Abstract
In this paper we investigate and compare the properties of two narrow-bandwidth free-electron laser (FEL) schemes, one using self-seeding and the other high gain harmonic generation (HGHG). The two systems have been thoroughly studied analytically and numerically in the past. The aim of this work is to compare their performances when the FEL is driven by an electron beam with nonideal properties, thus including effects such as shot-to-shot energy fluctuations and nonlinear energy chirp. In both cases nonlinearities produce a bandwidth larger than the Fourier transform limited value. However, our analysis indicates that, for approximately the same output power levels, the self-seeding scheme is less affected than the HGHG scheme by quadratic energy chirps in the electron beam longitudinal phase space. This is confirmed by a specific numerical example corresponding to SPARX parameters where the electron beam was optimized to minimize the FEL gain length. The work has been carried out with the aid of the time dependent FEL codes GENESIS 1.3 (3D) and PERSEO (1D).
Highlights
The unique characteristic of a free-electron laser (FEL), with respect to other sources of electromagnetic radiation based on the emission from relativistic electrons, is the large number of photons in the coherent volume
This means that the bandwidth is larger than the Fourier transform limit, except for the case when the electron bunch is short with respect to the cooperation length, and only a single spike is present in the radiation pulse
When the electron bunch is longer than the cooperation length, and many spikes are present in the output pulse, the number of photons in a coherent volume could be further increased by reducing the bandwidth to the transform limit or near to it
Summary
The unique characteristic of a free-electron laser (FEL), with respect to other sources of electromagnetic radiation based on the emission from relativistic electrons, is the large number of photons in the coherent volume. While a SASE source is practically diffraction limited [1], its bandwidth is determined by the FEL cooperation length [2] This means that the bandwidth is larger than the Fourier transform limit, except for the case when the electron bunch is short with respect to the cooperation length, and only a single spike is present in the radiation pulse. We will show that these deviations make the FEL bandwidth larger than the transform limit, and produce large pulse-to-pulse intensity fluctuations The results of these realistic beam studies show that the photon pulses produced in the self-seeding and HHG seeding schemes have similar characteristics, with the self-seeding having a lower sensitivity to the beam deviation from the ideal case. The scope of this paper is not to discuss optimization methods for the production of electron beams suitable for narrow-bandwidth FEL operation but rather that of describing some nonideal effects that are relevant for the production of narrow-bandwidth radiation
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