SINGLE-MODE AND MULTIMODE RAYLEIGH-TAYLOR EXPERIMENTS ON NOVA

Rayleigh-Taylor (RT) experiments have been conducted with planar CH(Br) foils accelerated by x-ray ablation from a shaped, low adiabat drive. The surface perturbations investigated consisted of single-mode, two-mode, and eight-mode sinusoids. The perturbation evolution begins during the shock transit phase, when perturbations show gradual growth due to Richtmyer-Meshkov-like dynamics. After shock breakout, the compressed foils accelerate and perturbation growth continues due to the Rayleigh-Taylor instability. Detailed comparisons with simulations indicate that in the linear Rayleigh-Taylor regime, the single-mode perturbations grow exponentially in time. In the nonlinear regime, the growth slows and the perturbation shape changes from sinusoidal to "bubble and spike" with the appearance of higher Fourier harmonics. In the multimode perturbations, the individual modes grow independently in the linear regime, but become coupled in the nonlinear regime. In addition to the higher harmonics of the individual modes, coupling leads to the appearance of ki,±kj "beat" modes. This results in a redistribution of the perturbation into a broader Fourier spectrum causing a change of shape: bubbles become broader and flatter and spikes narrower, in agreement with simulations and multimode theory. © 1995 American Institute of Physics.