Larry J. Suter

INDIRECTLY DRIVEN, HIGH GROWTH RAYLEIGH-TAYLOR IMPLOSIONS ON NOVA

Abstract Indirectly-driven implosions for which the predicted Rayleigh-Taylor (RT) instability growth rates of pre-imposed capsule surface perturbations are substantially increased by mid-Z-doping of the ablators have been fielded on the Nova laser. The multiple effects on implosion performance of the additional x-ray opacity provided by the ablator dopant is discussed. For best surface finish capsules, the addition of increasing ablator dopant levels is shown to improve the neutron yield. However, as capsule surface roughness is increased, so that RT instability growth increases, this trend is reversed, leading to decreasing yields with increased dopant content. The RT-induced mixing between shell and fuel is further investigated by diagnosing the x-ray emission levels and time histories from Ti and Ar dopants in capsules with predetermined surface roughness. The x-ray line ratios show the expected decrease in fuel temperature with increasing surface roughness. The spectral content, intensity and duration of the Ti spectra, however, suggest 2- or 3-D rather than just 1-D effects are important so that higher than 1-D models of the mix region may be needed.

Efficient multi-keV x-ray sources from Ti-doped aerogel targets

We have measured the production of hν equal to or greater than 4.5 keV x-rays from low-density Ti-doped aerogel targets at the OMEGA laser facility (University of Rochester). The targets were 2.2 mm long by 2 mm diameter beryllium cylinders filled with Ti-doped (3 atomic percent) SiO2 foam. The doped-foam density was ≈ 3 mg/cc. Forty beams of the OMEGA laser (λ = 351 nm) illuminated the two cylindrical faces of the target with a total power that ranged from 7 to 14 TW. The laser interaction fully ionizes the target (formula available in paper), and allows the laser-bleaching wave to excite, supersonically, the high-Z emitter ions in the sample. The heating of the target was imaged with a gated (200 ps time resolution) x-ray framing camera filtered to observe > 4 keV. 2-D radiative-hydrodynamic calculations predict rapid and uniform heating over the whole target volume with minimal energy losses into hydrodynamic motion. An x-ray streak camera, also filtered to observe > 4 keV, was used to measure the rate of heat propagation in the target. Ti K-shell x-ray emission was spectrally resolved with a two-channel crystal spectrometer and also with a set of filtered aluminum x-ray diodes, both instruments provide absolute measurement of the multi-keV x-ray emission. Back-scattered laser energy is observed to be minimal. We find between 100 to 400 J of output with 4.67 equal to or less than hv equal to or less than 5.0 keV, predicted target performance is a factor of 2 - 3 too low in this range.

Multi-Kilovolt X-Ray Conversion Efficiencies

X-ray sources in the 3-7 keV energy regime can be produced by laser-irradiating mid- and high-Z gas-filled targets with high-powered lasers. A series of experiments have been performed using underdense targets that are supersonically heated with approximately 35 kJ of 0.35 micrometers laser light. These targets were cylindrical Be enclosures that were filled with 1-2 atms of Xe or Ar gas. L-shell x-ray emission is emitted from the plasma and detected by Bragg crystal spectrometers and x-ray diodes. Absolute flux measurements show conversion efficiencies of approximately 10 percent in the multi-kilovolt x-ray emission. These sources can be used as bright x-ray backlighters or for material testing.

Modeling with backscatter and transmitted light of high-power smoothed beams with pF3D: a massively parallel laser plasma interaction code

Using the 3D wave propagation code, F3D, Berger et al., Phys. Plasmas 5,4337, and the massively parallel version pF3D, we have computed the transmitted and reflected light for laser and plasma conditions in experiments that simulated ignition hohlraum conditions. The frequency spectrum and the wavenumber spectrum of the transmitted light are calculated and used to identify the relative contributions of stimulated forward Brillouin and self- focusing in hydrocarbon-filled balloons, commonly called gasbags. The effect of beam smoothing, smoothing by spectral dispersion and polarization smoothing, on the stimulated Brillouin backscatter from Scale-1 NOVA hohlraums was simulated with the use of nonlinear saturation models that limit the amplitude of the driven acoustic waves. Other experiments on CO2 gasbags simultaneously measure at a range of intensities the SBS reflectivity and the Thomson scatter from the SBS-driven acoustic waves that provide a more detailed test of the modeling. These calculations also predict that the backscattered light will be very nonuniform in the near field which is important for specifying the backscatter intensities to be tolerated by the National Ignition Facility laser system.

High-power laser source evaluation

This document reports progress in these areas: EXPERIMENTAL RESULTS FROM NOVA: TAMPED XENON UNDERDENSE X-RAY EMITTERS; MODELING MULTI-KEV RADIATION PRODUCTION OF XENON-FILLED BERYLLIUM CANS; MAPPING A CALCULATION FROM LASNEX TO CALE; HOT X RAYS FROM SEEDED NIF CAPSULES; HOHLRAUM DEBRIS MEASUREMENTS AT NOVA; FOAM AND STRUCTURAL RESPONSE CALCULATIONS FOR NIF NEUTRON EXPOSURE SAMPLE CASE ASSEMBLY DESIGN; NON-IGNITION X-RAY SOURCE FLUENCE-AREA PRODUCTS FOR NUCLEAR EFFECTS TESTING ON NIF. Also appended are reprints of two papers. The first is on the subject of ``X-Ray Production in Laser-Heated Xe Gas Targets.`` The second is on ``Efficient Production and Applications of 2- to 10-keV X Rays by Laser-Heated Underdense Radiators.``

Production of multikilovolt x rays from laser-heated targets

Experiments to develop high photon energy x-ray sources were carried out on the Nova laser. Ten laser beams delivered approximately 39 kJ of energy in 2 ns into a Be cylinder filled with Xe gas. The conversion efficiency into x-rays > 4 keV was measured to be 5 - 15%, which is the highest measured in this photon regime for laser-produced plasmas. The temporal dependence of the x-ray emission indicates that the bulk of the emission is emitted in the first half of the 2 ns pulse. A set of diagnostics were fielded to image the volume in emission as well as provide spectra to measure conversion efficiency.

High‐yield direct drive experiments at Nova

A series of direct drive implosions of D‐T filled, glass microballoons has been initiated in an effort to produce intense neutron sources for diagnostics development. In particular, measurements such as neutron imaging, burn history using a neutron‐sensitive streak camera, and fuel  ρr  by both neutron activation and neutron spectroscopy could be more easily tested with these simple targets than high‐density targets that initially are not expected to produce such high yields. Targets with nominal dimensions of 1000×2 μm which contained D‐T at 12–14 atm were imploded with 18 kJ of 0.35‐μm light from the 10‐beam Nova system. For pulse widths of 1 ns in either converging or diverging geometry, fusion yields in excess of 1013 have been obtained. The yields are comparable to those predicted from numerous 1‐D model calculations in spite of the asymmetric illumination geometry of the driver. Work performed under the auspices of the U.S. DOE by Lawrence Livermore National Laboratory under contract number W‐7405‐E...