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Geometrical optics methods for moving anisotropic media: a tool for magnetized plasmas
[Méthodes d’optique géométrique pour les milieux anisotropes en mouvement : un outil pour les plasmas magnétisés]
Comptes Rendus. Physique, Volume 26 (2025), pp. 7-23.

Cet article fait partie du numéro thématique Ondes au service des plasmas, plasmas au service des ondes coordonné par Julien Hillairet.  

La propagation d’une onde dans un milieu est en général modifiée lorsque celui-ci est en mouvement. Les configurations d’équilibre d’un plasma reposant souvent sur un champ de vitesse, comme par exemple en astrophysique ou pour la fusion par confinement magnétique, une compréhension des effets du mouvement sur les ondes plasmas est particulièrement souhaitable. On s’intéresse ici à développer une méthode de lancer de rayon pour étudier la trajectoire des rayons se propageant dans un milieu anisotrope en mouvement dans l’approximation de l’optique géométrique. Une relation de dispersion effective pour le milieu en mouvement vu du laboratoire est identifiée en effectuant une transformation de Lorentz de la relation de dispersion du milieu au repos. Cette relation de dispersion est alors utilisée dans des équations de lancer de rayon, permettant ainsi de modéliser l’effet du mouvement sur la trajectoire des différents modes supportés par le milieu. Le potentiel de cette méthode est pour finir illustré en considérant le cas des ondes d’Alfvén basse fréquence et celui des modes ordinaire et extraordinaire classiques d’un plasma magnétisé.

The propagation of a wave in a medium is generally affected when the medium is moving with respect to the observer. Because plasma equilibria often involve plasma flows, for instance in astrophysics or in magnetic confinement nuclear fusion devices, understanding the effect of motion on plasma waves is important. Meanwhile, the presence of a background magnetic field in a plasma makes it anisotropic. To address this problem, we derive here ray tracing equations for the trajectory of rays propagating in a moving anisotropic medium. The proposed approach is to use an effective dispersion relation for the moving medium as seen from the laboratory, obtained by performing a Lorentz transformation of the dispersion relation known for the medium at rest. This formalism is illustrated by considering the low frequency Alfvén waves and the standard ordinary and extraordinary modes in a magnetized plasma at rest.

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DOI : 10.5802/crphys.218
Keywords: geometrical optics, ray tracing, light-dragging, plasma flow, moving dieletric
Mots-clés : optique géométrique, lancer de rayon, entraînement de la lumière, écoulement plasma, diélectrique en mouvement

Aymeric Braud 1 ; Julien Langlois 1 ; Renaud Gueroult 1

1 LAPLACE, Université de Toulouse, CNRS, INPT, UPS, 31062 Toulouse, France
Licence : CC-BY 4.0
Droits d'auteur : Les auteurs conservent leurs droits
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Aymeric Braud; Julien Langlois; Renaud Gueroult. Geometrical optics methods for moving anisotropic media: a tool for magnetized plasmas. Comptes Rendus. Physique, Volume 26 (2025), pp. 7-23. doi : 10.5802/crphys.218. https://comptes-rendus.academie-sciences.fr/physique/articles/10.5802/crphys.218/

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