Absolute X-Ray Yields from Laser-Irradiated Ge-Doped Aerogel Targets

Summary form only given. We have measured the production of 10 keV X-rays from low-density Ge-doped aerogel targets at the OMEGA laser. The targets were 1.2 mm long by 1.5 mm diameter beryllium cylinders filled with Ge-doped (20 atomic percent) SiO₂ foam. The doped-foam density was 5 or 7 mg/cc. These targets are a major advance over previous doped aerogels: instead of suspending the dopant in the SiO₂ matrix, the Ge atoms, with chemistry similar to Si, are incorporated directly in the matrix. Thus, the level of dopant is increased by more than a factor of six. Forty beams of the OMEGA laser (351 nm) illuminated the two cylindrical faces of the target with a total power that approached 20 TW. The laser interaction strongly ionizes the target (n_e/n_crit = 0.15-0.20), and allows the laser-bleaching wave to supersonically ionize the high-Z emitter ions in the sample. The heating of the target was imaged with a gated (200 ps time resolution) X-ray framing camera, filtered to observe >8 keV. 2-D radiative-hydrodynamic calculations predict rapid and uniform heating over the whole target volume with minimal energy losses into hydrodynamic motion. Ge K-shell X-ray emission was spectrally resolved with a two-channel crystal spectrometer and recorded with temporal resolution with a set of calibrated photoconductive devices (PCDs). The calculations predict 150-200 J of X-ray energy output with 10 keV. The effect of shaping and delaying the laser pulse is studied. A full description of the experiment and the preliminary results of our analysis will be presented.