The National Ignition Facility - Beam Area Increase

he National Ignition Facility (NIF) is the world's most energetic laser, having demonstrated in excess of 1.9MJ @351nm with Inertial Confinement Fusion pulse-shapes in July, 2012. First commissioned with 192 operational beamlines in March, 2009, NIF has since transitioned to routine operation for stockpile stewardship, inertial confinement fusion research, and basic high energy density science. The NIF design includes component placement and beam alignment tolerances to preclude laser beam clipping on components within the laser chain, indeed lengthy studies and analyses, including various statistical approaches, were done in the design phase as early as 1996. The margin between the available optical aperture and the beam was established to ensure, given beam centering variations and component placement errors, that we would achieve a confidence level such that even low-level clipping, which causes downstream modulation damage, would occur at an acceptably or even vanishingly low rate. With the completion of NIF and nearly 4 years of operational experience, it became apparent that we could increase the beam size to more optimally fill the available aperture, and gain an additional 5% to 10% or more energy and power delivered to targets. It was also shown that additional energy could be recovered by removing approximately 70% of our beam 'corner blockers' originally installed in May 2010 to prevent target-chamber 1 mu m counterpropagating light from leaking back through the extinction minimums at the corners of vacuum-loaded square optics. Subsequent analyses showed that only one and in some cases two of the corner blockers were really needed. Increasing the beam size was a challenging endeavor, however, as it fundamentally meant recommissioning the entire NIF laser chain to tailor all 192 beams to their specific available aperture, individual beam rotation (for the NIF square beam), beam centering offsets, change-out of the 48 front-end aperture (relay-plane "0"), and removal of 48 Laser Mirror #2 line replaceable units for corner-blocker removal. Some of this commissioning, such as tailoring beam sizes to their specific available aperture, had not been performed during the original commissioning. Furthermore, achieving this required precise diagnostics and rapid analysis of massive quantities of images and data in order to direct the changes and feed-back the achieved results. Completed on June 1, 2012, the beam area was increased by 7.5%, and was a significant contributing factor in NIF transitioning from a 1.6MJ laser to its present 1.9MJ capability