Coupling of pulsed 0.35-μm laser radiation to aluminum alloys

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Abstract
Experiments and theoretical analysis have been performed to investigate the thermal and mechanical coupling of pulsed 0.35‐μm laser radiation (τp≊10−6 s) to aluminum targets irradiated in vacuum. Thermal coupling coefficients measured for Al 2024 (as‐received and polished) and pure aluminum, and impulse coupling data for polished Al 2024 are presented. We find low fluence (≲5.0 J/cm2) thermal coupling coefficients of 0.4 for as‐received Al 2024, 0.2 for polished or laser‐cleaned Al 2024, and 0.1 for pure aluminum. Neglecting initial surface layer effects, impulse coupling is shown to result from bulk target vaporization for pulse fluences greater than 10 J/cm2. Measurements are also presented for the plasma formation threshold above aluminum targets irradiated in vacuum at 0.35 μm. An intensity threshold of 6×107 W/cm2 is indicated for a pulse duration of 0.5 μs. The impulse coupling and plasma ignition threshold data are shown to be in good agreement with theoretical model predictions.