Effect of beam prebunching on gain and efficiency in a free electron laser: Nonlocal theory

Abstract

Beam prebunching on the free electron laser (FEL) offers considerable enhancement in growth rate, efficiency, and gain when the prebunched beam electrons are in the retarding zone and the electron beam is highly modulated implying the largest beam oscillatory velocity due to wiggler. A nonlocal theory of this process has been developed. The growth rate, efficiency, and gain were evaluated based on existing experimental parameters and compared with available theoretical/experimental results. Nonlocal effects reduce the linear growth rate of the FEL instability. The growth rate, efficiency, and gain of the FEL instability increase with the modulation index and has the largest value when the modulation index approaching unity in addition to the frequency and wave number of the prebunched beam are comparable to that of the radiation wave. The growth rate of the FEL instability scales as one-third power of the beam current in the Compton regime and one-fourth power in the Raman regime.