## Section: Scientific Foundations

### High-frequency limit of the Helmholtz equation

Participant : François Castella.

The Helmholtz equation models the propagation of waves in a medium with variable refraction index. It is a simplified version of the Maxwell system for electro-magnetic waves.

The high-frequency regime is characterized by the fact that the typical wavelength of the signals under consideration is much smaller than the typical distance of observation of those signals. Hence, in the high-frequency regime, the Helmholtz equation at once involves highly oscillatory phenomena that are to be described in some asymptotic way. Quantitatively, the Helmholtz equation reads

 $\begin{array}{c}\hfill i{\alpha }_{\epsilon }{u}_{\epsilon }\left(x\right)+{\epsilon }^{2}{\Delta }_{x}{u}_{\epsilon }+{n}^{2}\left(x\right){u}_{\epsilon }={f}_{\epsilon }\left(x\right).\end{array}$ (10)

Here, $\epsilon$ is the small adimensional parameter that measures the typical wavelength of the signal, $n\left(x\right)$ is the space-dependent refraction index, and ${f}_{\epsilon }\left(x\right)$ is a given (possibly dependent on $\epsilon$) source term. The unknown is ${u}_{\epsilon }\left(x\right)$. One may think of an antenna emitting waves in the whole space (this is the ${f}_{\epsilon }\left(x\right)$), thus creating at any point $x$ the signal ${u}_{\epsilon }\left(x\right)$ along the propagation. The small ${\alpha }_{\epsilon }>0$ term takes into account damping of the waves as they propagate.

One important scientific objective typically is to describe the high-frequency regime in terms of rays propagating in the medium, that are possibly refracted at interfaces, or bounce on boundaries, etc. Ultimately, one would like to replace the true numerical resolution of the Helmholtz equation by that of a simpler, asymptotic model, formulated in terms of rays.

In some sense, and in comparison with, say, the wave equation, the specificity of the Helmholtz equation is the following. While the wave equation typically describes the evolution of waves between some initial time and some given observation time, the Helmholtz equation takes into account at once the propagation of waves over infinitely long time intervals. Qualitatively, in order to have a good understanding of the signal observed in some bounded region of space, one readily needs to be able to describe the propagative phenomena in the whole space, up to infinity. In other words, the “rays” we refer to above need to be understood from the initial time up to infinity. This is a central difficulty in the analysis of the high-frequency behaviour of the Helmholtz equation.