R. R. Berggren

Target diagnostic system for the national ignition facility (invited)

A review of recent progress on the design of a diagnostic system proposed for ignition target experiments on the National Ignition Facility (NIF) will be presented. This diagnostic package contains an extensive suite of optical, x ray, gamma ray, and neutron diagnostics that enable measurements of the performance of both direct and indirect driven NIF targets. The philosophy used in designing all of the diagnostics in the set has emphasized redundant and independent measurement of fundamental physical quantities relevant to the operation of the NIF target. A unique feature of these diagnostics is that they are being designed to be capable of operating in the high radiation, electromagnetic pulse, and debris backgrounds expected on the NIF facility. The diagnostic system proposed can be categorized into three broad areas: laser characterization, hohlraum characterization, and capsule performance diagnostics. The operating principles of a representative instrument from each class of diagnostic employed in t...

Nuclear diagnostics for the National Ignition Facility (invited)

The National Ignition Facility (NIF), currently under construction at the Lawrence Livermore National Laboratory, will provide unprecedented opportunities for the use of nuclear diagnostics in inertial confinement fusion experiments. The completed facility will provide 2 MJ of laser energy for driving targets, compared to the approximately 40 kJ that was available on Nova and the approximately 30 kJ available on Omega. Ignited NIF targets are anticipated to produce up to 1019 DT neutrons. In addition to a basic set of nuclear diagnostics based on previous experience, these higher NIF yields are expected to allow innovative nuclear diagnostic techniques to be utilized, such as neutron imaging, recoil proton techniques, and gamma-ray-based reaction history measurements.

Observation of d-t fusion gamma rays (invited)

Deuterium–tritium (DT) reaction rates of imploding capsules have historically been measured using neutron detectors. Temporal resolution is limited by the size of the detector and distance from the source to detector. The reaction rates can also be measured using the 16.7 MeV gamma ray, which is produced by the same DT reaction, but statistically far less often than the 14.1 MeV neutron. Cherenkov detectors detect gamma rays by converting the gamma rays to electrons, which in turn produce Cherenkov light and record this visible light using a fast optical detector. These detectors can be scaled to large volumes in order to increase detection efficiency with little degradation in time resolution, and placed well away from the source since gamma rays do not suffer velocity dispersion between the source and detector. Gas-based Cherenkov detectors can also discriminate against lower-energy photons produced in and around the target. A prototype gas Cherenkov detector has been built and tested for detector respo...

Development of a neutron imaging diagnostic for inertial confinement fusion experiments

Pinhole imaging of the neutron production in laser-driven inertial confinement fusion experiments can provide important information about the performance of various capsule designs. This requires the development of systems capable of spatial resolutions on the order of 5 μm or less for source strengths of 1015 and greater. We have initiated a program which will lead to the achievement of such a system to be employed at the National Ignition Facility (NIF) facility. Calculated neutron output distributions for various capsule designs will be presented to illustrate the information which can be gained from neutron imaging and to demonstrate the requirements for a useful system. We will describe the lines-of-sight available at NIF for neutron imaging and explain how these can be utilized to reach the required parameters for neutron imaging. We will describe initial development work to be carried out at the Omega facility and the path which will lead to systems to be implemented at NIF. Beginning this year, pr...

Multipurpose 10 in. manipulator-based optical telescope for Omega and the Trident laser facilities

We have recently designed and are building a telescope which acts as an imaging light collector relaying the image to an optical table for experiment dependent analysis and recording. The expected primary use of this instrument is a streaked optical pyrometer for witness plate measurements of the hohlraum drive temperature. The telescope is based on the University of Rochester’s 10 in. manipulator (TIM) which allows compatibility between Omega, Trident, and the NIF lasers. The optics capture a f/7 cone of light, have a field of view of 6 mm, have a spatial resolution of 5–7 μm per line pair at the object plane, and are optimized for operation at 280 nm. The image is at a magnification of 11.7×, which is convenient for many experiments, but can be changed using additional optics that reside outside the TIM.

Gamma-ray-based fusion burn measurements

A gas Cerenkov detector with a 12-MeV threshold for gamma-raydetection has been built for use on the OMEGA laser system to record high-energy gamma rays emitted during DT gas burn. Recording the 16.7-MeV gamma ray while discriminating against the lower energy 14-MeV neutron-induced gammas is an important objective using this detector system. Detector design, sensitivity, and background studies were possible using the Integrated Tiger Series Monte Carlo code modified to include Cerenkov production and full time-history of all particles. The results of this code were iterated with the ASAP optics code to optimize the light collection system, while providing the radiation shielding and stray light baffles to minimize backgrounds. As an initial test of the instrument, 8–20 MeV electrons from the Idaho State University linear accelerator were used in lieu of gamma rays. The primary results of these tests are that electron-produced Cerenkov has been observed and the Cerenkov threshold curve established for this instrument.

Neutron time-of-flight and emission time diagnostics for the National Ignition Facility

Current plans call for a system of current mode neutron detectors for the National Ignition Facility for extending the range of neutron yields below that of the neutron activation system, for ion-temperature measurements over a wide yield range, and for determining the average neutron emission time. The system will need to operate over a yield range of 106 for the lowest-yield experiments to 1019 for high-yield ignited targets. The requirements will be satisfied using several detectors located at different distances from the target. This article presents a conceptual design for the NIF nToF system.

Pad polishing for rapid production of large flats

Pad polishing is an efficient technique for polishing-out a ground surface and reaching a figure better than one wave, ready for completion with less than an hour on a planetary polisher. Recent work has shown success on 350 mm square parts; current work involves scaling the process to 1.4 meter diameter. For the 350 mm square piece of BK7, removal was one micrometer every 10 minutes. Polishing-out from a 5 micrometer grind took less than 3 hours, to a surface smoothness of one nm rms. Other tests verified that the pad leaves no unusual subsurface damage. Following completion on a pitch planetary polisher, surface finish is the same as obtained for conventional processing.

Full aperture backscatter station imager diagnostics system for far-field imaging of laser plasma instabilities on Nova

In ICF, the understanding of laser plasma scattering processes is essential for laser target coupling and for controlling the symmetry of indirect drive implosions. The existing Nova full aperture backscatter station has been useful in understanding laser plasma instabilities occurring in hohlraums by measuring the quantity, spectral distribution, and near-field spatial distributions of Brillouin and more recently Raman backscatter. Equally important is an understanding of the far-field spatial intensity distribution which could help in understanding filamentation, threshold and saturation processes. This article describes a broadband, color-corrected far-field imager and associated diagnostics capable of imaging the source of scattered light to better than 25 μm resolution. Brillouin and Raman backscatter can be imaged through the Nova beam-7 focusing lens or the imager can be used like a microscope to image side scatter from other beams.

Optical design of an imaging spatial heterodyne infrared spectrometer

The optical design for an IR imaging spatial heterodyne spectrometer is described. The evolution of the design from a paraxial layout to a final working system is detailed, showing how the basic ideas outlined by the inventors of the ISHS concept were realized in an instrument operating in the long wave IR. Several design constraints unique to operation in the LWIR posed challenges to the design; the choices made to solve these problems are discussed. The predicted imaging performance of the design is modeled, and improvements to the design of a second generation instrument are proposed.