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In this paper we describe the progress achieved recently in our laboratory in the field of X‐ray lasers. Both collisional excitation and recombination pumped systems are under investigation. We show that the 5g‐4f transition in lithium‐like ions could bring out a significant increase of the gain‐length accessible with recombination X‐ray lasers. We present preliminary results on an absorption spectroscopy experiment designed to probe the ionization state of recombination X‐ray laser plasmas. Finally we report on the observation of a strong amplified signal at 212 A, the wavelength of a 3p‐3s (J=0–1) in neon‐like zinc.

Investigating possibilities of attaining large gain‐length products on the recombination scheme using lithium‐like ions, we have examined two approaches aimed at overcoming the problem of plasma non‐uniformity susceptible to destroy gain by a number of processes. In the first approach we studied amplification on the transitions 5f‐3d and 4f‐3d in Li‐like Al10+ plasma column produced by smoothing optics using lens arrays. Employing this device resulted in the gain holding up significantly longer than when no smoothing optics was used. Second, we have investigated numerically and experimentally the 5g‐4f transition in Li‐like S13+, as the gain should be barely affected by the plasma nonuniformities. Encouraging results were obtained and their various aspects are discussed.

A large gain has been measured for the J=0–1 line of neonlike Zn at λ=21.2 nm. The time evolutions and the localization of emission zones of the J=0–1 and J=2−1 lines are compared. It is shown that a train of very small prepulses before the main pulse has an important role in the J=0=1 emission. A half‐cavity has been successfully used to attain a nearly saturated intensity with a 2 cm long plasma. The X‐UV pulse energy is of 400 μJ, the laser power of 5 MW. The driving laser is the 0.4 KJ, 600 ps laser of LULI.

An aluminium recombining X‐ray laser plasma was probed by keV radiation emitted by an auxiliary plasma source to yield absorption structures on Kα transitions. From this technique we obtain the spatial distribution of the relative abundances of He‐ to B‐like ions at different times during plasma cooling. He‐like and Li‐like ions are found to exist further from the target than predicted by the hydrocode, explaining previous gain measurements.

Recent experiments, performed with pumping laser of relatively moderate energy (E≤1KJ per pulse), have considerably increased the gain factor of X‐UV lasers in the wavelength region of 200 A. In this paper we recall the pumping mechanisms operating in laser‐produced plasmas and we present the large gain experiments performed with neonlike germanium, in using the laser VULCAN (E≊1KJ), as with neonlike zinc and lithiumlike sulphur, in using the laser of LULI (E≊0.5KJ).