STP1141

    Multipulse Laser-Induced Failure Prediction for Mo Metal Mirrors

    Published: Jan 1991


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    Abstract

    In combination with known thermomechanical fatigue data for Mo, we have applied the Transient Photothermal Deflection (TPD) technique to develop a model for the N-on-1 damage of Mo mirrors to predict their multipulse lifetimes. In laser-damage experiments to verify the model, mechanically polished Mo mirrors were irradiated with 10 ns Nd:YAG laser pulses at 1064 nm at a 10 Hz rep rate. In the TPD experiments, the approximately 600 μm diameter Nd/3YAG laser spot was probed off axis by a smaller HeNe laser beam whose deflection was detected by a fast bicell photodetector and amplifier. Digitized photodetector waveforms indicated that the surface angular deflection could be converted into surface displacement. In addition, thermal modelling of the vertical heat distribution enabled the peak surface-deflection signal to be converted into peak surface temperature. The thermomechanical model was verified by both the experimental and model results.

    Conventional mechanical fatigue data for Mo were used to derive a predictive equation for the laser-accumulation lifetime of Mo mirrors. Experiments were performed with one to 104 pulses per site yielding laser-damage thresholds and accumulation curves. The accumulation behavior predicted from measurements of mechanical fatigue was in excellent agreement with the measured behavior. It is possible that data on high-cycle (>106) mechanical fatigue can be used to predict the performance of optical surfaces at equally large values of N. Furthermore, a single-pulse TPD measurement of peak deformation at a subthreshold laser fluence, in conjunction with mechanical fatigue data, may be used to estimate the safe operating fluence for a component in a multipulse laser environment.

    Keywords:

    accumulation, N-on-1 damage, molybdenum damage, metal surface damage


    Author Information:

    Becker, MF
    The University of Texas at Austin, Austin, Texas

    Ma, C
    The University of Texas at Austin, Austin, Texas

    Walser, RM
    The University of Texas at Austin, Austin, Texas


    Paper ID: STP23625S

    Committee/Subcommittee: F01.02

    DOI: 10.1520/STP23625S


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