Intensity variation of reflected energy from spherical glass shell laser fusion targets

Abstract

The fraction of the energy reflected by spherical glass shell targets irradiated with 1.06-μm laser pulses is measured for average intensities up to 4×1015 W/cm2. The on-target energies ranged up to 230 J with laser pulse half-widths of from 0.03 to 1.0 nsec. The glass shells ranged in diameter from 57 to 553 μm. Using the KMS Fusion two-channel disk laser system, the shells were illuminated at 1.06 μm from opposite sides at near-normal incidence. Two illumination systems were investigated. In the first, the target was illuminated with a pair of f/0.6 lenses providing approximately 80° per side (full cone angle) of near-normal incidence. In the second, the f/0.6 lenses were used in conjunction with an ellipsoidal mirror system which provides 144° per side (full cone angle) of normal illumination to with 18°. The fraction of the energy reflected from the targets was found to vary as a power of the average intensity on target over the range 4×1013 to 4×1015 W/cm2. Within this range no sudden onset of enhanced energy absorption or reflection by the targets was observed. Measurements were also made of the 1.06-μm radiation scattered from the target at 45° and 90° to the laser axis. The scattering was roughly isotropic. The light reflected from the targets was found to be approximately 20% depolarized.