On the structure of quasi-stationary laser ablation fronts in strongly radiating plasmas

Abstract

The effect of strong thermal radiation on the structure of quasi-stationary laser ablation fronts is investigated under the assumption that all the laser flux is absorbed at the critical surface. Special attention is paid to adequate formulation of the boundary-value problem for a steady-state planar ablation flow. The dependence of the laser-to-x-ray conversion efficiency / r on the laser intensity I L and wavelength k L is analyzed within the non-equilibrium diffusion approximation for radiation transfer. The scaling of the main ablation parameters with I L and k L in the strongly radiative regime 1 À / r ( 1 is derived. It is demonstrated that strongly radiating ablation fronts develop a characteristic extended cushion of "radiation-soaked" plasma between the condensed ablated material and the critical surface, which can efficiently suppress perturbations from the instabilities at the critical surface. V C 2015 AIP Publishing LLC. [http://dx.

Power-law scaling of plasma pressure on laser-ablated tin microdroplets

“…Another reason is departure from the ideal-gas equation of state (EOS) due to multiple temperature-dependent ionization of the target material. These two effects are of major importance for tin (Z = 50) targets at the here considered irradiation intensities 20 . A significant further issue is the non-trivial geometry of the laser-target configuration in our experiments, where a spherical target is irradiated from only one side and an essentially twodimensional (2D) ablation flow develops.…”

Efficient Generation of Extreme Ultraviolet Light From Nd :YAG-Driven Microdroplet-Tin Plasma

“…Nd:YAG-driven light sources have a much higher, near complete laser absorptivity due to the strong increase of the critical plasma electron density n c with decreasing wavelength λ (n c ∝ λ −2 ). The cross section of the here dominant [26] inverse bremsstrahlung absorption mechanism strongly increases with electron density [27,28]. However, the density increase may cause self-absorption of the emitted EUV light, leading to opacity-broadened emission [29][30][31][32].…”

Laser-to-droplet alignment sensitivity relevant for laser-produced plasma sources of extreme ultraviolet light

“…The time scale of this plasma generation is typically much shorter than the laser pulse. As a result, most of the laser energy is absorbed just before the plasma critical density surface 13 where the plasma electron frequency equals that of the laser light and no further light penetration is possible. This gives the system an effective radius R eff ≥ R 0 , see Fig.…”

“…In the remainder of this work, we take δ = 0.6 in accordance to Ref. [13]. Furthermore we neglect the small offset energy E 0 , since typically E 0 E (see Ref.…”

“…Here, we present and experimentally validate an intuitive model describing L2D alignment sensitivity. The model is based on a single parameter: the dimensionless ratio of the laser spot size to the effective size of the droplet target, which we relate to the position of the plasma critical density surface [13][14][15] and sets the typical length scale for the problem. Our model enables the development of straightforward scaling arguments, which in turn enable the minimization of detrimental alignment sensitivity.…”