Super-radiance in a prebunched beam free electron maser

Abstract

It is well known that electrons passing through a magnetic undulator emit partially coherent radiation: “Undulator Synchrotron Radiation”. Radiation from electrons, entering the undulator at random, adds incoherently. If the electron beam is periodically modulated (bunched) to pulses shorter than the radiation wavelength, electrons radiate in phase with each other, resulting in super-radiant emission at the bunching frequency. Introduction of a signal at the input of the prebunched beam FEL, results in stimulated super-radiant emission. The interaction between the electromagnetic wave and a synchronous modulated e-beam results in amplification of the signal wave in addition to the spontaneous super-radiant emission. We demonstrated and measured the super-radiant emission in a wide band of frequencies from 3.15 to 5.5 GHz using the mini-FEM of Tel-Aviv University, wherein pre-bunching at the radiation frequency is accomplished with the aid of a traveling-wave prebuncher. The measured upper synchronous frequency is centered about 4.5 GHz and the lower synchronous frequency is just above cutoff (near 3.153 GHz). Analytical models, computer simulations and experimental results of a pre-bunched free-electron laser operation are presented and compared. The power levels that can be achieved are discussed. The measured results agree well with results predicted theoretically and obtained by a 3D simulation code.