The short-wavelength laser - Progress and prospects

A review is presented of theoretical and experimental work on developing a soft X-ray laser. The goal of all the efforts has been to achieve energetic population inversions, either by cooling the excited atoms through hydrodynamic expansion of the plasma or by radiative cooling of a magnetically confined plasma. Recombining electrons with photons at high energy quantum levels have also yielded positive results when combined with an ionization of lower-energy state thermalized electrons to facilitate the inversion. Very few useful resonance states have been identified for photoexcitation methods, although a staged approach, i.e., first populating an excited metastable state and then pumping the metastable state ions to a higher state, has shown promise, but at high power requirements. Other avenues being explored to obtain EUV wavelengths include charge transfer, free electron and electron collisional excitation lasers, all of which are briefly discussed.