sensitive to these phenomena. The goal of this thesis is to study the initiation and growth of damage sites on thick optical components with a laser beam that have similar properties to those of a LMJ beam. Such a beam can be delivered by the MELBA facility which can reach similar energy densities to those that can be found on the LMJ. The beam energy is evenly distributed on a centimeter scale circular aperture and the pulse shape and duration can be defined by the user. Three main axes have been explored in this work. The measurement of damage initiation on thick fused silica optical components have firstly been studied. The thickness of these optical components is likely to perturb the beam energy density via nonlinear propagation. These measurements therefore requires an accurate metrology of the energy density after the propagation through the optical component, thus taking Kerr small scale self focusing into account. A dedicated imaging setup has been implemented and proved to be able to accurately measure the energy density on the exit face. This setup has therefore been used to measure the damage density of thick optical components. The growth of a population of damage sites have secondly been studied as a function of the laser fluence and pulse duration. These two parameters have shown to be of critical importance to describe damage growth. Finally, the growth of damage sites up to millimeter scales have been studied. Atypical behaviors were observed and explained via the observation and quantification of the damage volumic morphology. Hypotheses based on the results of these measurements have been advanced to explain the results of the measurements of the growth of a population of damage sites.
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Submitted on : Friday, November 15, 2019 - 5:48:25 PM
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Matthieu Veinhard. Endommagement surfacique de la silice avec des faisceaux laser type LMJ. Optique [physics.optics]. Aix Marseille Université, 2019. Français. ⟨tel-02363517⟩

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