Abstract
For a planar FEL configuration we study stimulated coherent spontaneous emission driven by a gradient of the bunch current in the presence of different levels of noise in bunches. To perform a vast amount of simulations required for obtaining statistically valid results, we developed a memory and time efficient one-dimensional simulation code based on the integral solution to a Klein-Gordon equation describing the field evolution. The longitudinal granularity of the electron bunch density originating from shot noise is maintained throughout the analysis. Three-dimensional effects like transverse emittance and diffraction are taken into account in simulations via an effective FEL parameter calculated from Xie's fitting formula. Calculations are performed for an FEL model with the SwissFEL injector bunch parameters. It turns out that a reduction of noise by several orders of magnitude below the level of shot noise is required to mitigate the noise effect. We propose a novel scheme that allows for 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 shot noise suppression section as well as a bunch compressor. The noise factor and microbunching gain with and without laser heater are estimated. We found that shot noise reduction by three orders of magnitude can be achieved for a finite transverse size electron bunch.
Highlights
Operating free electron lasers in the vacuum ultraviolet (VUV) and x-ray regions are usually based on the self-amplified spontaneous emission (SASE) process starting from spontaneous emission of undulator radiation initiated by shot noise [1]
Recent experiments on self-seeded free electron laser (FEL) using x rays from the first half of the undulator to seed the second half through a diamond-based monochromator [6] and FEL operating in a SASE mode with a chirped electron beam and a tapered undulator [7] showed a substantial reduction of the relative FEL bandwidth
In view of the shot noise reduction schemes, the intrinsic seeding driven by the current gradient of quiet electron bunches becomes an attractive way of obtaining pulse-topulse stable stimulated CSE radiation without external seeding lasers
Summary
Operating free electron lasers in the vacuum ultraviolet (VUV) and x-ray regions are usually based on the self-amplified spontaneous emission (SASE) process starting from spontaneous emission of undulator radiation initiated by shot noise [1]. In view of the shot noise reduction schemes, the intrinsic seeding driven by the current gradient of quiet electron bunches becomes an attractive way of obtaining pulse-topulse stable stimulated CSE radiation without external seeding lasers. In our scheme bunches from the injector pass successively through a laser heater, a bunch compressor, and a noise suppression section The latter is used to obtain quiet bunches whereas the former two are employed to achieve a high current gradient at the bunch tail using the controlled microbunching instability in a way similar to that used in the longitudinal space-charge amplifier (LSCA) [36]. The obtained results are summarized in the Conclusion, and, the numerical solution to the self-consistent FEL model and the feasibility of FELs based on CSE in the VUV region are discussed in Appendices A and B, respectively
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