We present a laser-target scaling model which permits approximate prediction of the dependence of ablation pressure, mechanical coupling coefficient, and related parameters in vacuum upon single-pulse laser intensity (I), wavelength (λ), and pulse width (τ) over extremely broad ranges. We show that existing data for vacuum mechanical coupling coefficient for metallic and endothermic nonmetallic, surface-absorbing planar targets follows this empirical trend to within a factor of 2 over 7 orders of magnitude in the product (Iλ(τ)1/2). The comparison we present is valid for intensity equal to or greater than the peak-coupling intensity Imax, where dense plasma formation mediates laser-target coupling. Mechanical coupling coefficients studied ranged over two orders of magnitude. The data supporting this trend represent intensities from 3 MW/cm2 to 70 TW/cm2, pulse widths from 1.5 ms to 500 ps, wavelengths from 10.6 μm to 248 nm, and pulse energies from 100 mJ to 10 kJ. With few exceptions, data approximating one-dimensional or planar expansions were selected. Previously, meaningful scaling of ablation pressure parameters with I, λ, τ was not possible because existing data concentrated in a small range of these parameters. Our own data, obtained in the low- and midrange of (Iλ(τ)1/2), completes the experimental picture. Since this new data was derived from five separate experiments with specialized character and purpose, detailed accounts of this work will appear separately. In this paper, we summarize the experimental conditions and select only those data which are relevant to the scaling issue. We find that laboratory-scale laser experiments can often give impulse coupling data which agree with results from much higher-energy experiments without much error, and at much lower cost. We review a theory of vacuum laser ablation, specialize it to a quantitative description of mechanical coupling, and show that the resulting model provides a simple physical description which comes quite close to the observed empirical trend. This is accomplished with minor elaborations of the theory as originally presented to account for the temperature dependence of plasma ionization states, while adhering to the premise that a simple and generally applicable treatment of laser impulse production should be available. The theoretical model can quantitatively predict vacuum ablation pressure for opaque targets without adjustable parameters to the factor-of-2 accuracy in which we are interested. Other published scaling models omit one or more of the important variables, lack broad applicability, or deviate more noticeably from the observed trend.

The results of multiple-pulse damage tests on copper mirrors using 1.7-ns CO2 lasers are reported. The measured reduction in the brightness reflectivity of the mirrors is shown to be correlated to the dramatic appearance of fine scale microstructure on the mirror surface. Scanning electron micrographs of this surface structure are presented and possible explanations of the effects are discussed.

Because copper optics for CO2-ICF system applications will be highly stressed, the single-shot damage thresholds which have been reported [1,2], provide only part of the information needed for accurate performance predictions. We report here a performance comparison for copper mirrors finished by different techniques, including single-point diamond turning (SPDT), showing for the first time the relative decrease in 10-μm damage thresholds corresponding to multiple shots, and the loss of reflected beam brightness arising at high fluence levels. We also report the intensity and energy loss due to plasma formation when Cu mirrors are used in air.

Measurements of damage to metal surfaces induced by intense nanosecond pulses of ir radiation are reported. Single-shot damage thresholds of Cu, stainless steel, molybdenum and aluminum surfaces have been measured for various angles of incidence and the predicted increase in damage thresholds for grazing incidence optical components have been experimentally verified for the first time at 10-μm. Multiple-shot damage tests have also been performed and practical lifetime curves for Cu mirrors have been established. Scanning electron micrographs of the mirror show the damage mechanism to be the development of a fine scale (1 μm) microstructure on the copper surface.