Modeling the interaction between a few-cycle relativistic laser pulse and a plasma mirror : from electron acceleration to harmonic generation

Abstract : When a laser pulse with a relativistic intensity is focused onto a solid target, the material is instantly ionized and forms a plasma mirror, namely an overdense plasma with a short density gradient on its front side. During the laser pulse reflection, high harmonics are generated in the reflected pulse, and electrons can be accelerated out of the target. While the mechanisms for high harmonic generation are well-known, the acceleration of electrons remained unclear. Based on experimental results from two ultraintense femtosecond laser systems (the "Salle Noire" laser at LOA and the UHI100 laser at CEA), this theoretical and numerical thesis unravels the mechanisms for ejection and acceleration of electrons, following three research lines. First, using particle-in-cell numerical simulations, we identify the ejection mechanism occuring during every laser period at the plasma surface. In particular, the role of the fields inside the plasma is highlighted, and the scale length of the plasma density gradient is shown to be a key parameter. Second, after being ejected from the plasma surface, electrons can be accelerated by the laser fields in the reflected pulse. This so-called "vacuum laser acceleration" had not been studied extensively in experiments, the biggest hurdle being to inject electrons directly inside an ultraintense laser pulse. Plasma mirrors offer an answer to this question and serve as electron injectors. In this thesis, we develop a model to interpret experimental results obtained on the UHI100 laser at CEA. In particular, we show that these experiments lead to the first observation of vacuum laser acceleration of a high-charge (3 nC) electron beam to relativistic energies (10 MeV). Finally, high harmonic generation may occur via two mechanisms: coherent wake emission at low intensity and the relativistic oscillating mirror effect at high intensity. Comparing electron ejection with each of these mechanisms brought new insights into the nanoscale dynamics of the plasma surface.
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  • HAL Id : tel-01485500, version 1

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Maxence Thévenet. Modeling the interaction between a few-cycle relativistic laser pulse and a plasma mirror : from electron acceleration to harmonic generation. Plasma Physics [physics.plasm-ph]. Université Paris-Saclay, 2016. English. ⟨NNT : 2016SACLX112⟩. ⟨tel-01485500⟩

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