Ways to discharge-based soft X-ray lasers with the wavelength λ<15 nm

Two basic ways to amplification of spontaneous emission in the soft X-ray region are described. The first is based on the electron-collisional recombination pumping scheme, which uses recombination of fully stripped ions into hydrogen-like ions to create (in the case of sufficiently fast cooling) a population inversion on energy levels belonging to the Balmer-alpha transition. We test this scheme on nitrogen, for which the lasing wavelength is 13.4 nm. The second way to amplification of spontaneous emission is based on the electron-collisional excitation pumping scheme: this uses for creation of population inversion a fast excitation of Ne- or Ni-like ions. However, for wavelength below 15 nm it is necessary to use Ni-like ions of some metal vapors. Feeding metal vapors into a capillary is difficult, and if being fed they deposit on the capillary wall and significantly reduce the capillary lifetime. That is why we prepare metal vapor plasma in a capillary with liquid wall – by wire explosion in water. For slowdown of the plasma-channel expansion a local-water-compression by linearly focused shock wave is being developed.

Analytical study of recombination X-ray lasers in an adiabatic expanding plasma

The collisional recombination process in highly ionized plasma and relevant reheating of the plasma are analyzed in the parameter regime appropriate for X-ray lasing. A method for describing the rate of collisional recombination is proposed based on a compact threelevel atomic model. Reheating of the plasma due to collisional recombination is described by a time-dependent adiabatic exponent γ. Using a self-similar description of the free expanding plasma, the gain of recombination lasing is investigated. In comparison with an open two-level atomic model, the present model gives lower gains appearing later in time. The inclusion of plasma reheating reduces the peak gain and shifts it to later times. Although the three-level model is shown to give a good qualitative description of the atomic processes, a considerable discrepancy was observed when compared with simulations for hydrogenlike carbon.