C. A. Back

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

X-ray backlighting is a powerful tool for diagnosing a large variety of high-energy-density phenomena. Traditional area backlighting techniques used at Nova and Omega cannot be extended efficiently to NIF-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 SNR and hence quality by switching from film to CCD-based recording media and by removing the fixed-pattern noise of MCP-based cameras.

Laser–plasma interactions in ignition‐scale hohlraum plasmas

Scattering of laser light by stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) is a concern for indirect drive inertial confinement fusion (ICF). The hohlraum designs for the National Ignition Facility (NIF) raise particular concerns due to the large scale and homogeneity of the plasmas within them. Experiments at Nova have studied laser–plasma interactions within large scale length plasmas that mimic many of the characteristics of the NIF hohlraum plasmas. Filamentation and scattering of laser light by SBS and SRS have been investigated as a function of beam smoothing and plasma conditions. Narrowly collimated SRS backscatter has been observed from low density, low‐Z, plasmas, which are representative of the plasma filling most of the NIF hohlraum. SBS backscatter is found to occur in the high‐Z plasma of gold ablated from the wall. Both SBS and SRS are observed to be at acceptable levels in experiments using smoothing by spectral dispersion (SSD).

Opacity measurements: Extending the range and filling in the gaps

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.

Multi-keV x-ray conversion efficiency in laser-produced plasmas

X-ray sources are created at the Nova and Omega laser by irradiating a confined volume of Ar and Xe gas. The gas is heated by 20–35 kJ of 0.35 μm laser light and becomes a highly ionized mm-sized x-ray source which emits K-shell or L-shell x rays. The radiator is “underdense,” meaning that the initial electron density is lower than the critical density of the laser, nc∼1022 cm−3. It is heated primarily by inverse bremsstrahlung, which produces a supersonic ionization wave. In this paper, x-ray conversion efficiency and imaging from time-resolved and time-integrated diagnostics are compared over a range of experimental parameters. This work represents an important, new method for development of efficient, large-area, tailored multi-keV x-ray sources.

Diffusive, supersonic x-ray transport in radiatively heated foam cylinders

Diffusive supersonic radiation transport, where the ratio of the diffusive radiation front velocity to the material sound speed >2 has been studied in experiments on low density (40 mg/cc to 50 mg/cc) foams. Laser-heated Au hohlraums provided a radiation drive that heated SiO2 and Ta2O5 aerogel foams of varying lengths. Face-on emission measurements at 550 eV provided clean signatures of the radiation breakout. The high quality data provides new detailed information on the importance of both the fill and wall material opacities and heat capacities in determining the radiation front speed and curvature. The Marshak radiation wave transport is studied in a geometry that allows direct comparisons with analytic models and two-dimensional code simulations. Experiments show important effects that will affect even nondiffusive and transonic radiation transport experiments studied by others in the field. This work is of basic science interest with applications to inertial confinement fusion and astrophysics.

Time-resolved x-ray imaging of high-power laser-irradiated underdense silica aerogels and agar foams

This article presents the results of experiments in which a high‐power laser was used to irradiate low density (4–9 mg/cm3) silica aerogel and agar foam targets. The laser–solid interaction and energy transport through the material were monitored with time‐resolved imaging diagnostics, and the data show the production and propagation of an x‐ray emission front in the plasma. The emission‐front trajectory data are found to be in significant disagreement with detailed simulations, which predict a much more rapid heating of the cold material, and the data suggest that this discrepancy is not explainable by target inhomogeneities. Evidence suggests that energy transport into the cold material may be dominated by thermal conduction; however, no completely satisfactory explanation for the discrepancies is identified, and further experimental and theoretical research is necessary in order to resolve this important problem in laser–plasma interaction physics.

Observation of multiple mechanisms for stimulating ion waves in ignition scale plasmas

The laser and plasma conditions expected in ignition experiments using indirect drive inertial confinement have been studied experimentally. It has been shown that there are at least three ways in which ion waves can be stimulated in these plasmas and have significant effect on the energy balance and distribution in the target. First ion waves can be stimulated by a single laser beam by the process of Stimulated Brillouin Scattering (SBS) in which an ion acoustic and a scattered electromagnetic wave grow from noise. Second, in a plasma where more than one beam intersect, ion waves can Lie excited at the `beat` frequency and wave number of the intersecting beams,, causing the side scatter instability to be seeded, and substantial energy to be transferred between the beams [R. K. Kirkwood et. al. Phys. Rev. Lett. 76, 2065 (1996)]. And third, ion waves may be stimulated by the decay of electron plasma waves produced by Stimulated Raman Scattering (SRS), thereby inhibiting the SRS process [R. K. Kirkwood et. al. Phys. Rev. Lett. 77, 2706 (1996)].

Gas‐filled targets for large scale‐length plasma interaction experiments on Nova

Stimulated Brillouin backscatter from large scale‐length gas‐filled targets has been measured on the Nova laser. These targets were designed to approximate conditions in indirect drive ignition target designs in underdense plasma electron density (ne∼1021/cm3), temperature (Te≳3 keV), and gradient scale lengths (Ln∼2 mm, Lv≳6 mm) as well as calculated gain for stimulated Brillouin scattering (SBS). The targets used in these experiments were gas‐filled balloons with polyimide walls (gasbags) and gas‐filled hohlraums. Detailed characterization using x‐ray imaging and x‐ray and optical spectroscopy verifies that the calculated plasma conditions are achieved. Time‐resolved SBS backscatter from these targets is <3% for conditions similar to ignition target designs.

Imaging backscattered and near to backscattered light in ignition scale plasmas (invited)

Diagnostics have been developed and fielded at the Nova laser facility that, for the first time, image nearly all the light scattered within 20° of the laser axis, including the light collected by the laser focusing lens as well as that just outside the lens. Absolute calibration of optical components exposed to the target debris have been achieved by a combination of routine in situ calibration and maintenance. Measurements from plasmas relevant to ignition experiments indicate that scattering is peaked in the direction of backscatter with significant energy scattered both into the lens and just outside the lens. The scattering outside the lens is found to be dominant when the f number is large (f/8).

Supersonic propagation of ionization waves in an underdense, laser-produced plasma

A laser-driven supersonic ionization wave propagating through a millimeter-scale plasma of subcritical density up to 2–3keV electron temperatures was observed. Propagation velocities initially ten times the sound speed were measured by means of time-resolved x-ray imaging diagnostics. The measured ionization wave trajectory is modeled analytically and by a two-dimensional radiation-hydrodynamics code. The comparison to the modeling suggests that nonlocal heat transport effects may contribute to the attenuation of the heat-wave propagation.