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

We have measured the production of $h\nu$ $\geq$~10~keV x rays from low-density, Ge-doped aerogel targets at the OMEGA laser (LLE, U. of Rochester). The targets were 1.2mm long by 1.5mm diameter Be cylinders filled with Ge-doped (20 atomic{\%}) SiO$_2$ 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 SiO$_2$ matrix, the Ge atoms are incorporated directly in the matrix. Forty beams of the OMEGA laser ($\lambda$ = 351~nm) illuminated the cylindrical faces of the target with a total power of $\approx$~18~TW. The laser strongly ionizes the target ($n_e$/$n_{cr}$ $\leq$ 0.1--0.2), and allows the laser-bleaching wave to ionize supersonically the high-Z 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 x-ray framing camera. 2-D rad-hydro calculations predict rapid and uniform heating over the target volume with minimal energy losses into hydrodynamic motion. The calculations predict 150--200~J of x-ray output with $h\nu$ $\ge$~10~keV, in agreement with the measurements.\newline *This work performed under auspices of the U.S. DoE by LLNL under Contract W-7405-ENG-48.