Simulating Laser-Material Interactions

Lawrence Livermore National Laboratory (LLNL) is a premier research institution located in California, USA. It is renowned for its cutting-edge research in various scientific fields, including materials science, astrophysics, and nuclear research. LLNL oversees the National Ignition Facility (NIF), which houses the world’s largest and most energetic laser system. This facility is instrumental in advancing our understanding of the universe through technologies like inertial confinement fusion (ICF). The laboratory employs a team of highly skilled researchers and scientists who are dedicated to exploring innovative solutions for complex scientific challenges. One of the key areas of focus at LLNL is the repair and maintenance of fused silica optics used in high-power laser systems. The laboratory leverages advanced simulation tools and techniques to optimize laser-material interactions, ensuring the longevity and efficiency of their laser systems. With a strong commitment to scientific excellence and innovation, LLNL continues to make significant contributions to various fields of research and technology.

California-based LLNL oversees the National Ignition Facility (NIF), home to the world’s largest and most energetic laser. The giant machine—with 192 separate beams and 40,000 optics that focus, reflect, and guide those beams— can amplify emitted laser-pulse energy by as much as ten billion times and direct it towards a target about the size of a pencil eraser. The laser produces temperatures, pressures, and densities that are similar to those found in the cores of stars, supernovae, and large planets. Astrophysics and nuclear researchers use the giant laser to better understand the universe, utilizing such technologies as inertial confinement fusion (ICF), where hydrogen fuel is heated and compressed to the point where nuclear fusion reactions take place. However, repeated use of this powerful laser can damage the optics within the system. “The optics can be quite expensive,” says Matthews. “The high-power laser light generated by the NIF can damage some of the fused silica optics, creating little pits in the surface—similar to the ding you get when a rock hits your car’s windshield. We do everything we can to repair and recycle the damaged ones.” An example of two damaged optic surfaces before and after repair is shown in Figure 1.

Matthews and his team have used simulation to explore three techniques for repairing damaged optics: infrared (IR) pulsed laser microshaping/ micromachining, slow annealing, and laser chemical vapor deposition (L-CVD). In a first research cycle, they focused on the basic underlying physics and material science of how fused silica behaves when exposed to laser light at varying temperatures. There were several milestones in their temperature-tiered campaign: The first was to understand the thermalelastic response of the material up to the glass transition temperature of 1,300 K, where fused silica exhibits a sudden increase in elastic response and becomes less resistant to flow. They continued by examining the molecular relaxation of glass under viscous flow between the glass transition and the evaporation point at ~2,200 K. The final objective was to investigate the evaporation and redeposition of the material over temperatures between 2,200 and 3,400 K. To explore specific techniques for repairing the damaged optics, Matthews turned to the COMSOL Multiphysics® software. “I decided to use COMSOL to get a better understanding of what was going on,” says Matthews. “All the necessary physics were already available in the software, so I could readily try out ideas and avoid the time and effort that would be needed to develop my own code from scratch.” According to Matthews, COMSOL has been instrumental in helping them understand how lasers interact with fused silica, as well as in refining their specific repair methods. “A high-power laser system can’t tolerate much surface roughness in the optics. Controlling flatness to such high standards required extensive simulation,” he says. His simulations include heat transfer in fluids, chemical reactions, and structural mechanics, as well as mass transport and fluid flow.

• The team has implemented CO2 laser-based surface microshaping at NIF, optimized using multiphysics simulation, as part of the facility’s optics mitigation program.
• Their laser-material interaction research does not stop at optics repair. Mathews and his team are also supporting a laboratory-wide Additive Manufacturing Initiative by further developing an additive process for 3D printing known as selective laser melting (SLM).
• Simulation has been instrumental in helping them understand how lasers interact with fused silica, as well as in refining their specific repair methods.

• Over 130,000 damage sites have been repaired using IR microshaping and other techniques through 2014.