Felix P. Garcia

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.

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.

Fabrication of a 3X3 neutron pinhole array

Abstract Neutron imaging diagnostics are needed for understanding the principles of fusion ignition. Current experiments on the University of Rochester OMEGA laser facility and future experiments at the NIF require a new level of complexity in neutron diagnostics that has not yet been achieved. Previous shots have fielded a one dimensional pinhole array to gather an image of a sphere’s neutron emission during the implosion. This one dimensional pinhole array that consisted of two pinholes on a plane was a challenging manufacturing task and was a substantial accomplishment for its time. Future neutron imaging diagnostics will require a two dimensional pinhole array to gather a more comprehensive set of data. This two dimensional pinhole array, consisting of 3 pinholes one three planes to form a 3x3 array of pinholes, added a new level of complexity to the manufacturability. A method for fabricating this pinhole array was developed and the finished instrument was fielded in July and October 2006. This paper describes the fabrication process to producing this pinhole array and shows some of the early data taken with it at the Omega facility.

SPECIALIZED MACHINING TECHNIQUES FOR TARGET AND DIAGNOSTIC FABRICATION

Abstract During the course of machining targets for various experiments, it sometimes becomes necessary to take fixtures or machines that are designed for one function and adapt them to another function. When adapting a machine or fixture is not adequate, it may be necessary to acquire a machine specifically designed to produce the component required. In addition to the above scenarios, the features of a component may dictate that multistep machining processes are necessary to produce the component. This paper discusses the machining of four components where adaptation, specialized machine design, or multistep processes were necessary to produce the components.

Improvements in ICF target fabrication through high precision assembly and nondestructive characterization

Current ICF and HED targets are fielded on Omega, Z, and Trident; future campaigns will also be fielded on NIF. NIF will field less than 2 shots per day. With such few experiments, target fabrication and alignment accuracy, enhanced metrology and advanced component machining will be even more important. Future target designs are also becoming more complex and more stringent in terms of both manufacturing accuracy and precision. Several steps have been taken to improve the fabrication and characterization of targets, such as instituting an automated assembly station with 3 μm tolerances, utilizing non-destructive characterization tools for rapid component metrology and target assembly, and advancing machining capabilities. Recapitalization of target fabrication infrastructure is continuous.