Theory of laser heating of solids: Metals

Abstract

Calculations of the transient and steady‐state temperature rise T of laser‐irradiated metals indicate that values of the intensity (irradiance) I_f that cause failure change drastically under different experimental conditions, such as the degree of plasma ignition on the surface and the relative magnitudes of the laser‐beam diameter, sample thickness l, lateral dimension, and thermal diffusion distance. The dependence of I_f on material parameters such as thermal conductivity and heat capacity is also quite different, depending on the experimental conditions. The results suggest that the highest of the recently measured copper‐damage thresholds of 125–750 J/cm² for 0.6‐μsec pulses at 10.6 μm are likely to be at or at least quite near the intrinsic limit set by the simple process of melting that results from the intrinsic absorption. The theoretical intensity at which the cavity mirrors of recently developed xenon uv lasers fail is in good agreement with the experimental value. The theoretical value of T of metals irradiated for 20 sec with 10.6‐μm radiation is two orders of magnitude too small to explain recent experimental results. It is suggested that the discrepancy is related to plasma ignition at the sample surface. The steady‐state value of T for metals cooled with a surface‐heat‐transfer coefficient h is not reduced substantially by increasing the cooling efficiency past a certain point (h≳h_l≡K/l). For short time t<τ, where the characteristic time τ depends on both l and h, cooling the metal is not effective is preventing the temperature rise.