Study of a noise degradation of amplification process in a multistage HGHG FEL

Abstract

Recent advances in free-electron laser (FEL) physics and accelerator techniques led to the possibility of generating coherent X-ray radiation with self-amplified spontaneous emission (SASE) FEL. Despite the fact that the SASE FEL is capable of providing much higher peak brilliance than spontaneous synchrotron radiation sources, there is a great potential for improvements. The brilliance of the output radiation from the SASE FEL is mainly limited by the poor longitudinal coherence of the radiation. One of the approaches to obtain fully coherent X-ray radiation uses frequency multiplication, a scheme known as the high-gain harmonic generation (HGHG) FEL. In the HGHG FEL the radiation output is derived from a coherent subharmonic laser seed pulse. Consequently, the optical properties of the HGHG FEL are expected to reflect the characteristics of the high-quality seed laser. This paper is devoted to the investigation of the physical processes in the HGHG FEL. Our studies have shown that the frequency multiplication process produces noise degradation proportional at least to the square of the frequency multiplication factor. This prevents operation of HGHG FEL at a very short wavelength range.