Self-amplified coherent spontaneous emission in a free electron laser with “quiet” bunches

For a planar free electron laser (FEL) configuration we study self-amplified coherent spontaneous emission driven by a gradient of the bunch current in the presence of different levels of noise in bunches. The longitudinal granularity of the electron bunch density originating from shot noise is maintained throughout the analysis. For the FEL model with the SwissFEL injector bunch parameters, we calculate the probability density distribution of the maximum power of the radiation pulses for different levels of shot noise. It turns out that the temporal coherence quickly increases as the noise level reduces. We also show that the FEL based on coherent spontaneous emission produces almost Fourier transform limited pulses. The analysis indicates that the time-bandwidth product is mainly determined by the bunch length and the interaction distance in an undulator. Calculations of the FEL characteristics for different rise times of the front of the current pulse are performed, and it is found that a reduced level of the power fluctuations is preserved for the bunch current pulse with a front duration up to several FEL wavelengths. We also propose a novel scheme that permits the formation of electron bunches with a reduced level of noise and a high gradient of the current at the bunch tail to enhance coherent spontaneous emission. The presented scheme uses effects of noise reduction and controlled microbunching instability and consists of a laser heater, a bunch compressor, and a shot noise suppression section. We show that shot noise reduction by 2 orders of magnitude in electron bunches produced by the SwissFEL injector can be achieved in a compact noise suppression section. The noise factor and microbunching gain of the overall proposed scheme with and without laser heater are estimated