Computer models of micrometeoroid impact on fused silica glass mirrors

Abstract

Analysis of impacts on fused silica was motivated by the design of the mirror for the James Webb Space Telescope (JWST). The JWST is expected to have a large-diameter mirror shaded from sunlight but open to space. One of the candidate mirror materials is fused silica; the other candidate materials are ultra-low expansion (ULE) glass and O-30 beryllium. Historically, most meteoroid tests have concerned themselves with the degraded strength of the substrate; JWST is primarily concerned with maintaining its precise optical figure. Micrometeoroids can have velocities of 50 km/s or higher. Although the velocity in the analysis did not exceed 10 km/s, the computer model included a fit to shock Hugoniot data for fused silica with pressures as high as 700 GPa, typical of impacts at velocities higher than 10 km/s. The computer model also included the first-order phase transformation observed in fused silica. The calculations were made with the AUTODYN hydrocode [1], which includes smooth particle hydrodynamic (SPH) and Lagrange solvers with other modules and features for coupling them together. For the analysis of impact, both SPH and Lagrange methods were used. Using the static relaxation option, the analysis continued from crater formation to a late-time static solution from which the effect of the impact on the optics could be determined. Both low and high energy impacts were computed using the fused silica model. For the low energy impact, the computed crater depth, crater diameter, and spall diameter closely matched both historical data for fused silica and values measured by Auburn University/ Hypervelocity Impact Facility (AU/HIF) for two typical fused silica sites and one ULE site. For the high energy impact, the computed values matched historical data for fused silica but not the data measured by AU/HIF for three typical ULE sites. The discrepancy can be explained by the difference in the response of fused silica and ULE under impact, as seen in historical trends and as observed by AU/HIF. Optical interferometry of the surfaces of one of the mirrors revealed deformation in the vicinity of particle impacts that matched that estimated with a static model, and the deformation was consistent with that computed by AUTODYN.