Alignment commissioning of the neutron imager for the national ignition facility

Abstract

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.