Modeling of mirror surface damage effects on beam performance in a laser-driven IFE power plant

Grazing incidence metal mirrors in laser-driven IFE power plants are subject to a variety of threats that result in damages leading to increased laser absorption, beam quality degradation and reduced laser-induced damage threshold. In this paper, we analyze the mirror reflectivity changes and wavefront distortions incident on the target using several modeling approaches, depending on the nature and size of the damage. We have developed a four-layer Fresnel solver to quantify the dependence of reflectivity on the thickness of surface contaminant and mirror protective coating, and their material properties, for a relevant range of incident angles. With a lossy contaminant like carbon, it is found that reflectivity decreases with thickness mainly due to surface dissipation, but this deleterious effect is diminished towards grazing incidence. For defect size small with respect to a wavelength, we have used Kerchhoff's wave scattering theory to evaluate degradation of the beam performance. For a damaged surface characterized by Gaussian statistics, we found that the average damage size needs to be less than one percent of the wavelength to avoid loss of beam intensity at 80° grazing incidence. Ray tracing techniques have been used to assess distortion in beam illumination profiles when the surface defect size is large compared to the incident wavelength. Simple bulk deformations of the mirror surface, typical of swelling due to thermal and gravity loads, have been studied.