Absorption of Short Pumping Pulses for Grazing Incidence Pumped X-ray Lasers

Abstract

The study presented here has proposed a way to introduce the NIB mechanism in the complex refractive index formalism for understanding the propagation of short light pulses in plasmas. It was applied to the specific case of GRIP XRL efficiency analysis. However, it could also be useful for a broader range of applications related to the behaviour of short pulses in plasmas, such as plasma mirrors and the propagation of ultra-short light pulses in plasma channels. The complex refractive index formalism was applied to two specific XRL arrangements. In the first, GRIP XRLs, the results show that at very small grazing angles little energy is deposited in the plasma due to the reduced opacity. Slightly larger grazing angles, in the range 25–30° instead of 18° for a wavelength of 1.054 µm, and longer, but still short, pulses can circumvent the opacity problem and increase the XRL efficiency since the pumping energy is then efficiently deposited in the pre-plasma. In the second case, sub-10nm XRL systems, the required pump energy is significantly higher, around 20 J for a Ni-like lanthanum XRL to a few kilojoules for water-window XRLs. Increasing the opacity of the plasma by tuning the plasma parameters can significantly increase the temperature of the plasma formed by a short pulse and reduce the pumping energy requirements, making shorter wavelength XRLs possible at limited pump energies.