Abstract—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 o 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. I. INTRODUCTION The grazing incidence metal mirror (GIMM) [1] has been proposed as the final element of an optical system that provides optimum illumination of the DT fusion targets by the driver laser beams in order to achieve a full target implosion. Exposure to a variety of threats, including prompt neutron and gamma fluxes, x-ray and ionic emissions, and contamination from condensable target and chamber materials, could cause damage to the mirror resulting in increased laser absorption, beam quality degradation and reduced laser- induced damage threshold. This paper is aimed at addressing and quantifying the effects of these damages on the mirror performance, such as reflectivity and target illumination profile, using various wave modeling approaches. For this study, a final optics arrangement based on the Prometheus-L Reactor design [2] will be employed. In this design there are 60 beamlines all focusing on the target at the center of the chamber. Each laser beam is collimated and refocused at a 90 o bend, and reflected off a flat GIMM at a grazing angle of 80 o to the mirror normal before reaching the target. The focal length of the focusing mirror is 30 m. with the distance from the GIMM to the target being 20 m. The beam source is a KrF laser with a wavelength of 248 nm, while other sources such as the diode-pumped solid state laser (DPSSL) [3] at 345.5 nm have also been considered. The GIMM lowers the effective laser power flux without much loss of reflectivity, thus prolonging the mirror lifetime, and helps reducing neutron irradiation of the rest of the components in the optical train. The mirror reflecting surface is typically composed of a thin coating of aluminum on a silicon carbide substrate that provides structural support. In this paper, modeling results are presented on damage effects on mirror and beam performance using the Prometheus-L final optics design as a reference. It is noted that the modeling tools developed will also provide analysis of laser-target experimental results [4] at UCSD, and help define design windows for the GIMM in a laser-driven IFE power plant in the ARIES study. II. MIRROR DEFECTS AND ANALYSIS APPROACHES Depending on the type of threat, the mirror can sustain damages (or defects) that are dimensional or compositional in nature, each of which can be analyzed using a suitable approach. Dimensional defects involve induced geometrical changes in the mirror surface, and can be classified into two types according to the size d of the defects relative to the incident wavelength l. Gross deformations (d > l) can be the direct consequence of faulty fabrication, neutron- and thermal- induced swelling, and gravity loads. This type of damage can best be analyzed by the ray tracing technique. On the other hand, for surface morphology with d < l, that are caused by laser-induced damage or thermomechanical load, the best analysis approach is by applying Kirchhoff’s theory of light scattering by rough surfaces [5]. Compositional defects of the mirror are the result of surface contamination and involve local modifications of the material composition of the surface that alter its optical properties. For gross surface contamination, caused by neutron-induced transmutations or bulk re-deposition of impurities on the surface, a Fresnel solver for a one-dimensional multiple-layer medium [6] appears quite adequate. In the case of localized contaminants on the surface, such as aerosol, dust or debris from the surrounding, the Mie theory of particle scattering of light appears to be an appropriate technique. Analyses on mirror dimensional defects and on gross surface contamination have been carried out and their results are Modeling of Mirror Surface Damage Effects on Beam Performance In a Laser-Driven IFE Power Plant T.K. Mau, M.S. Tillack and M.R. Zaghloul Center for Energy Research, University of California-San Diego, La Jolla, CA 92093-0417, USA