Julien Langlois’s research while affiliated with French National Centre for Scientific Research and other places

Light dragging phenomena in accelerated media have classically been modelled neglecting the effect of inertia on the dielectric response of the media. Here, we show that the inertial corrections due to a rotating motion can have a profound impact on light-dragging manifestations, leading notably to birefringence in media that are isotropic at rest. By applying these findings to a rotating unmagnetized plasma, we further reveal how inertia plays, in this case, a dominant role, offering unique opportunities to expose these new effects. Inertia is notably demonstrated to be the source of non-zero drag and enhanced polarization drag, pointing to fundamental differences between linear and angular momentum coupling. In taking advantage of the singular properties of plasmas, this work more generally underlines how rest-frame properties are affected by an accelerated motion, and how these modifications can carry over to wave dynamics.

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 standard ordinary and extraordinary modes in a magnetized plasma at rest. Although we work here at lowest order in the geometrical optics approximation, this method is a first step towards higher order expansions, as required for instance to capture polarization effects.

DOI:https://doi.org/10.1103/PhysRevE.110.019901

The change in direction of the wavevector and group velocity experienced by a wave refracted at the interface of an anisotropic medium in uniform linear motion are determined analytically. These transmission conditions, which are shown to be consistent with generalized Snell's law written in the laboratory frame, are then used to examine the effect of motion on waves incident on a magnetized plasma. For an incident wave in the plane perpendicular to the magnetic field the motion is observed to lead to non negligible deviation of the low-frequency X-mode, as well as to non-symmetrical total reflection angles. These effects are shown to be further complicated when the magnetic field is in the plane formed by the incident wavevector and the medium's velocity, as the anisotropy now competes with the motion-induced drag. Although obtained in simplified configurations, these results suggest that accounting for motion when modeling plasma waves trajectory could be important under certain conditions, calling for a more detailed quantification of the effect of motion in actual diagnostics and plasma control schemes.

The signature of light dragging in a rotating unmagnetized plasma is studied analytically. In contrast with previous work which focused exclusively on the drag effects arising from rigid rotation, we examine here the supplemental contribution of inertia to the rest-frame dielectric properties of a rotating medium. We reveal, for the first time, that these so far neglected contributions actually play a dominant role on light dragging in rotating unmagnetized plasmas. Besides birefringence and enhanced polarization drag, inertia is notably demonstrated to be the cause of a non-zero drag, pointing to fundamental differences between linear and angular momentum coupling. We finally discuss how, thanks to the more favourable scaling elicited here, it may be possible to observe these effects in recently proposed laser driven rotating plasmas, identifying new promising directions for experimental investigations.

Models for polarization drag—mechanical analog of the Faraday effect—are extended to include inertial corrections to the dielectrics properties of the rotating medium in its rest frame. Instead of the Coriolis-Faraday term originally proposed by Baranova and Zel'dovich [Proc. R. Soc. London A: Math. Phys. Sci. 368, 591 (1979)], inertia corrections due to the fictitious Coriolis and centrifugal forces are here derived by considering the effect of rotation on both the Lorentz and plasma dielectric models. These modified rest-frame properties are subsequently used to deduce laboratory properties. Although elegant and insightful, it is shown that the Coriolis-Faraday correction inferred from Larmor's theorem is limited in that it can only capture inertial corrections to polarization drag when the equivalent Faraday rotation is defined at the wave frequency of interest. This is notably not the case for low-frequency polarization drag in a rotating magnetized plasma, although it is verified here using the more general phenomenological models that the impact of fictitious forces is, in general, negligible in these conditions.

Models for polarization drag - mechanical analog of the Faraday effect - are extended to include inertial corrections to the dielectrics properties of the rotating medium in its rest-frame. Instead of the Coriolis-Faraday term originally proposed by Baranova & Zel'dovich, inertia corrections due to the fictitious Coriolis and centrifugal forces are here derived by considering the effect of rotation on both the Lorentz and plasma dielectric models. These modified rest-frame properties are subsequently used to deduce laboratory properties. Although elegant and insightful, it is shown that the Coriolis-Faraday correction inferred from Larmor's theorem is limited in that it can only capture inertial corrections to polarization drag when the equivalent Faraday rotation is defined at the wave frequency of interest. This is notably not the case for low frequency polarization drag in a rotating magnetized plasma, although it is verified here using the more general phenomenological models that the impact of fictitious forces is in general negligible in these conditions.