Experimental studies on recombination pumped soft x-ray lasers with helium-like ions are presented. The four-beam multi-TW neodymium-doped phosphate glass laser system at the Institute for Solid State Physics of the University of Tokyo was used in this work. From simultaneous time-and space-resolved spectra of the resonance lines of helium-like aluminium ions, population inversions between the n= 3 and 2 levels were observed. Details on the first observation of amplification in a recombination-pumped helium-like soft x-ray laser, exhibiting a time-resolved small-signal gain of 3.3+ 0.5cm -1 for the 3 1D-2 1p transition of helium-like nitrogen, are also given.
2 ~-9 / 2 rates are proportional to ne/l e [1]. Adiabatic expansion [2, 3] and radiative cooling [4] are often used to rapidly decrease the electron temperature. For sufficiently cold plasmas, three-body recombination dominates, which selectively populates the highly excited states of the lasing ion. The upper laser level is then efficiently pumped by the subsequent collisional-radiative cascade from these levels. The lower level rapidly decays radiatively to the ground state, thereby generating large population inversions.
Recombination schemes require smaller pump energy compared with collisional excitation schemes for generating soft x-ray lasers with the same wavelength. One reason for this is that inversions in recombination schemes are produced between An = 1 transitions, and hence the ratio of transition energy to the ionization potential is larger than that of the present collisional scheme, which uses An = 0 transitions. Therefore, recombination type soft x-ray lasers can be highly efficient, and also show rapid scaling to shorter wavelength with increasing atomic number of the target. Most works up to now have focused on the Balmer a transition of H-like ions [4,5] and the 4-3, 5-3 transitions of Li-like ions [6,7]. These schemes use recombination from completely ionized and He-like ions, respectively, which both have a closed-shell nature. An abundance of such ions can initially be produced in a laser plasma, which condition is favourable for generating large population inversions. Compared with these schemes, the He-like system has not been as extensively studied, and the lack of gain 0306-8919 9 1996 Chapman & Hall