Impulse coupling to targets in vacuum by KrF, HF, and CO2 single-pulse lasers

Abstract

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.

Theoretical peak performance and optical constraints for the deflection of an S-type asteroid with a continuous wave laser

“…Our results agree qualitatively with the theory of Phipps et al (1988) for short pulses up to a time of approximatively 1 millisecond 5 , beyond which significant deviation occurs as 3D expansion becomes the limiting factor for plasma ignition and the ignition threshold is only dependent on laser wavelength and intensity as predicted by the model of Poueyo-Verwaerde et al (1993). The intensity threshold for plasma ignition found by our model is around 50GW/m 2 at a 10.6µm wavelength for aluminium, which is again in agreement with the values that we derived based on works of PoueyoVerwaerde et al (1993) and Phipps et al (1988).…”

“…The FVM curves compare quantitatively well with the predictions of the analytical model. It is worthwhile noting that a somewhat similar result was obtained by Phipps et al (1988) for the case of pulsed laser systems. …”

“…According to this expression, for laser frequencies in the infrared and typical values of ionization potentials and atomic masses, the plasma ignition threshold is at intensities ranging between 10GW/m 2 and 1TW/m 2 . For pulsed lasers, an empirical value of the plasma ignition threshold was found by Phipps et al (1988) as…”

A Study of Polycrystalline Silicon Damage Features Based on Nanosecond Pulse Laser Irradiation with Different Wavelength Effects

“…Plasma shock wave propagates against the incident direction in the form of ultrasound, which will have a certain pressure effect on irradiated area of the target. According to Phipps's pressure load analysis formula, we can get the pressure formula of the plasma shock wave on the surface of laser target area [11] :…”

Statistical multi-criteria evaluation of non-nuclear asteroid deflection methods

“…Plasma effects are also not expected to play any role under CW laser irra- diation below intensity levels of 10 GW/m 2 , which are required to accelerate the free electrons in the vapor by inverse Bremsstrahlung until their kinetic energy becomes sufficient to ionize the atoms of the vapor by an avalanche process, according to Poueyo-Verwaerde et al (1993). A model to predict the thrust coupling coefficient in the Plasma regime has been developed by Phipps et al (1988) for pulsed laser systems and it is interesting to compare the peak coupling predicted by this model with the values predicted by our CW model. In this model, the plasma coupling C mp coefficient was empirically found to follow a power law:…”