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Energy deposition in air from femtosecond laser filamentation for the control of high voltage spark discharges

Abstract : Laser filamentation is a spectacular optical propagation regime appearing for pulses of which peak power exceeds a few GW in air. Filament forms due to the optical Kerr effect, which tends to self-focus the beam until intensity reaches the medium ionization threshold by multiphoton absorption. A complex dynamic competition is then established between the Kerr effect on the one hand, and diffraction, nonlinear absorption and plasma defocusing effect on the other hand. This results in a reorganization of the beam profile, characterized by a thin (100 µm) and intense (10^18 W/m²) core able to propagate over a distance much longer than the Rayleigh length. When the initial pulse peak power largely exceeds filamentation threshold, several co-propagating filaments are formed in the same beam, with each of these multifilaments sharing physical properties of isolated single filaments. While propagating in air, filaments transfer a portion of the laser energy to the medium, mainly through Raman rotational excitation of air molecules, ionization and inverse Bremsstrahlung in the plasma. This energy is redistributed in one nanosecond and almost entirely converted into air molecule translational energy, that is heat. The medium reacts to this rapid heating by launching a cylindrical pressure wave that brings the system back to pressure equilibrium by ejecting matter from the center. This results in the formation of a hot underdense air channel, which slowly resorbs by diffusion at timescales > 1 ms. My work as a Ph. D. student first focused on the study and the optimization of laser energy deposition in air by filamentation. Thus, I investigated the influence of laser parameters such as pulse energy, focusing strength or pulse duration on deposited energy. To this purpose, I used several complementary diagnostics: study of pressure waves using microphones, characterization of the filamentation plasma by means of spectroscopy and time resolved study of underdense air channels using interferometry. I demonstrated in the single filamentation regime that above a given pulse energy, energy deposition becomes so important that the medium generates a shock wave instead of a sound wave, and that underdense channels can last for more than 100 ms. I also studied and characterized the high energy multifilamentation regime, showing that moderately focusing the pulse leads to a reorganization of filaments in the focal zone, generating large structures with a resulting plasma ten times denser than filaments. Filamentation-induced hydrodynamic effects lead to a transient reduction of the air breakdown voltage along the path of the laser pulse, enabling one to trigger and guide electric discharges. The second part of my thesis focused on the study and the optimization of such guided discharges for the design of a radio-frequency plasma antenna, contactless high-voltage switches or a laser lightning rod. To this purpose I developed and built an interferometric plasma diagnostic, allowing to measure the lifetime of generated plasmas. I also contributed to the proof of principle for a filament induced plasma antenna emitting RF signal. Finally, I took part to prospective experimental studies for the development of a laser lightning rod.
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Contributor : Guillaume Point <>
Submitted on : Tuesday, September 22, 2015 - 9:45:20 AM
Last modification on : Thursday, March 5, 2020 - 6:31:11 PM
Document(s) archivé(s) le : Tuesday, December 29, 2015 - 6:55:53 AM

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  • HAL Id : tel-01202982, version 1

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Guillaume Point. Energy deposition in air from femtosecond laser filamentation for the control of high voltage spark discharges. Optics [physics.optics]. Ecole Polytechnique, 2015. English. ⟨tel-01202982⟩

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