Observation of strong oscillations of areal mass in an unsupported shock wave produced by a short laser pulse*

The first experimental study of hydrodynamic perturbation evolution in a strong unsupported shock wave, which is immediately followed by a rarefaction wave, is reported. Our planar solid polystyrene laser-machined targets, 50 to 100 $\mu $m thick, rippled from the front side with a single-mode wavelength 30 or 45 $\mu $m and peak-to-valley amplitude 4 to 6 $\mu $m, were irradiated with a 350 ps long Nike KrF laser pulse at peak intensity of up to 330 TW/cm$^{2}$. The perturbation evolution in the target was observed using face-on monochromatic x-ray radiography while the pulse lasted and for 3 to 4 ns after it ended. While the driving pulse was on, the areal mass modulation amplitude in the target was observed to grow by a factor of up to $\sim $4 due to the ablative Richtmyer-Meshkov instability. After the end of the pulse, while the strong unsupported shock wave propagated through the unperturbed target, the theoretically predicted large oscillations of the areal mass [A. L. Velikovich \textit{et al}., Phys. Plasmas \textbf{10}, 3270 (2003)] were observed. Multiple phase reversals of the areal mass modulation have been detected.