Multiple cutoff wave numbers of the ablative Rayleigh-Taylor instability

The cutoff wave number of the incompressible ablative Rayleigh-Taylor instability is calculated using the physical optics approximation of the Wentzel-Kramers-Brillouin theory. It is found that a single value of the wave number k can correspond to multiple modes with different eigenfunctions and growth rates γ. In the γ-k plane the unstable spectrum is characterized by multiple branches with different cutoff wave numbers, and eigenfunctions with different number of zeros. The theory provides a formula for the cutoff wave number, valid in the regimes of interest for inertial confinement fusion capsules