Shape optimisation of the first solenoidal Maxwell eigenvalue

Abstract

We study the minimisation of the first solenoidal Maxwell eigenvalue $λ_1^{\mathrm{Max}}(Ω)$ in perfectly conducting cavities through the scale-invariant functional $J_{\mathrm{Per}}(Ω):=\mathrm{Per}(Ω)λ_1^{\mathrm{Max}}(Ω)$ in $\mathbb R^3$. While the corresponding unconstrained problem is known to be ill-posed, we focus on bounded convex cavities and prove that $J_{\mathrm{Per}}$ has a positive infimum in this class. The proof relies on the study of the three main possible behaviours for sequences of convex domains with bounded perimeter: convergence to a bounded convex domain with non-empty interior, collapse to a planar domain or collapse to a one-dimensional domain. The case of a planar collapse is handled through projection arguments, yielding a lower bound, while that of a one-dimensional collapse is controlled by a different argument, showing that in that case $J_{\mathrm{Per}}$ must diverge to $+\infty$. Although the question of existence of an optimal shape (that is, ruling out planar collapse) remains open, we derive several negative results and obstructions to existence; typically, no minimiser with a $\mathscr C^{3,+}$ boundary exists. This relies on a fine analysis of first-order optimality conditions.