C56-32.aRAR Nano-Textured Fused Silica Optics with Multi-Gigashot Stability

Lawrence Livermore National Laboratory’s (LLNL) National Ignition Facility (NIF) recently announced the achievement of fusion ignition for the first time, by delivering 2.05 MJ of energy to the target, resulting in 3.15 MJ of fusion energy output. Further power scaling of the system will require addressing a number of power-limiting technical issues. One such limitation is the anti-reflection coatings applied to transmissive optics in the high-energy lasers, which fundamentally exhibit both elevated absorption and defects that result in beam quality issues, and more critically, lifetime concerns. Developing commercially viable laser drivers for an internal fusion energy (IFE) powerplant will require engineered beam transport and focusing optical systems capable of operating robustly 24/7 at high energy, high average power, and improved Mean Time to Failure (MTTF) in the multi-Gigashot Regime. TelAztec has developed a replacement technology for multilayer thin-film AR coatings, by plasma etching a random anti-reflection (RAR) nano-texture directly into the optical component. Greater than 99.9% transmission is routinely achieved over broad bandwidths, with low scatter and an absence of power and lifetime-limiting absorption and defects. In addition, RAR nano-textures have shown a 2–5x higher laser damage threshold than coatings for both CW and pulsed lasers, and long-term beam stability due an absence of material fatigue at the air/optic interface. In the Phase I R&D effort to further power scale high-energy laser systems, TelAztec will coordinate with LLNL, the Stanford Linear Accelerator Center (SLAC), and the Texas Petawatt facility partners to refine a detailed series of experiments to isolate fundamental failure mechanisms with nano-textured fused silica optics. Fused silica bulk material issues such as sub-surface polishing damage (Beilby layer), polishing contamination, UV plasma degradation and nano-texture related characteristics such as surface contamination from plasma etching, nano-texture design, and electric field effects on the nano-texture will be among variables isolated. Photon backscatter measurements and pulsed laser damage testing at 355 nm will define the dominant factors in the onset of laser damage, and substrate polishing and/or plasma etch process modifications will be brought into play to mitigate the issue(s) and enhance laser damage threshold. A second batch of nano-textured optics processed with damage mitigation processes to verify increased power handling, as quantified through a second round of characterization and pulsed laser damage testing. The results of the Phase I will demonstrate a surface treatment for improved beam transport and focusing optical systems capable of operating robustly and continuously at high energy, high average power, and MTTFs in the multi-Gigashot Regime—addressing all of the aforementioned critical drivers for an IFE powerplant. On the commercial side, nano-textured optics are now being integrated into high-power industrial and medical lasers due to improved power handling, lifetime, beam stability, and operational cost. Further power scaling of these systems will enable higher efficiency manufacturing that will lead to significant energy savings and introduction to new markets. Finally, for NIF, potential improvement in reducing reflection losses and increasing optic lifetime may be a key to ultimately realizing economic fusion energy, a milestone towards green energy and a zero-emission future.