Dan E. Bower

Full-aperture backscatter measurements on the National Ignition Facility

The National Ignition Facility’s full-aperture backscatter station (FABS) is described. The FABS uses five independent diagnostics on each of the four laser beams in the initial National Ignition Facility quad to measure the energy, power, spectrum, and near-field amplitude modulations of the stimulated Brillouin and stimulated Raman backscattered light. In initial tests CO2 and C5H12 gas-filled targets were used to create various laser–plasma interaction conditions which have shown the capability of producing ignition size laser plasmas with reflectivites on the order of 10%. Results are presented for tests in which 16 kJ on target produced between 0.3 and 2.5 kJ of backscattered light.

Laser coupling to reduced-scale hohlraum targets at the Early Light Program of the National Ignition Facility

A platform for analysis of material properties under extreme conditions, where a sample is bathed in radiation with a high temperature, is under development. Depositing maximum laser energy into a small, high-Z enclosure produces this hot environment. Such targets were recently included in an experimental campaign using the first four of the 192 beams of the National Ignition Facility [J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, 755 (1994)], under construction at the University of California Lawrence Livermore National Laboratory. These targets demonstrate good laser coupling, reaching a radiation temperature of 340 eV. In addition, there is a unique wavelength dependence of the Raman backscattered light that is consistent with Brillouin backscatter of Raman forward scatter [A. B. Langdon and D. E. Hinkel, Phys. Rev. Lett. 89, 015003 (2002)]. Finally, novel diagnostic capabilities indicate that 20% of the direct backscatter from these reduced-scale targets is in the polarization or...

Full aperture backscatter station measurement system on the National Ignition Facility

A full aperture backscatter station (FABS) target diagnostic has been activated on the first four beams of the National Ignition Facility. FABS measures both stimulated Brillouin scattering and stimulated Raman scattering with a suite of measurement instruments. Digital cameras and spectrometers record spectrally resolved energy for both P and S polarized light. Streaked spectrometers measure the spectral and temporal behavior of the backscattered light. Calorimeters and fast photodetectors measure the integrated energy and temporal behavior of the light, respectively. This article provides an overview of the FABS measurement system and detailed descriptions of the diagnostic instruments and the optical path.

Progress toward the development and testing of source reconstruction methods for NIF neutron imaging.

Development of analysis techniques for neutron imaging at the National Ignition Facility is an important and difficult task for the detailed understanding of high-neutron yield inertial confinement fusion implosions. Once developed, these methods must provide accurate images of the hot and cold fuels so that information about the implosion, such as symmetry and areal density, can be extracted. One method under development involves the numerical inversion of the pinhole image using knowledge of neutron transport through the pinhole aperture from Monte Carlo simulations. In this article we present results of source reconstructions based on simulated images that test the methods effectiveness with regard to pinhole misalignment.

Calibration of initial measurements from the full aperture backscatter system on the National Ignition Facility

The full aperture backscatter system provides a measure of the spectral power, and integrated energy scattered by stimulated Brillouin (348–354 nm) and Raman (400–800 nm) scattering into the final focusing lens of the first four beams of the NIF laser. The system was designed to provide measurements at the highest expected fluences with: (1) spectral and temporal resolution, (2) beam aperture averaging, and (3) near-field imaging. This is accomplished with a strongly attenuating diffusive fiber coupler and streaked spectrometer and separate calibrated time integrated spectrometers, and imaging cameras. A new technique determines the wavelength dependent sensitivity of the complete system with a calibrated Xe lamp. Data from the calibration system are combined with scattering data from targets to produce the calibrated power and energy measurements that show significant corrections due to the broad band calibrations.

One-dimensional neutron imager for the Sandia Z facility.

A multiinstitution collaboration is developing a neutron imaging system for the Sandia Z facility. The initial system design is for slit aperture imaging system capable of obtaining a one-dimensional image of a 2.45 MeV source producing 5x10(12) neutrons with a resolution of 320 microm along the axial dimension of the plasma, but the design being developed can be modified for two-dimensional imaging and imaging of DT neutrons with other resolutions. This system will allow us to understand the spatial production of neutrons in the plasmas produced at the Z facility.

The 27.3 meter neutron time-of-flight system for the National Ignition Facility

One of the scientific goals of the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory, Livermore CA, is to obtain thermonuclear ignition by compressing 2.2 mm diameter capsules filed with deuterium and tritium to densities approaching 1000 g/cm3 and temperatures in excess of 4 keV. Thefusion reaction d + t → n + a results in a 14.03 MeV neutron providing a source of diagnostic particles to characterize the implosion. The spectrum of neutrons emanating from the assembly may be used to infer the fusion yield, plasma ion temperature, and fuel areal density, all key diagnostic quantities of implosion quality. The neutron time-of-flight (nToF) system co-located along the Neutron Imaging System line-of-site, (NIToF), is a set of 4 scintillation detectors located approximately 27.3 m from the implosion source. Neutron spectral information is inferred using arrival time at the detector. The NIToF system is described below, including the hardware elements, calibration data, analysis methods, and an example of its basic performance characteristics.

The neutron imaging diagnostic at NIF (invited).

A neutron imaging diagnostic has recently been commissioned at the National Ignition Facility (NIF). This new system is an important diagnostic tool for inertial fusion studies at the NIF for measuring the size and shape of the burning DT plasma during the ignition stage of Inertial Confinement Fusion (ICF) implosions. The imaging technique utilizes a pinhole neutron aperture, placed between the neutron source and a neutron detector. The detection system measures the two dimensional distribution of neutrons passing through the pinhole. This diagnostic has been designed to collect two images at two times. The long flight path for this diagnostic, 28 m, results in a chromatic separation of the neutrons, allowing the independently timed images to measure the source distribution for two neutron energies. Typically the first image measures the distribution of the 14 MeV neutrons and the second image of the 6-12 MeV neutrons. The combination of these two images has provided data on the size and shape of the burning plasma within the compressed capsule, as well as a measure of the quantity and spatial distribution of the cold fuel surrounding this core.A neutron imaging diagnostic has recently been commissioned at the National Ignition Facility (NIF). This new system is an important diagnostic tool for inertial fusion studies at the NIF for measuring the size and shape of the burning DT plasma during the ignition stage of Inertial Confinement Fusion (ICF) implosions. The imaging technique utilizes a pinhole neutron aperture, placed between the neutron source and a neutron detector. The detection system measures the two dimensional distribution of neutrons passing through the pinhole. This diagnostic has been designed to collect two images at two times. The long flight path for this diagnostic, 28 m, results in a chromatic separation of the neutrons, allowing the independently timed images to measure the source distribution for two neutron energies. Typically the first image measures the distribution of the 14 MeV neutrons and the second image of the 6-12 MeV neutrons. The combination of these two images has provided data on the size and shape of the burning plasma within the compressed capsule, as well as a measure of the quantity and spatial distribution of the cold fuel surrounding this core.

Performance characteristics of the neutron imaging diagnostic at NIF

The neutron imaging diagnostic has recently been commissioned at the National Ignition Facility. We will present the diagnostic performance characteristics, which have been measured with the collection of these first neutron images. The goal for this diagnostic is to collect two pinhole images at two different times. The long flight path results in a chromatic separation of the neutrons, the first image will be of the 14 MeV neutrons and the second image of the 10–12 MeV neutrons. The combination of these two images will provide data on the size and shape of the compressed capsule as well as a measure of the quantity and spatial distribution of the cold fuel surrounding this core. The imager uses an array of 20 pinholes and three mini-penumbra machined in 20 cm of layered gold and tungsten, with an apex at 32.5 cm from the source to produce images in a scintillator array at 2800 cm. This geometry provides a magnification factor of 85 at the scintillator. The scintillator is a coherent array of scintillating fibers, which is viewed from the two ends by two fast-gated image collection systems. The first neutron images, collected in February, 2011, have provided the first measure of system performance at NIF. These results will be presented along with an interpretation of future system performance.

Alignment commissioning of the neutron imager for the national ignition facility

Summary form only given. We have installed the National Ignition Facility Neutron Imaging System. The imaging system provides information about the areal density of fuel in the various regions of the capsule implosion. A long line of sight enables imaging both the primary 14 MeV neutrons as well as the down-scattered neutrons with energies in the range of 9-12 MeV. The imager is a pinhole camera where the pinhole is located 325 mm from the target and the imaging plane is located 28 m from the target. The long absorption length of neutrons requires precise alignment of these extended imager components in order to have high detection efficiency and strong background rejection. The imaging plane is a 150 mm square scintillating fiber bundle 50 mm thick. The bundle consists of 250 μm square fibers. The fiber bundle is aligned such that the fibers point at the target. The pointing is accomplished using a retro reflection of a laser alignment device aligned to the imager line of sight. The 200 mm long pinhole is aligned to the imager line of sight using the Opposed Port Alignment System. The imager line of sight was registered to the alignment system to calculate the precise position of the pinhole. The ability of the positioning manipulator to both place and maintain the precise location of the pinhole, were monitored using the positioning imaging system as well as laser alignment device. Here we describe the various aspects of the imaging system alignment to enable the acquisition of neutron images as well as the estimated pointing errors of the first shot imaged.