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High-power/high-energy (HEL) systems include an optical train consisting of mirrors and windows, which must be capable of transporting and directing the beam without seriously degrading the nominal performance of the laser. Since catastrophic failure modes are not a major threat at beam-power levels of current interest, the system's performance as measured in terms of achievable target irradiances can degrade as a result of thermal lensing, that is, the wavefront distortion caused by thermally induced phase aberrations. The purpose of this paper is to present an analytical investigation that addresses the problem of evaluating the impact of laser-driven mirror distortions; in this context it is shown how to obtain simple figures of merit (FoM) for rating the thermal lensing performance of mirror-faceplate material candidates. The performance of cooled HEL mirrors reflects their ability to minimize irradiance-mapping wavefront distortions, which leads to defining a thermal distortion coefficient (Xi) equals (alpha) (1+v) that controls the out-of-plane growth of the faceplate. It is then straightforward to derive equations for characterizing the RMSsed surface deformation and to assess the merits of mirror-faceplate material candidates in a pulsed or a CW environment. Figures of merit for CW operation must take into account the requirement that the faceplate should be as thin as possible but still able to minimize coolant-induced pressure ripples; the modulus of elasticity, therefore, must be properly factored into FoM expressions. Since water-cooled HEL mirror heat-exchangers exhibit relatively modest Biot numbers (Bi < 1), the thermal conductivity of the faceplate is not a critical material parameter. Numerical evaluations demonstrate that the ranking of faceplate-material candidates does not depend on the laser mode of operation or the efficiency of the heat exchanger. It is the thermal expansion coefficient (alpha) that determines the performance if optical distortions are of concern. For this reason, diamond shows much promise, which will attract attention as CVD-diamond fabrication technologies mature.
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