Measurement of the mass ablation rate for 0.53 mu m random phased laser irradiation of spherical targets

Abstract

The mass ablation rate has been measured using the 12-beam GEKKO XII glass laser. An X-ray pinhole camera coupled to a streak camera was used to provide both spatial and temporal history of the emission from buried maker layers. The camera was filtered by 0.75 mu m of Al to enhance the image of the cool, denser plasma near the ablation front. The targets (diameter approximately 500 mu m) were both polymer shells and solid polymer pellets and were overcoated with up to four maker layers sandwiched between CH ablator layers. Laser energies of up to 8 kJ in 1.7 ns were incident on the targets, with absorbed irradiances of up to 4.0*10/sup 14/ W-cm/sup -2/. Good agreement has been found between the experiment and a one-dimensional hydrodynamic code in which the heat flow is obtained from the Fokker-Planck equation. The mass data were consistent with the temporal behavior of spectral line emissions observed by a streaked crystal spectrography. The introduction of random phase plates had no significant influence on the mass ablation rate for imploding targets. The results indicate that a highly uniform drive pressure has been achieved for the GEKKO XII laser. >