The influence of aperture averaging on the two-wavelength intensity covariance function was experimentally determined for visible (0.63 microm) and infrared (1.06 microm) collinear, approximately spherical beams which propagated through the earth's turbulent atmosphere. Range varied from 1300 to 3250 m, and due to the prevailing atmospheric conditions, most measurements were made in the strong turbulence regimes. Results show that (1) the covariance function monotonically decreases as the receiver aperture size increases; (2) the correlation coefficient attains high values > or = 0.7) even for a relatively small aperture size of 5 mm; (3) while the single wavelength probability distribution of the intensity is approximately lognormal, the experimental two-wavelength conditional probabilities are higher than those predicted by the lognormal model.
Aluminum foils 1–75 μm thick were irradiated by 500-psec Nd-glass laser pulses with intensities 6×1012 to 1014 W/cm2. The reflected and transmitted light and the produced x rays were measured using PIN photodiodes and crystal spectrometers. Two torsion pendula were used to measure the target and the plasma momenta. Both measurements are consistent with a simple hydrodynamic model. We obtain plasma pressures in the range 1.5–13 Mbars, shock-wave velocities between 0.9×106 and 2.6×106 cm/sec, penetration depths of the ablation surface in the domain of 3–10 μm for laser intensities in the range 6×1012 to 1014 W/cm2. The burn-through times (i.e., the times that a hole is opened in the foil) for foils 25, 50, and 75 μm thick are measured to be 8±5, 18±5, and 25±5 nsec, respectively.
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