Early-time measurements of laser-plasma conditions in OMEGA-Upgrade ICF Targets. Final report, April 1, 1997--March 31, 1998

摘要

Under this FY-97 NLUF grant, we primarily carried out spectral line and continuum diagnostics at early times and in the coronal region of the plasma using our flat-field grazing-incidence spectrograph, improved to incorporate time resolution at wavelengths extending below the carbon K-absorption edge using a gated microchannel plate detector. These experiments were carried out on the OMEGA facility. Fifty-nine beams were focused onto the target, providing nominally 18 kJ of energy in a 1 ns pulse for an irradiance of (approximately)2(times)10(sup 14)/cm(sup 2). Some beam smoothing, provided by spectral dispersion, was used, but may not have been particularly effective alone, i.e., without the presence of distributed phase plates in the beams. The plastic microballoon targets were nominally 900 (mu)m in diameter with 10- and 20-(mu)m thick walls, and were filled with neon to a pressure of 10 atm. Overcoatings of Mg and Al in thicknesses ranging from 0.2 to 4 (mu)m were applied. A 1-(mu)m thick layer of CH was added in some early shots to reduce the rate of expansion of the metallic coatings. In the extreme ultraviolet (euv) spectral region, we observed n=3 to n=2 emissions from Li-, He- and H-like ions from the Mg and Al coatings. We also obtained evidence confirming our previously-published laser-field-induced satellites lines at 53.1 (Angstrom) and 62.8 (Angstrom), apparently at the peak of the Gaussian drive pulse. Both the Mg-line and the continuum euv emissions are high during the radial collapse. The metallic coating materials appear to be in place to some degree during the compression phase, i.e., are not all blown away as coronal plasma at earlier times as modeled. This also is apparent in the Al Lyman-(alpha) x-ray measurements before and after compression. Here, however, higher line opacity made it difficult to track the resonance lines through the compression phase. This illustrates the importance of euv measurements of less opaque lines at high densities.