Electron temperature versus laser intensity times wavelength squared: a comparison of theory and experiments

The peak temperature in the corona of plasma ejected by a laser-irradiated slab is discussed in terms of a one-electron-temperature model. Both heat-flux saturation and pulse rise-time effects are considered; the intensity in the rising half of the pulse is approximated by a linear function of time, I(t) ≡ I₀t/τ. The temperature is found to be proportional to (I₀λ²)^2/3 and a function of I₀λ⁴/τ. Above a certain value of I₀λ⁴/τ, the plasma presents two characteristic temperatures (at saturation and at the critical surface) which can be identified with experimentally observed cold- and hot-electron temperatures. The results are compared with extensive experimental data available for both Nd and CO₂ lasers, I₀(W·cm⁻²) λ² (μm) starting around 10¹². The agreement is good if substantial flux inhibition is assumed (flux-limit factor f ≅ 0.03), and fails for I₀λ² above 1O¹⁵. Results for both ablation pressure and mass ablation rate are also given.