Abstract
The Shenguang-II Upgrade (SG-II Up) facility is an under-construction high-power laser driver with eight beams, 24 kJ energy, 3 ns pulse duration and ultraviolet laser output, in the Shanghai Institute of Optics and Fine Mechanics, China. The prototype design and experimental research of the prototype final optics assembly (FOA), which is one of the most important parts of the SG-II Up facility, have been completed on the ninth beam of the SG-II facility. Thirty-three shots were fired using 1- ${\it\omega}$ energy from 1000 to 4500 J and 3- ${\it\omega}$ energy from 500 to 2403 J with a 3 ns square pulse. During the experiments, emphasis was given to the process of optical damage and to the effects of clean-gas control. A numerical model of the FOA generated by the Integrated Computer Engineering and Manufacturing code for Computational Fluid Dynamics (ICEMCFD) demonstrated that a flux within $1{-}5~\text{l s}^{-1}$ and a 180 s period is effectual to avoid contaminant sputtering to the optics. The presence of surface ‘mooning’ damage and surface spots located outside the clear aperture are induced by contaminants such as wire, silica gel and millimeter order fiber and metal.
Highlights
The Shenguang-II Upgrade (SG-II Up) is a kilojoule-class solid-state laser and targeting facility under construction by the National Laboratory on High Power Lasers and Physics
The aperture sizes of these optics are as large as 370 mm × 370 mm
Beam sampling grating (BSG) and main debris shield (MDS) – providing a low-efficiency grating to the input surface for energy sampling and large-object protection for upstream optics
Summary
The Shenguang-II Upgrade (SG-II Up) is a kilojoule-class solid-state laser and targeting facility under construction by the National Laboratory on High Power Lasers and Physics. The amplified 1053 nm beams from the Nd:glass driver are transported (at 40 kJ for 3 ns) in 2 × 2 quads with eight beams with aperture size 310 mm × 310 mm to the 2.4 mdiameter target chamber where eight final optics assemblies (FOAs) convert each beam to the third harmonic, separate the residual 1053 and 527 nm beams, and focus the 351 nm beam onto the target. 6. Beam sampling grating (BSG) and main debris shield (MDS) – providing a low-efficiency grating to the input surface for energy sampling (calorimetry) and large-object protection for upstream (more expensive) optics. The BSG and MDS are made with 8 mm-thick fused silica possessing a 0.2% low-efficiency grating on the input surface for energy sampling. This setup provides transitions from the near-vacuum FOA environment to the hard-vacuum target chamber environment. Inside the prototype FOA, the pressure is 10 Torr with 10 SLPM before the laser shot and 50 Torr with 40 SLPM after the laser shot using clean dry nitrogen to purge
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