| A series of experiments have been conducted to investigate the dynamic over-pressure stabilization of the ablative Richtmyer-Meshkov (RM) instability in inertial confinement fusion (ICF) targets during the start-up phase of implosion. Theory and hydrodynamic simulations predict that due to localized thermal flux variations modulating the dynamic pressure, the components of the perturbation spectrum at the ablation front oscillate in time. The oscillation frequency depends on the mode wavenumber, ablation velocity and density scalelength or ablation front thickness. These predictions were verified on the 30-kJ OMEGA laser facility by measuring the perturbation amplitudes and frequencies directly, through face-on, x-ray radiography. A high-resolution, Ir-coated Kirkpatrick-Baez microscope, coupled to a high-current streak tube provided a continuous record of the target areal density during shock transit, when it is dominated by the evolution of the ablative RM instability. Planar plastic targets with variable thickness (30--60 mum) and single mode (lambda = 10 - 30 mum) perturbations on the front surface were irradiated by 1.5 ns square UV laser pulses with intensities ranging from 5 x 10 13 W/cm2 to 4 x 1014 W/cm2. Results clearly indicate a phase reversal in the evolution of the target areal density perturbations, in agreement with theory and simulation. The predicted dependence of the oscillation period on laser intensity and modulation wavelength was verified. |
