Analysis of the confluence of three patterns using the Centering and Pointing System (CAPS) images for the Advanced Radiographic Capability (ARC) at the National Ignition Facility

The Advance Radiographic Capability (ARC) at the National Ignition Facility (NIF) is a laser system that employs up to four petawatt (PW) lasers to produce a sequence of short pulses that generate X-rays which backlight highdensity internal confinement fusion (ICF) targets. Employing up to eight backlighters, ARC can produce an X-ray "motion picture" to diagnose the compression and ignition of a cryogenic deuterium-tritium target with tens-ofpicosecond temporal resolution during the critical phases of an ICF shot. Multi-frame, hard-X-ray radiography of imploding NIF capsules is a capability which is critical to the success of NIF's missions. The function of the Centering and Pointing System (CAPS) in ARC is to provide superimposed near-field and far-field images on a common optical path. The Images are then analyzed to extract beam centering and pointing data for the control system. The images contain the confluence of pointing, centering, and reference patterns. The patterns may have uneven illumination, particularly when the laser is misaligned. In addition, the simultaneous appearance of three reference patterns may be co-incidental, possibly masking one or more of the patterns. Image analysis algorithms have been developed to determine the centering and pointing position of ARC from these images. In the paper we describe the image analysis algorithms used to detect and identify the centers of these patterns. Results are provided, illustrating how well the process meets system requirements.