Measurements and simulation of the radiation build-up process in a prebunched free-electron maser oscillator

Experiments on the radiation build-up process obtained using a prebunched e-beam free-electron maser at Tel-Aviv University (TAU) are compared to results of theoretical studies carried out at TAU and at the University of Maryland (UMD). Two computer codes were developed and employed for simulation of FEM operation. A non-linear three-dimensional "amplifier" code based on a coupled-mode approach was employed for calculations of small-signal gain, extraction efficiency and saturation power. This code (FEM3D) allows simulation of FEL operation taking into account space-charge effects. The power evolution of several longitudinal modes was also studied numerically using a one-dimensional, multi-frequency simulation code MALT1D; it was also observed in initial experiments. Numerical calculations of extraction efficiency for the TAU-FEM were made for all resonator eigen-frequencies lying under the FEM net gain curve. It was found that for a constant set of parameters the maximum efficiency is obtained at an eigen-frequency, which differs from the maximum gain frequency. Prebunching of the e-beam provides a unique opportunity to choose any desired oscillator eigen-frequency and thus to select the highest efficiency mode. This makes it possible to obtain efficiency enhancement of the oscillator by a factor of about 2.