Population-inverted medium for stimulated emission in vacuum ultraviolet

The populations of excited states of multiply charged He-like ions are calculated for a high-temperature plasma. It is shown that, if the optical thickness of the plasma $\tau_p$, measured at the center of a resonance line, is not too great ($\tau_p\lesssim30$), a population inversion is established between a series of triplet and singlet levels. If the ionic charge is $Z\sim10$, inverted transitions give rise to radiation of wavelengths $\lambda\sim$ 5–15 nm. The dependences of the population inversion and gain on the plasma density and temperature are derived. Consideration is given to the possibility of generation of coherent radiation as a result of stimulated anti- Stokes scattering of visible laser radiation in plasma systems of this kind.