Absolute x-ray yields from laser-irradiated Ge-doped aerogel targets

Abstract

We have measured the production of <i>h</i>&#957; &ges; 10 keV x rays from low-density, Ge-doped aerogel targets at the OMEGA laser (Laboratory for Laser Energetics, University of Rochester). The targets were 1.2mm long by 1.5mm diameter beryllium cylinders filled with Ge-doped (20 atomic percent) SiO₂ aerogel. The doped-aerogel density was 4.8 or 6.5 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. Forty beams of the OMEGA laser (&#955; = 351 nm) illuminated the two cylindrical faces of the target with a total power of approximately 18 TW. The laser interaction strongly ionizes the target (<i>n_e</i>/<i>n_cr</i>&les; 0.1-0.2), and allows the laser-bleaching wave to ionize supersonically the high-Z emitter ions in the sample. 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 heating of the target was imaged with a gated (60 ps time resolution) x-ray framing camera, filtered to observe > 4 keV. 2-D radiative-hydrodynamic calculations predict rapid and uniform heating over the whole target volume with minimal energy losses into hydrodynamic motion. The calculations predict 150-200 J of x-ray energy output with <i>h</i>&#957; &ges; 10 keV. Good agreement between measurements and the calculations is found.