KMSF low preheat implosion experiments

Laser-driven inertial confinement fusion (ICF) experiments are being performed at several laboratories throughout the world. Nearly all are seeking to demonstrate compressed fuel densities of several tens g/cm3. For ICF to be an efficient process, it is desirable to compress the fuel along a near-Fermi degenerate adiabat to densities of several hundred g/cm3. Addition of thermal energy prior to achieving the compressed state is detrimental in that a significantly larger driver would be required to achieve ignition. Preheating of the fuel prior to compression may result from radiation or thermal electron conduction, or non-isentropic processes such as shock heating. Recent experiments at KMSF sought to control these by a combination of target and laser parameters. Using a spherical illumination system, the target is irradiated with a carefully prescribed temporal pulse of frequency doubled neodymium glass laser light. Low preheat targets are designed with thick polymer (poly vinyl alcohol) shells and cryogenic fuel layers. The implosion process is recorded by a number of optical and x-ray diagnostics. A four-frame holographic interferometer records the temporal evolution of the ablated plasma while an x-ray pinhole camera records the symmetry of the compressed core. The most important diagnostic is the combined x-ray backlighter/streak camera combination which records the implosion trajectory in one dimension. A comparison of the data from these three major instruments with the results from the hydrodynamic simulation code allows one to interpret the final state the fuel reached and thus, the preheat level. Assuming spherical symmetry, record fuel densities in excess of 35 g/cm3 have been achieved