Studies of plastic-ablator compressibility for direct-drive inertial confinement fusion on OMEGA

The compression of planar plastic targets was studied with x-ray radiography in the range of laser intensities of I similar to 0.5 to 1.5x10(15) W/cm(2) using square (low-compression) and shaped (high-compression) pulses. Two-dimensional simulations with the radiative hydrocode DRACO show good agreement with measurements at laser intensities up to I similar to 10(15) W/cm(2). These results provide the first experimental evidence for low-entropy, adiabatic compression of plastic shells in the laser intensity regime relevant to direct-drive inertial confinement fusion. A density reduction near the end of the drive at a high intensity of I similar to 1.5x10(15) W/cm(2) has been correlated with the hard x-ray signal caused by hot electrons from two-plasmon-decay instability.