X-ray backlighting is a powerful tool for diagnosing a large variety of high-density phenomena. Traditional area backlighting techniques used at Nova and Omega cannot be extended efficiently to National Ignition Facility scale. New, more efficient backlighting sources and techniques are required and have begun to show promising results. These include a backlit-pinhole point-projection technique, pinhole and slit arrays, distributed polychromatic sources, and picket-fence backlighters. In parallel, there have been developments in improving the data signal-to-noise and, hence, quality by switching from film to charge-coupled-device-based recording media and by removing the fixed-pattern noise of microchannel-plate-based cameras.

https://www.z-beamlet.sandia.gov/media_lib/docs/pdfs/sci_papers/rosi-1.pdf

X-ray backlighting for the National Ignition Facility (invited)

Results of a niobium absorption experiment are presented that represent a major step in the development of techniques necessary for the quantitative characterization of hot, dense matter. The general requirements for performing quantitative analyses of absorption spectra are discussed. Hydrodynamic simulations are used to illustrate the behavior of tamped x-ray-heated matter and to indicate potential two-dimensional problems inherent in the technique. The absorption spectrum of a low-Z material, in this case aluminum, mixed with niobium provides a temperature diagnostic, which together with radiography as a density diagnostic fully characterizes the sample. A discussion is presented of opacity calculations and a comparison to the measurements is given that illustrates the need for experiments to provide a critical test of theory. The experimental technique is placed in context with a review of previous measurements using absorption spectroscopy to probe hot, dense matter. It is shown that the overall experimental concepts, although understood, were not always achieved in previous experiments. \textcopyright{} 1996 The American Physical Society.

Absorption experiments on x-ray-heated mid-Z constrained samples.

A series of experiments to explore Ge opacity at temperatures where the M-shell is almost filled will be discussed. Data are obtained at lower temperatures than previously explored and allow us to investigate the role of atomic structure calculations and their impact on opacity scalings. The experiment uses the Nova laser to irradiate a gold hohlraum within which a CH-tamped Ge sample is radiatively heated. A Nd backlight probes the sample 2 ns later to produce Ge spectral absorption features in the 1.2-1.5 keV energy range. Temperature is monitored by the use of an Al dopant and density is monitored by measuring the edge-on expansion of the sample. Temporal resolution of about 200 ps is obtained by using a short pulse backlight. Calculations in this photon energy region show significant changes in the spectral features.

/pdf/opacity-measurements-extending-the-range-and-filling-in-the-yvp08dtbrj.pdf

Opacity measurements: Extending the range and filling in the gaps

Absorption spectroscopy of mid and neighboring Z plasmas: Iron, nickel, copper and germanium

Measurements of the opacity of silicon at high temperature and high density are reported A silicon dioxide foam was heated by eight nanosecond laser beams while a backlighter X-ray source was produced with a picosecond laser Absorptions of the 1-2 transitions of Si XII through Si VI were observed in the wavelength range from 66 to 71 A The experimental results are simulated with theoretical calculations under local thermodynamic equilibrium using a detailed level accounting model and can be reproduced in general when the effects of the oxygen in the SiO2 are taken into account

https://iopscience.iop.org/article/10.1086/588750/pdf

Opacity Studies of Silicon in Radiatively Heated Plasma

Results of recent experiments measuring x-ray absorption by a hot, dense, germanium plasma are presented. A general discussion of the experimental technique is given showing the requirements that must be met in order to extract quantitative transmission data. The resulting spectrally resolved absorption measurements can then be used to test the capabilities of LTE opacity codes. Meaningful comparisons require that the sample be in LTE, and that the temperature and density of the sample be uniform and accurately measured. Comparisons between the experiment and calculations are shown.

R. Cauble

Papers

Quantitative measurement of mid-z opacities