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    Relationship Between Coating Defects and the Limiting Flux Density a Cooled Laser Mirror Can Withstand

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    The flux density which a cooled laser mirror can handle without catastrophic damage is predicted to be limited in typical situations by absorbing defects on the optical surface rather than by mirror reflectance. In this case, failure is predicted to occur either through coating damage in the vicinity of the defect or by burnthrough of the cooled faceplate following vapor barrier formation at the coolant-faceplate interface. Expressions are developed which predict the limiting defect size for a given flux density as a function of (1) thickness and composition of the faceplate, (2) transport properties and fluid mechanics of the liquid coolant, and (3) conditions generating a hydrodynamic crisis at the liquid coolant stagnation boundary beneath the defect. This parametric analysis can be used to optimize faceplate thickness as a function of mirror material, coolant flow, coating damage threshold, heat exchanger design, and the expected maximum size and absorption of multilayer film coating defects. According to this analysis, for thin faceplates the optimum faceplate thickness may be determined by the coating process rather than by average heat transfer considerations.


    coating defects, cooled laser mirrors, defect damage, hot-face design, limiting flux density, mirror damage, vapor-barrier-induced burnthrough

    Author Information:

    Palmer, JR
    TRW One Space Park, Redondo Beach, California

    Bennett, HE
    TRW One Space Park, Redondo Beach, California

    Committee/Subcommittee: F01.02

    DOI: 10.1520/STP37276S