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Solid-state optical fiducial timing pulse generators provide a convenient and accurate method to include timing fiducials in a streak-camera image for time-base correction. Current commercially available vertical-cavity surface-emitting lasers (VCSELs) emitting in the visible range can be amplitude modulated up to 5 GHz. An optically passive method is utilized to interleave a time-delayed path of the 5-GHz pulsed light with itself, producing a 10-GHz pulsed fiducial, or comb. Comb pulse rates at 0.5, 1.0, and 2.5 GHz can also be selected. The instrument presented is a self-contained and portable generator with primary use for streak-camera temporal calibration. Applications can also be extended to many other optical timing needs. The VCSEL output is fiber optic coupled at a wavelength of 680 nm (visible red) with a nearly Gaussian pulse shape. The peak power of each ∼50-ps full width at half maximum (FWHM) comb pulse at 5-GHz operation, or picket, is approximately 5 mW. The low phase noise of the internal microwave modulation drive source provides low pulse to pulse jitter. An external reference frequency standard can be utilized to synchronize the output to external timing equipment. A selectable internal reference frequency crystal oscillator is incorporated for stand-alone operation. [ABSTRACT FROM AUTHOR]

Localized absorption in hafnium dioxide used as a high-index component in multilayer coatings for near-ultraviolet, nanosecond-pulse-laser applications is directly linked to laser-induced damage. The nature of the absorbing species and their physical properties remains unknown because of their extremely small sizes. Previous experimental evidence provided by the atomic force microscopy mapping of damage morphology points to a few-nanometer scale of these absorbers. This work demonstrates the submicrometer-resolution mapping of 355-nm absorption in HfO2 monolayers using a recently developed photothermal heterodyne imaging technique. The comparison of absorption maps with the atomic force microscopy investigation of pulsed-laser-induced damage morphology allows one to better estimate the spatial distribution of nanoscale absorbing defects in hafnia thin films. Possible defect-formation mechanisms are discussed. [ABSTRACT FROM AUTHOR]

Time-resolved x-ray self-emission imaging of hot spots in inertial confinement fusion experiments along several lines of sight provides critical information on the pressure and the transient morphology of the hot spot on the University of Rochester's OMEGA Laser System. At least three quasi-orthogonal lines of sight are required to infer the tomographic information of the hot spots of deuterium–tritium cryogenic layered implosions. OMEGA currently has two time-gated x-ray hot-spot imagers: the time-resolved Kirkpatrick–Baez x-ray microscope and the single-line-of-sight, time-resolved x-ray imager (SLOS-TRXI). The time-gated x-ray hot-spot imager (XRHSI) is being developed for use on OMEGA as the third line of sight for the high-yield operation of up to 4 × 1014 neutrons. XRHSI follows the SLOS-TRXI concept; however, it will have improved spatial and temporal resolutions of 5 μm and 20 ps, respectively. The simultaneous operation of the three instruments will provide 3-D reconstructions of the assembled hot-spot fuel at various times through peak thermonuclear output. The technical approach consists of a pinhole array imager and demagnifying time-dilation drift tube that are coupled to two side-by-side hybrid complementary metal–oxide semiconductor (hCMOS) image sensors. To minimize the background and to harden the diagnostics, an angled drift-tube assembly shifting the hCMOS sensors out of the direct line of sight and neutron shielding will be applied. The technical design space for the instrument will be discussed and the conceptual design will be presented. [ABSTRACT FROM AUTHOR]

Here, we present evidence, in the context of OMEGA cryogenic target implosions, that laser imprint, known to be capable of degrading laser-direct-drive target performance, plays a major role in generating fuel–ablator mix. OMEGA cryogenic target implosions show a performance boundary correlated with acceleration-phase shell stability; for sufficiently low adiabats (where the adiabat is the ratio of the pressure to the Fermi pressure) and high in-flight aspect ratios (IFAR's), the neutron-weighted shell areal density and neutron yield relative to the clean simulated values sharply decline. Direct evidence of Rayleigh–Taylor fuel–ablator mixing was previously obtained using a Si Heα backlighter driven by an ∼20-ps short pulse generated by OMEGA EP. The shadow cast by the shell shortly prior to stagnation, as diagnosed using backlit radiographs, shows a softening near the limb, which is evidence of an ablator–fuel mix region for a low-adiabat implosion (α ∼ 1.9, IFAR = 14) but not for a moderate adiabat implosion (α ∼ 2.5, IFAR = 10). We find good agreement between experimental and synthetic radiographs in simulations that model laser imprint and account for uncertainty in the initial ablator thickness. We further explore the role of other mechanisms such as classical instability growth at the fuel–ablator interface, species concentration diffusion, and long-wavelength drive and target asymmetries. [ABSTRACT FROM AUTHOR]

Backlighting is a powerful technique to observe the flow of cold and dense material in high-energy-density-plasma experiments. High-performance, direct-drive cryogenic deuterium-tritium (DT) implosions are a challenging backlighting configuration because of the low opacity of the DT shell, the high shell velocity, the small size of the stagnating shell, and the very bright self-emission of the hot core. A crystal imaging system with a Si Heα backlighter at 1.865 keV driven by ~20-ps short pulses from OMEGA EP was developed to radiograph the OMEGA cryogenic implosions. The high throughput of the crystal imaging system makes it possible to record high-quality images with good photon statistics and a spatial resolution of ~15 µm at 10% to 90% modulation. This imager has been used to study the evolution of preimposed mass-density perturbations in the ablator, to quantify the perturbations caused by the stalk that is used to mount the target, and to study the mix caused by laser imprint or small-scale debris on the target surface. Because of the very low opacity of DT relative to carbon, even 0.1% of mix of carbon into the DT ice can be reliably inferred from the images. With the current implosion designs, mix is only observed for an adiabat below α = 4. [ABSTRACT FROM AUTHOR]