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Étude théorique et numérique de la génération d’harmoniques XUV à l’aide de lasers ultra-intenses sur feuilles minces

Abstract : When focusing an ultra-intense femtosecond laser pulse (Iλ0² > 10¹⁶ W.cm⁻ ²) on to a solid, the incident laser field is sufficiently high to ionise almost entirely the target at the very beginning of the pulse.Thus the most part of the laser field is reflected in the specular direction by the overdense plasma created until then : This is what we call a plasma mirror. The electrons, accelerated by the ultra-intense laser field ,are pulled out of the plasma with speeds which are almost equal to the speed of light c. For each laser period, electrons are the sources of a high-frequency radiation that can extend to the Extreme Ultra Violetor X domains. This periodicity in the generation process leads to the emergence of a harmonic spectrum of the laser frequency ω₀. Eventually, electrons are pushed back into the plasma with speeds always very close to c. Even though mechanisms of the XUV radiation are well known in the specular direction today, too few studies were conducted to understand generations process in the transmitted direction.The objective of this PhD thesis is to deepen understanding the laser-plasma interaction for the case where the target thickness is of the wavelength order. In particular, we will study the role of relativistic electronsjets in the forward radiation, when they fly across the target rear side.The first part of this manuscript will essentially deal with the already well-known mechanisms which explain the radiation in the specular direction. We can wonder to what extent these models are not sufficient to describe the forward radiation, in the transmitted direction.The second part of this manuscript deal with FDTD methods (Finite Difference Time Domain) in use in the "Particle-in-Cell" codes, especially two numerical effects induced by these methods potentially harmful for simulations in order to achieve physical results with meaning : Numerical dispersion and Numerical Cherenkov Instabilities. We will try to make an improvement to the algorithms in order to mitigate these two annoying effects. Eventually, we will identify a new coherent XUV radiation mechanism : The coherent plasma bremsstrahlung or coherent plasma braking radiation. When electrons leave the plasma, a several TV space-charge field appear on the target rear side. This braking longitudinal field accelerate electrons jets transversally which was created by next optical cycles. These jets radiate intenselight pulses of hundreds attoseconds.
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Submitted on : Wednesday, October 14, 2020 - 5:57:14 PM
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Guillaume Bouchard. Étude théorique et numérique de la génération d’harmoniques XUV à l’aide de lasers ultra-intenses sur feuilles minces. Physique des plasmas [physics.plasm-ph]. Université Paris-Saclay, 2020. Français. ⟨NNT : 2020UPASS037⟩. ⟨tel-02967252⟩

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