The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) uses the world’s largest and most energetic laser system to explore High-Energy-Density (HED) physics. Historically, experiments at the NIF could not radiograph an Inertial Confinement Fusion (ICF) experiment at late times due to self-emission from the capsule. The Crystal Backlighter Imager diagnostic (CBI) fielded on NIF in 2017 and has allowed radiography of ICF capsules at late times. This capability is due to the very narrow bandwidth of the imaging system, which eliminates much of the self-emission. X-rays from a backlighter source (driven by NIF beams) pass through the experiment, and the CBI uses a spherically curved crystal to reflect these x-rays at near-normal incidence (Bragg angle close to 90°) onto the detector, resulting in a very narrow bandwidth microscope.
The geometry of a near-normal-incidence microscope is challenging to implement at the NIF, since the crystal must be positioned and aligned to high precision on the opposite side of the target relative to the detector. The in-chamber alignment procedure cannot take significantly longer than a simple pinhole imager, since demand for NIF shots is high and a given experiment is allotted a strict time limit. Avoiding any collision between diagnostic hardware and the target is paramount and any instrument that is placed in close proximity to a target must be able to withstand the debris produced by a 2.0 MJ NIF shot.
CBI overcomes these challenges by mounting the detector and crystal on a single diagnostic instrument manipulator (DIM). The crystal is mounted on an arm that passes around the target, positioning the crystal on the opposite side of the target to the detector. This allows much of the crystal alignment to be done before the instrument is inserted into the NIF chamber, saving time. The arm that supports the crystal is mechanized so that, during insertion of the CBI, the risk of collision with the target is minimized. The CBI is designed as a robust platform that is capable of maintaining alignment tolerances of <200 microns relative to the target, as well as survive the harsh loading on the mechanical components during a NIF 2.0 MJ energy experiment. This paper discusses the engineering challenges of
the CBI system.