Inverse population of levels of multiply charged ions

A theoretical analysis is made of the possible use of a multiply charged expanding plasma for laser action in far ultraviolet. It is shown that a plasma containing ions with low-energy resonance states (ΔE~kT_e) may cool strongly by the emission of lines. The populations of excited states in a lithiumlike ion Al XI is considered by way of example. It is shown that cooling and expansion of a plasma should result in a population inversion of the 4→3 transition (λ~150 Å) and the gain should 0.1–1cm^–1.