Experimental investigation of the nonlinear Rayleigh-Taylor instability in CH foils irradiated by UV light

In direct-drive inertial confinement fusion (ICF), nonuniformities in the laser drive can create (or imprint) mass perturbations in the target and seed hydro-dynamic instabilities. Understanding of this imprinting and the temporal evolution of resulting features is of utmost importance to high-gain target designs. Time-integrated optical measurements of a single OMEGA laser beam show that smoothing techniques such as distributed phase plates (DPP's) and smoothing by spectral dispersion (SSD) generate a broadband spectrum of laser nonuniformities. These nonuniformities are imprinted into targets, then, during a laser-driven acceleration, the Rayleigh-Taylor instability amplifies these features. The temporal evolution of these mass perturbations is studied using x-ray through-foil radiography of planar CH targets employing pinholes and framing cameras. To study these phenomena, it is important that the spatial response of the instrumentation be properly characterized. The noise in these experiments is limited by the photon statistics of the uranium backlighter x rays and the system resolution is limited by 8-mum pinhole. Using the measured system resolution, noise, and sensitivity, a Wiener filter has been constructed to filter the noise from the images and deconvolve the system MTF in order to recover the target's areal density modulations. The Fourier analysis of the radiographic images shows that the perturbations evolve to longer wavelengths and the amplitudes of the shorter wavelengths saturate. The saturation amplitudes and the rates of growth of these features are consistent with the predictions of Haan for broadband spectrum [1]. Using high-contrast, thin 5-mum teflon targets, the imprint has been measured at the time of the shock breakout using the same radiographic system.