Automated alignment of the Advanced Radiographic Capability (ARC) target area at the National Ignition Facility

The Advanced Radiographic Capability (ARC) at the National Ignition Facility (NIF) is a petawatt-class, short-pulse laser system designed to provide x-ray backlighting of NIF targets. ARC uses four NIF beamlines to produce eight beamlets to create a sequence of eight images of an imploding fuel capsule using backlighting targets and diagnostic instrumentation. ARC employs a front end that produces two pulses, chirps the pulses out to 2 ns, and then injects the pulses into the two halves of each of four NIF beamlines. These pulses are amplified by NIF pre- and main amplifiers and transported to compressor vessels located in the NIF target area. The pulses are then compressed and pointed into the NIF target chamber where they impinge upon an array of backlighters. The interaction of the ARC laser pulses and the backlighting material produces bursts of high-energy x-rays that illuminate an imploding fuel capsule. The transmitted x-rays are imaged by diagnostic instrumentation to produce a sequence of radiograph images. A key component of the success of ARC is the automatic alignment system that accomplishes the precise alignment of the beamlets to avoid damaging equipment and to ensure that the beamlets are directed onto the tens-of-microns scale backlighters. In this paper, we describe the ARC automatic alignment system, with emphasis on control loops used to align the beampaths. We also provide a detailed discussion of the alignment image processing, because it plays a critical role in providing beam centering and pointing information for the control loops.