Self-bunching in an electron beam during passage through a resonantly absorbing medium

It is shown that self-bunching may occur during propagation of an electron beam through a resonantly absorbing medium. The repetition period of the bunches is determined by the resonance absorption frequency and by the electron beam velocity. The dependence of the perturbation growth rate in the beam on the electron energy is obtained for the relativistic case. The feasibility of using this effect to generate and amplify radiation is analyzed.