Absolute x-ray yields from laser-irradiated germanium-doped low-density aerogels

The x-ray yields from laser-irradiated germanium-doped ultra-low-density aerogel plasmas have been measured in the energy range from sub-keV to approximate to 15 keV at the OMEGA laser facility at the Laboratory for Laser Energetics, University of Rochester. The targets' x-ray yields have been studied for variation in target size, aerogel density, laser pulse length, and laser intensity. For targets that result in plasmas with electron densities in the range of approximate to 10% of the critical density for 3 omega light, one can expect 10-11 J/sr of x rays with energies above 9 keV, and 600-800 J/sr for energies below 3.5 keV. In addition to the x-ray spectral yields, the x-ray temporal waveforms have been measured and it is observed that the emitted x rays generally follow the delivered laser power, with late-time enhancements of emitted x-ray power correlated with hydrodynamic compression of the hot plasma. Further, the laser energy reflected from the target by plasma instabilities is found to be 2%-7% of the incident energy for individual beam intensities approximate to 10(14)-10(15) W/cm(2). The propagation of the laser heating in the target volume has been characterized with two-dimensional imaging. Source-region heating is seen to be correlated with the temporal profile of the emitted x-ray power.