Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

We report a study of the effect of prepulses on XUV lasing of Ne-like germanium for an irradiation geometry where ≈20 mm long germanium slab targets were irradiated at ≈1.6 × 1013 W cm-2 using ≈0.7 ns (1.06 μm) pulses from the VULCAN glass laser. Prepulses were generated at fractional power levels of ≈2 × 10-4 (low) and ≈2 × 10-2 (high) and arrived on target 5 and 3.2 ns respectively in advance of the main heating pulse. For both the low and high prepulses the output of the 3p-3s, J=0-1, line at 19.6 nm was enhanced such that the peak radiant density (J/st) for this line became greater than that for the normally stronger J=2-1 lines at 23.2 and 23.6 nm. The 7=0-1 line, whose FWHM duration was reduced from ≈450 ps to ≈100 ps, delivered ≈6X more power (W) than the average for the combined J=2-1 lines, whose FWHM duration was ≈500 ps for both levels of prepulse. The higher prepulse was more effective, yielding ≈2X more radiant density and ≈7X more power on both the J=0-1 and J=2-1 transitions compared to the low prepulse case. The most dramatic observation overall was the ≈40X increase of power in the J=0-1 line for the high prepulse (≈2%) case compared with the zero prepulse case. These observations, coupled with measurements of beam divergence and beam deviation through refractive bending, as well as general agreement with modelling, lead us to conclude that, for germanium, the main influence of the prepulse is (a) to increase the energy absorbed from the main pulse, (b) to increase the volume of the gain zone and (c) to relax the plasma density gradients, particularly in the J=0-1 gain zone.

Abstract:

Recombining plasmas produced by picosecond laser pulses are characterized by measuring ratio of intensities of resonance lines of H- and He-like ions in the plasmas. It is found that the rapidly recombining plasmas produced by picosecond laser pulses are suitable for high-gain operation.

Abstract:

A theoretical and experimental time-resolved spectroscopic investigation of indirectly driven microsphere implosions is described. The plasma dynamics is studied for several fill gases with a trace amount of argon. Through an analysis of the line profile of Ar XVII 1[Formula Presented]–1s3p[Formula Presented]P, with a line center position at [Formula Presented]=3684 eV, the evolution of the plasma density and temperature as a function of fill gas is examined. The theoretical calculations are performed with a fast computer code, which has been previously benchmarked through the analysis of specific complex ionic spectra in hot dense plasmas. The experimental aspect of the work utilizes the Lawrence Livermore National Laboratory Nova 10 beam laser facility to indirectly drive the implosion of a gas filled plastic microsphere contained in a gold Holhraum target. The dynamical density measurement is derived from a streak camera linewidth measurement and a comparison with the computed profile. Calculations demonstrate that in certain cases there can be a substantial ion dynamics effect on the line shape. The frequency fluctuation model is used to compute the effect on the line profile and a comparison with the experimental spectra provides evidence that ion dynamics may be affecting the line shape. This study provides a method for obtaining an improved understanding of the basic processes dominating the underlying plasma physics of matter compressed to a state of high energy density. © 1996 The American Physical Society.