Resonance conversion of waves in modulated electron beams and in systems with modulated traveling parameters

An analysis is made of phase-matched (Bragg) reflection of waves from density-modulated relativistic electron bunches and from regions with a modulated traveling parameter. A mechanism is suggested and calculated for parametric resonance modulation of the density of relativistic electrons in a beam subjected simultaneously to incident radiation and a spatially periodic static magnetic field. Quantitative estimates are obtained for experimentally feasible devices.