Enhanced super-radiance from energy-modulated short electron bunch free-electron lasers

In this paper we show that free-electron laser (FEL) super-radiant (SR) emission can be greatly enhanced by means of energy modulation of the radiating electron bunch. In FELs, super-radiant emission from short electron bunches is strongly reduced as the temporal duration of the bunch is increased. This prevents the realization of SR FELs operating at submillimeter wavelengths. We analyze the performance of a SR FEL source operating in the THz regime, which is driven by an energy-modulated short duration electron bunch. We carried out analytical evaluations and numerical simulations utilizing a space-frequency 3D model. These show that a remarkable improvement in super-radiant emission intensity can be achieved by the use of appropriate linear modulation (chirp) of the beam energy within the bunch. Possible limitations in applying this method are discussed, as well as the spectral purity of the emitted radiation.