The characteristics of 22–40keV Kα x-ray sources are measured. These high-energy sources are produced by 100TW and petawatt high-intensity lasers and will be used to develop and implement workable radiography solutions to probe high-Z and dense materials for the high-energy density experiments. The measurements show that the Kα source size from a simple foil target is larger than 60μm, too large for most radiography applications. The total Kα yield is independent of target thicknesses, verifying that refluxing plays a major role in photon generation. Smaller radiating volumes emit brighter Kα radiation. One-dimensional radiography experiments using small-edge-on foils resolved 10μm features with high contrast. Experiments were performed to test a variety of small volume two-dimensional point sources such as cones, wires, and embedded wires, measured photon yields, and compared the measurements with predictions from hybrid-particle-in-cell simulations. In addition to high-energy, high-resolution backlighte...

/pdf/high-energy-ka-radiography-using-high-intensity-short-pulse-51ll2v098v.pdf

High-energy Kα radiography using high-intensity, short-pulse lasersa)

Large-scale directional outflows of supersonic plasma, also known as ''jets'', are ubiquitous phenomena in astrophysics [1]. The interaction of such jets with surrounding matter often results in spectacular bow shocks, and intense radiation from radio to gamma-ray wavelengths. The traditional approach to understanding such phenomena is through theoretical analysis and numerical simulations. However, such numerical simulations have limited resolution, often assume axial symmetry, do not include all relevant physical processes, and fail to scale correctly in Reynolds number and perhaps other key dimensionless parameters. Additionally, they are frequently not tested by comparison with laboratory experiments. Recent advances in high-energy-density physics using large inertial-confinement-fusion devices now allow controlled laboratory experiments on macroscopic volumes of plasma of direct relevance relevant to astrophysics [2]. In this Letter we report the first results of experiments designed to study the evolution of supersonic plasma jets and the bow shocks they drive into a surrounding medium. Our experiments reveal both regular and highly complex flow patterns in the bow shock, thus opening a new window--complementary to computer simulations--into understanding the nature of three-dimensional astrophysical jets.

/pdf/high-energy-density-laboratory-astrophysics-studies-of-jets-54x3rcra2a.pdf

High-Energy-Density, Laboratory-Astrophysics Studies of Jets and Bow Shocks

In the context of target design for multi-keV x-ray laser-produced experiments, the concept of exploding metallic thin foils by two laser pulses delayed in time has been tested at the OMEGA laser facility [J. M. Soures, R. L. McCrory, C. P. Verdon et al., Phys. Plasma 3, 2108 (1996)]. The first laser pulse creates an underdense plasma (ne∕nc≈0.2), and the second laser pulse heats the plasma plume which produces strong line emission from the titanium K shell (Heα at 4.7 keV and Hα at 4.9 keV). Six OMEGA beams (500-ps duration) for the prepulse and nine beams (1-ns duration) for the heating pulse irradiate one side of the foil. Different experimental conditions have been investigated in order to optimize the conversion efficiency enhancement on titanium foils. The influences of the foil thicknesses (5 and 6 μm), the delays (3, 4, and 5 ns) between the laser pulses, and the laser intensities (1.3 and 2.2×1015Wcm−2) have been tested. The absolute output power was measured by a set of filtered x-ray diodes, gi...

Multi-keV x-ray conversion efficiencies of laser-preexploded titanium foils

As multi-keV x-ray radiators, hohlraums and halfraums with inner walls coated with metallic materials (called liner) have been tested for the first time with laser as the energy drive. For titanium, conversion efficiencies (CEs) are up to ∼14% for emission into 4π, integrating between 4.6 and 6.5 keV when a large diameter hohlraum is used. Germanium CE is ∼0.8% into 4π between 9 and 13 keV. The highest CEs have been obtained with a 1 ns squared pulse and phase plates giving laser absorption near 99%. These high CEs are due to long-lasting, good plasma conditions for multi-keV x-ray production maintained by plasma confinement inside the plastic cylinder and plasma collision leading to a burst of x rays at a time that depends on target size. As photon emitters at 4.7 keV, titanium-lined hohlraums are the most efficient solid targets and data are close to CEs for gas targets, which are considered as the upper limit for x-ray yields since their low density allows good laser absorption and low kinetics losses....

Titanium and germanium lined hohlraums and halfraums as multi-keV x-ray radiators

1D spectral imaging was used to characterize the K-shell emission of Z ≈ 30–35 and Z ≈ 40–42 laser-irradiated foils at the National Ignition Facility. Foils were driven with up to 60 kJ of 3ω light, reaching laser irradiances on target between 0.5 and 20 × 1015 W/cm2. Laser-to-X-ray conversion efficiency (CE) into the Heα line (plus satellite emission) of 1.0%–1.5% and 0.15%–0.2% was measured for Z ≈ 30–32 and Z ≈ 40–42, respectively. Measured CE into Heα (plus satellite emission) of Br (Z = 35) compound foils (either KBr or RbBr) ranged between 0.16% and 0.29%. Measured spectra are compared with 1D non-local thermodynamic equilibrium atomic kinetic and radiation transport simulations, providing a fast and accurate predictive capability.

X-ray area backlighter development at the National Ignition Facility (invited).

High-energy-density (HED) experiments are often diagnosed using x-ray backlighting. Recently, experiments have been designed and fielded that require x-ray backlighting having large fields of view and high x-ray energies. These types of experiments will be even more prevalent on the National Ignition Facility laser. Point backlighting offers the potential to obtain higher-energy x rays using less laser energy while giving a large, uniform field of view (millimeters). We present recent results from Los Alamos National Lab, Lawrence Livermore National Lab, and the University of Rochester’s Laboratory for laser energetics obtained on the OMEGA laser at the University of Rochester on the development of such bright sources. We include discussion of the challenges and successes to date.

Development of intense point x-ray sources for backlighting high energy density experiments (invited)

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Development of intense point x-ray sources for backlighting high energy density experiments (invited)