Oxford Centre for High Energy Density Science (OxCHEDS)

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 μm of Al to enhance the image of the cool, denser plasma near the ablation front. The targets (diameter approximately 500 μm) 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 × 1014 W-cm-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 spectrograph. 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.

Abstract:

An optically pumped laser that operates in the VUV region of the spectrum with pulse energies in the millijoule range is proposed. The laser is expected to oscillate on ten to 13 new laser lines between 158 and 250 n. The transmission of the F2 pump laser output was measured as a function of NO pressure to deduce the absorption cross section. The absorption was also modeled using spectral rate equations. The model reproduces the observed transmission very well and predicts that the broadening of the absorption line is due to rapid quenching of the upper level. The measurements are not consistent with the results reported by T.J. McKee The predicted upper level population is of the order of 8 × 1014 cm-3 for an NO pressure of 800 mbar, which would give a small signal gain of ≈ 1 cm-1 for the strongest line. On the basis of this it appears to be possible to realize the proposed laser.