On the Instabilities of the Collisionless and Non-Magnetized Active Plasmas

Spontaneously excited instabilities in a uniform, collisionless and non-magnetized active plasma are studied, under the assumption that the polarisability of its active constituent is due to transitions of only one optical electron between two inversely populated energy levels. The attention is focused on non-resonant situations in which the modal frequencies of the excited waves differ significantly from the frequency of coherent electromagnetic radiation arising due to the presence of the active constituent. The longitudinal (‘acoustic’) waves in both low and high phase velocity domains, as well as the transverse (‘optical’) waves, are investigated. It is shown that the instabilities excited are most frequently aperiodical, and appear within, a comparatively large wavelength range. Their amplitude increments are evaluated. Under certain particular conditions, specified in the paper, the instabilities turn out to be periodical and to appear in very narrow wavelength bands only. Their modal frequencies and amplitude increments are also evaluated.