[en] This article describes 250, 280, and 350 eV drive temperature copper-doped Be [Be(Cu)] two-dimensional (2-D) capsule-hohlraum designs for the National Ignition Facility (NIF) [Paisner , Laser Focus World 30, 75 (1994)]. These capsule-hohlraum designs are a follow-on to the previous one-dimensional capsule designs of Bradley and Wilson [Phys. Plasmas 6, 4293 (1999)]. It is shown that a 2-D 350 eV capsule-hohlraum design scaled from the successful 330 eV design does not ignite, mostly due to poor symmetry. In addition, the 350 eV capsule hohlraum design requires the full 500 TW of the NIF design and 1.66 MJ of the maximum 1.8 MJ designed energy output. It is possible to design a capsule-hohlraum combination that achieves ignition and burn with peak radiation temperatures of 250, 280, and 330 eV. These designs use from 1.3 to 1.6 MJ of laser energy and the successful designs have yields of 16--20 MJ. Changes in symmetry and yield due to changing the focal point of the inner and outer laser cones were examined. The 280 eV capsule can tolerate pointing changes of 40--100 μm before the yield drops by 50%, while even a 40 μm pointing change for the 250 eV capsule causes the yield to drop by a factor of 2 to 100

[en] We are developing a sensitive neutron spectrometer for the National Ignition Facility laser at Livermore. The spectrometer will consist of a 1020 channel single-neutron-interaction time-of-flight detector array fielded 23 m from the neutron-producing target. It will use an existing detector array together with upgraded electronics for improved time resolution. Measurements of neutron yield, ion and electron temperatures, and density-radius product are all possible under certain conditions using one-, two-, or three-step reaction processes. The locations of the most important potential sources of scattered neutron backgrounds are determined as the first step in designing collimation to reduce these backgrounds

[en] At Los Alamos, we have been working on two-dimensional capsule-Hohlraum implosion calculations for the NIF that utilize a laser drive pulse that peaks at ∼300 eV and uses about 1 MJ of laser energy. We use a 0.3 at. % uniformly Cu doped beryllium ablator capsule that has an inner ice radius of 753 μm, inner ablator radius of 825 μm, and an outer ablator radius of 1000 μm. Our Hohlraum has a diameter of 5.02 mm and a length of 8.56 mm. We use a Hohlraum gas fill density of 1.3 mg/cm³ and obtain ignition with yields up to 9.8 MJ, but we also have some failures due to bad shock timing or different pointing positions of the inner and outer laser cones. We describe the salient features of our implosions and show postprocessed diagnostic signatures from x-ray and neutron images, along with reaction history plots to show how we can diagnose 'failures' and how to correct for them based on these diagnostics

[en] Fusion neutrons streaming from a burning inertial confinement fusion capsule generate gamma rays via inelastic nuclear scattering in the ablator of the capsule. The intensity of gamma-ray emission is proportional to the product of the ablator areal density (ρR) and the yield of fusion neutrons, so by detecting the gamma rays we can infer the ablator areal density, provided we also have a measurement of the capsule's total neutron yield. In plastic-shell capsules, for example, ¹²C nuclei emit gamma rays at 4.44 MeV after excitation by 14.1 MeV neutrons from D+T fusion. These gamma rays can be measured by a new gamma-ray detector under development. Analysis of predicted signals is in progress, with results to date indicating that the method promises to be useful for diagnosing imploded capsules.

[en] Generation of debris from targets and by x-ray ablation of surrounding materials will be a matter of concern for experimenters and National Ignition Facility (NIF) operations. Target chamber and final optics protection, for example debris shield damage, drive the interest for NIF operations. Experimenters are primarily concerned with diagnostic survivability, separation of mechanical versus radiation induced test object response in the case of effects tests, and radiation transport through the debris field when the net radiation output is used to benchmark computer codes. In addition, radiochemical analysis of activated capsule debris during ignition shots can provide a measure of the ablator <ρr>. Conceptual design of the Debris Monitor and Rad-Chem Station, one of the NIF core diagnostics, is presented. Methods of debris collection, particle size and mass analysis, impulse measurement, and radiochemical analysis are given. A description of recent experiments involving debris collection and impulse measurement on the OMEGA and Pharos lasers is also provided

[en] Initial experiments were performed at the OMEGA laser [J. Soures et al., Phys. Plasmas 3, 2108 (1996)] to investigate the physics associated with inertial confinement fusion capsule fill tubes and holes. These experiments were performed in planar geometry and examined the hydrodynamics of a 6.7:1 aspect ratio fill-hole. X-ray radiographs at 310 eV show a jet has formed due to the interaction between the temperature drive and the beryllium (Be) washer

[en] Results are shown from recent experiments at the Omega laser facility, using 40 Omega beams driving the hohlraum with 3 cones from each side and up to 19.5 kJ of laser energy. Beam phasing is achieved by decreasing the energy separately in each of the three cones, by 3 kJ, for a total drive energy of 16.5 kJ. This results in a more asymmetric drive, which will vary the shape of the imploded symmetry capsule core from round to oblate or prolate in a systematic and controlled manner. These results show the sensitivity of capsule implosion symmetry for implosions in 'high temperature' (275 eV) hohlraums at Omega. Dante measurements confirmed the predicted peak drive temperatures of 275 eV. Implosion core time dependent x-ray images were obtained from framing camera data which show the expected change in symmetry due to beam imbalance and which also agree well with post processed hydro code calculations.

[en] We present an evaluation of the effects of a modified final laser pulse on capsule energetics and Laser-Plasma Interactions (LPI) for a National Ignition Facility cryogenic ignition target. A point design laser pulse is well optimized for the capsule and has reasonably good expected LPI performance. However, we demonstrate that reduced exposure to LPI backscatter is possible for an ignition target with a reduction in capsule margin by reducing the laser peak power and slightly re-balancing the beam energy. The benefit of this reduced LPI exposure is expected to be highest if the ignition target LPI exhibits a threshold intensity near the laser pulse peak intensity. In this case, it may be possible to reduce LPI exposure while delivering adequate energy into the hohlraum and achieve adequate symmetry to ignite the capsule

[en] Both neutron images and spectra will diagnose ignition implosions at the National Ignition Facility. From the integrated Hohlraum and capsule calculations of copper-doped beryllium capsules using ∼1 MJ of laser energy we have postprocessed neutron spectra and both energy-gated and time integrated neutron images displaying the observable consequences of two-dimensional Hohlraum asymmetries. If low signal precludes multiple down scattered images, we suggest a 6-12 MeV gate. With asymmetries, it is noted that the neutron yield, spectra, and images vary with observation direction. The yield varies only several percent when observed at different angles. Since most asymmetries are expected about the Hohlraum axis, a perpendicular view has the highest priority. The next most informative view would be along the Hohlraum axis, but may be precluded by target chamber structures. We present images at the available port angles and discuss their utility. To facilitate detailed diagnostic simulations with real pinhole geometries or penumbral apertures, we offer a compact disk containing neutron spectra and gated images from various integrated calculations

[en] Neutron imaging is currently being developed as a primary diagnostic for inertial fusion studies at the National Ignition Facility (NIF). It is an attractive diagnostic for measuring asymmetries in the burn region and will be able to operate at neutron fluences found during ignition scale implosions. The most straightforward technique for imaging of the spatial distribution of deuterium-tritium (DT) fusion neutrons utilizes a simple pinhole aperture, which blocks all neutrons outside of the solid angle defined by the pinhole and results in a blurred image at the detector. We are currently investigating source image reconstruction techniques from detector images. Source reconstructions from Monte Carlo neutron transport (MCNP) calculations are shown to emulate hydrodynamic simulations with imposed Legendre asymmetries to high accuracy.