David D. Clark

The National Ignition Facility Neutron Imaging System

The National Ignition Facility (NIF) is scheduled to begin deuterium-tritium (DT) shots possibly in the next several years. One of the important diagnostics in understanding capsule behavior and to guide changes in Hohlraum illumination, capsule design, and geometry will be neutron imaging of both the primary 14 MeV neutrons and the lower-energy downscattered neutrons in the 6-13 MeV range. The neutron imaging system (NIS) described here, which we are currently building for use on NIF, uses a precisely aligned set of apertures near the target to form the neutron images on a segmented scintillator. The images are recorded on a gated, intensified charge coupled device. Although the aperture set may be as close as 20 cm to the target, the imaging camera system will be located at a distance of 28 m from the target. At 28 m the camera system is outside the NIF building. Because of the distance and shielding, the imager will be able to obtain images with little background noise. The imager will be capable of imaging downscattered neutrons from failed capsules with yields Y(n)>10(14) neutrons. The shielding will also permit the NIS to function at neutron yields >10(18), which is in contrast to most other diagnostics that may not work at high neutron yields. The following describes the current NIF NIS design and compares the predicted performance with the NIF specifications that must be satisfied to generate images that can be interpreted to understand results of a particular shot. The current design, including the aperture, scintillator, camera system, and reconstruction methods, is briefly described. System modeling of the existing Omega NIS and comparison with the Omega data that guided the NIF design based on our Omega results is described. We will show NIS model calculations of the expected NIF images based on component evaluations at Omega. We will also compare the calculated NIF input images with those unfolded from the NIS images generated from our NIS numerical modeling code.

First Light with RATIR: An Automated 6-band Optical/NIR Imaging Camera

The Reionization and Transients InfraRed camera (RATIR) is a simultaneous optical/NIR multi-band imaging camera which is 100% time-dedicated to the followup of Gamma-ray Bursts. The camera is mounted on the 1.5-meter Johnson telescope of the Mexican Observatorio Astronomico Nacional on Sierra San Pedro Martir in Baja California. With rapid slew capability and autonomous interrupt capabilities, the system will image GRBs in 6 bands (i, r, Z, Y, J, and H) within minutes of receiving a satellite position, detecting optically faint afterglows in the NIR and quickly alerting the community to potential GRBs at high redshift (z>6-10). We report here on this Springs first light observing campaign with RATIR. We summarize the instrumental characteristics, capabilities, and observing modes.

Automation of the OAN/SPM 1.5-meter Johnson telescope for operations with RATIR

The Reionization And Transients Infra-Red (RATIR) camera is intended for robotic operation on the 1.5-meter Harold Johnson telescope of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico. This paper describes the work we have carried out to successfully automate the telescope and prepare it for RATIR. One novelty is our use of real-time absolute astrometry from the finder telescopes to point and guide the main telescope.

Neutron imaging development for megajoule scale inertial confinement fusion experiments

Neutron imaging of Inertial Confinement Fusion (ICF) targets is useful for understanding the implosion conditions of deuterium and tritium filled targets at Mega-Joule/Tera-Watt scale laser facilities. The primary task for imaging ICF targets at the National Ignition Facility, Lawrence Livermore National Laboratory, Livermore CA, is to determine the asymmetry of the imploded target. The image data, along with other nuclear information, are to be used to provide insight into target drive conditions. The diagnostic goal at the National Ignition Facility is to provide neutron images with 10 μm resolution and peak signal-to-background values greater than 20 for neutron yields of ~ 1015. To achieve this requires signal multiplexing apertures with good resolution. In this paper we present results from imaging system development efforts aimed at achieving these requirements using neutron pinholes. The data were collected using directly driven ICF targets at the Omega Laser, University of Rochester, Rochester, NY., and include images collected from a 3 × 3 array of 15.5 μm pinholes. Combined images have peak signal-to-background values greater than 30 at neutron yields of ~ 1013.

Neutron imaging for inertial confinement fusion experiments

Neutron imaging of Inertial Confinement Fusion (ICF) targets provides a powerful tool for understanding the implosion conditions of deuterium and tritium filled targets at Mega-Joule/Tera-Watt scale laser facilities. The primary purpose of imaging ICF targets at that National Ignition Facility (NIF), sited at Lawrence Livermore National Laboratory, Livermore, California, is to determine the asymmetry of the fuel in an imploded ICF target. The image data are then combined with other nuclear information to gain insight into the laser and radiation conditions used to drive the target. This information is requisite to understanding the physics of Inertial Confinement Fusion targets and provides a failure mode diagnostic used to optimize the conditions of experiments aimed at obtaining ignition. We present an overview of neutron aperture imaging including a discussion of image formation and reconstruction, requirements for the future (NIF) neutron imaging systems, a description of current imaging system capabilities, and ongoing work to affect imaging systems capable of meeting future system requirements.

Mechanical design and integration of the support structure for the Reionization And Transients InfRared Instrument RATIR

In this article we present the mechanical design and the manufacturing of the support structure for the Reionization And Transients InfraRed (RATIR) camera. The instrument is mounted at the f/13 Cassegrain focus of the 1.5-meter Harold Johnson telescope of the Observatorio Astronómico Nacional at San Pedro Mártir (OAN/SPM) in Mexico. We describe the high-level requirements and explain their translation to the mechanical specifications and requirements. We describe the structural finite-element analysis and the boundary conditions, loads, and general assumptions included in the simulations. We summarize the expected displacements, rotations and stresses. We present the optomechanical components and the elements used to attach the instrument to the telescope. Finally, we show the instrument installed on the telescope.

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.

Weld quality evaluation using a high-temperature SQUID array

This paper presents preliminary data for evaluating weld quality using high temperature SQUIDS. The SQUIDS are integrated into an instrument known as the SQUID Array Microscope, or SAMi. The array consists of ll SQUIDs evenly distributed over an 8.25 mm baseline. Welds are detected using SAMi by using an on board coil to induce eddy currents in a conducting sample and measuring the resulting magnetic fields. The concept is that the induced magnetic fields will differ in parts of varying weld quality. The data presented here was collected from three stainless steel parts using SAMi. Each part was either solid, included a good weld, or included a bad weld. The induced magnetic fields magnitude and phase relative to the induction signal were measured. For each sample considered, both the magnitude and phase data were measurably different than the other two samples. These results indicate that it is possible to use SAMi to evaluate weld quality.

Digital spatial filter made from a SQUID array

A linear array of high transition temperature SQUID, has been configured to work as digital spatial filters. The SQUID filters can be configured to obtain a desired spatial frequency response. We describe their design using a technique known as Frequency Sampling, which consists of specifying a discrete spatial frequency response for the SQUID array spatial filter. We then apply that sequence to the inverse Discrete Fourier Transform to get the spatial domain gain of each SQUID and the baseline. Several designs for spatial band-pass filters are proposed and checked experimentally. The results show that while keeping or even increasing the noise rejection obtained with conventional gradiometers, the SQUID filters preserve the original signal characteristics of the source.

Testing of a gamma ray imaging system at Omega

Successful images of hard x-rays were taken at the OMEGA Laser at the Laboratory for Laser energetics ant he University of Rochester. This facility served as a surrogate for the National Ignition Facility for which this system was designed. Eleven plastic shells filled with 3He pellets were imploded producing soft and hard x-rays. As the system was designed to image 4.44MeV gammas the hard x-rays were of particular interest. These bremsstrahlung x-rays were emitted for the outer plastic shell and imaged using the gamma ray imaging system 13 meters away. A number of filtering arrangements were used to do transmission radiography of the source providing spectrum information. A 200-micron pinhole aperture was used to image the source. These shots provide information critical in characterizing the performance of the system