Characteristics of lateral and axial transport in laser irradiations of layered-disk targets at 1.06 and 0.35 μm wavelengths

Abstract

Results and analysis are presented for Be-on-Al disk target irradiations at 1.06 and 0.35 μm laser wavelengths with 600–700 psec pulses, 240 μm spot diameter, and 1×1014 W/cm2 absorbed intensity. Absorptions of 32%–39% (1.06 μm) and 90% (0.35 μm) are largely due to inverse bremsstrahlung. The hard x-ray spectra indicate low hot-electron fractions of 10−2 (1.06 μm) and 10−4 (0.35 μm). Backreflected light shows strong hot spots for 0.35 μm irradiations. Multiple absolute and relative x-ray measurements are compared with one- and two-dimensional computer hydrodynamics calculations. Only weak indications of lateral transport are found and limits are set from x-ray imaging and spectral data from targets with and without a surrounding Ti shield. Axial transport appears strongly inhibited at 1.06 μm and mildly inhibited at 0.35 μm wavelength. Measured shock-wave transit times and velocities imply ablation pressures of 7 Mbar (1.06 μm) and 11 Mbar (0.35 μm).