Linear model formulation for superradiant and stimulated superradiant prebunched e -beam free-electron lasers

We present a unified model and analysis of radiation modes excitation in free-electron lasers with a periodically premodulated electron beam. The formulation characterizes superradiant coherent radiation emission from electron beam prebunched at the synchronous frequency, including the case of stimulated superradiant where in addition to a prebunched electron beam an electromagnetic wave is injected into the undulator region. The derived formulation of radiation mode excitation by currents is applicable to general structures that can support orthogonal eigenmodes.