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Plasma formation at the surface of a metal mirror under intense pulsed laser irradiation in an ambient atmosphere usually leads to enhanced thermal and mechanical coupling with consequently increased surface damage.
Exo-electron emission associated with plastic micro-yielding of the metal surface under thermomechanical stress induced by a surface temperature rise (ΔT) is proposed here as a plasma initiation mechanism. Emission starts when ΔT exceeds a relatively small critical value. Rapid buildup of the plasma proceeds via nonequilibrium ionization cascade in the ambient atmosphere when the incident laser flux exceeds a threshold value. Formation of a highly absorbing plasma layer requires delivery of a minimum fluence while the incident flux is higher than the above-mentioned flux threshold. At high values of incident flux, the required fluence decreases to an asymptotic minimum.
The critical temperature rise, threshold flux, and asymptotic minimum fluence required for surface plasma formation are defined here in terms of the pertinent radiation, material, and atmosphere parameters.
Cu mirrors, surface plasma, air breakdown, laser-induced electron emission, absorption waves, mirror surface damage
Lockheed Palo Alto Research Laboratories, Palo Alto, California