Transport mirror laser damage mitigation technologies on the National Ignition Facility

摘要

There are 830 transport mirrors with a combined surface area of approximately 255 m~2 of precision multilayer coatings deposited on 50 metric tons of BK7 glass in the high fluence transport section of the National Ignition Facility (NIF). With peak fluences over 20 J/cm2 at 1053 nm, less than five percent of these mirrors are exchanged annually due to laser damage since full system operations began in 2009. Multiple technologies have been implemented to achieve these low exchange rates. The coatings are complex dichroics designed to reflect the fundamental wavelength (1053 nm) and an alignment beam (374 nm) while suppressing target backscatter wavelengths (351 nm & 400-700 nm) from backward propagation up the beamlines. Each optic is off-line laser conditioned to nominally 50% over the average fluence and nominally 90% of the peak fluence allowing the final laser conditioning to occur on-line during NIF operations. Although the transport section of NIF is sealed in a clean argon environment, air knives were installed on upward facing transport mirrors to blow off particulates that could accumulate and initiate laser damage. Beam dumps were installed in between the final optics assembly and the final transport mirrors to capture ghost reflections from the anti-reflection coated surfaces on the transmissive optics used for polarization rotation, frequency conversion, and focusing the 192 laser beams on target. Spot blockers, normally used for the final optics, are sometimes used to project a shadow over transport mirror laser damage in an effort to arrest laser damage growth and extend transport mirror lifetime. Post analysis of laser-damaged mirrors indicates that the dominant causes of laser damage are from surface particulates and the 351-nm wavelength target backscatter.