How to make an X-ray laser

An exciting feature of the x-ray laser is that its development could greatly extend our knowledge of the structure of biological macromolecules. The problem in building x-ray lasers lies in the difficulty in creating a population inversion on a closed shell transition. Several schemes to demonstrate x-ray lasing which have appeared recently in the literature are considered. These include methods which involve the removal of electrons from the inner electron shells in atoms, those which involve a highly ionised plasma, those using ion beams, and a method of laser frequency conversion where laser light incident on some medium is coherently scattered giving light at a changed frequency. (U.K.)