LLE Review 147

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Highlights

This volume of the LLE Review, covering April–June 2016, features “Measurements of the Effect of Adiabat on Shell Decompression in Direct-Drive Implosions on OMEGA.” This article examines the maximum in-flight shell thickness in direct-drive implosions on OMEGA for various shell adiabats. Two-dimensional simulations that included laser imprint, nonlocal thermal transport, cross-beam energy transfer, and first-principles equation-of-state models reproduced the measured shell thickness, shell trajectories, minimum core radius, and neutron yield and showed that the increased shell thickness for α ≤ 3 is caused by laser imprint.

Additional highlights of research presented in this issue include the following:

• Experiments have been performed with a prototype multi-FM smoothing by spectral dispersion (SSD) system integrated into the National Ignition Facility (NIF)-like beamline of the OMEGA EP Laser System to verify the smoothing performance by measuring Rayleigh–Taylor growth rates in planar targets of laser-imprinted and preimposed surface modulations. Multi-FM 1-D SSD has been observed to reduce imprint levels by ~50% compared to the nominal OMEGA EP SSD system. The experimental results agree with 2-D DRACO simulations using realistic, time-dependent, far-field spot-intensity calculations that emulate the effect of SSD.
• Simultaneous measurements of ion-acoustic and electron plasma wave spectra were obtained using a 263.25-nm. Thomson-scattering probe beam, which is an important diagnostic technique for measuring the plasma conditions in laser-plasma experiments. A fully reflective collection system was used to record light scattered from electron plasma waves at electron densities greater than 10²¹ cm^–3, which produced scattering peaks near 200 nm. An accurate analysis of the experimental Thomson-scattering spectra was performed, taking into account plasma gradients, instrument sensitivity, optical effects, and background radiation. Including these effects when fitting Thomson-scattering spectra to the measured spectra significantly improves the plasma characterization.
• A nine-channel image-plate–based detector absolute x-ray spectrum was used to infer the temperature of the hot electrons from nanosecond laser– plasma interaction experiments. The measured temperatures are consistently lower than those measured by a three channel fluorescence-photomultiplier detector (by a factor ~1.5 to 1.7). The measurements were supplemented with three experiments that measured the hot-electron temperature using K_α emission from high-Z target layers, independent of the hard x-ray bremsstrahlung emission. These experiments yielded temperatures that were consistent with those measured by the nine-channel image-plate–based detector. For a given x-ray emission in inertial confinement fusion compression experiments, this result would lead to a higher total energy in hot electrons, but to a lower preheat of the compressed fuel, because of the reduced hot-electron range.
• A high-throughput, broadband optical spectrometer coupled to the Rochester Optical Streak System was designed to record time-resolved spectra with 1-ps time resolution. A spectral resolution of 0.8 nm has been achieved over a wavelength coverage range of 480 to 580 nm. The overall pulse-front tilt across the beam diameter generated by the diffraction grating is reduced by delaying discrete segments of the collimated input beam using a 34-element reflective echelon optic. The resulting spectrometer design balances resolving power and pulse-front tilt while maintaining high throughput.
• An ultrafast (<2-ps), streaked, extreme-ultraviolet (XUV) spectrometer (5 to 20 nm) has been developed to measure the temperature dynamics in rapidly heated samples irradiated by a high-intensity picosecond laser. The surface-temperature measurement constrains models for the release of high-energy-density material.
• A study is presented on the influence of surface modifications on the adsorption and absorption of tritium into stainless steel. The effect of altering the metal surface by mechanical polishing, electropolishing, Fe or Cr oxidation, gold plating, and nitric-acid treatments was studied using linear thermal desorption and plasma-induced ion sputtering. The results demonstrate that altering the metal surface can reduce tritium absorption by ≥35%. Finally, a quantitative migration model accurately describes the migration of tritium out of the stainless-steel lattice after the surface is cleaned.