X-Ray Lasers

Abstract

Multicharged ions in laser-produced plasmas are able to provide soft X-ray amplification. Conditions under which hot plasma columns act as X-ray amplifyers are investigated in several laboratories. Plasma optical properties, especially in relation with refractive index gradient and radiation trapping, are of importance to reach large gain-length product values. Multilayer mirrors are now available for designing X-ray laser cavities. Regarding X-ray laser pumping, the theory of which requires detailed atomic-level population calculation to be joined to hydrodynamical plasma modelling, experiments set off now two types of mechanisms amongst many possible theoretical schemes. On the one hand, recombination, during fast plasma cooling, populates high-lying ion levels more rapidly than the first resonance levels. This is observed for the 2–3 transition of H-like ions, especially for the line at 182-A wavelength in carbon which has shown gain with a coefficient attaining 4 cm−1. This is also observed for 3–5 and 3–4 transitions of lithium-like ions, near 100-A wavelength. Gain coefficients are between 0.5 cm−1 and 2.5 cm−1. On the other hand, the balance between collisional excitation by plasma free electrons and fast radiative decay produces population inversions between 3s- and 3p-levels in Ne-like ions and between 4p- and 4d-levels in Ni-like ions. Ne-like selenium has provided a gain coefficient of 5 cm−1 at 206.3 A. Ni-like ions allow to extend this scheme to wavelenghts below 100 A. In view of their possible applications, X-ray lasers are expected to be by far the brightest X-ray sources in laboratory.KeywordsPlasma ColumnPopulation InversionGain CoefficientRadiation TrappingPlasma LengthThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.