Superradiant and stimulated superradiant emission in a prebunched beam free-electron maser

An electron beam, prebunched at. the synchronous free-electron laser freque ncy and passing through a magnetic undulator, emits coherent (superradiant) synchrotron undulator radiation at the bunching frequency. If an external electromagnetic wave is-introduced into the interaction region,at the same frequency and at a proper phase, the radiation process will be stimulated ( stimulated prebunched beam radiation). We report first experimental measure ments of stimulated superradiant emission in a prebunched free-electron mas er. Measurements are in good agreement with theory.