Abstract
In order to generate coherent hard x-ray free-electron laser ( FEL), cascading stages of high-gain harmonic generation ( HGHG) scheme usually employ seed pulse with longitudinal length down to tens of femtoseconds. Such a short pulse length ( SPL) seed laser, with broad bandwidth in the spectral domain, can no longer be characterized by monochromatic light in FEL equations. In this paper, a set of self-consistent, multifrequency FEL equations is derived to describe the interaction between the seed laser and the electrons in the modulator of HGHG. Moreover, the SPL effects of the seed laser on the electron beam's energy modulation, the output pulse length, the output peak power, and the output wavelength tuning in HGHG are theoretically and numerically investigated. Study demonstrates that the SPL seed laser seriously and significantly influences the HGHG performances, and these influences have to be taken into account in the design and optimization of the seeded FEL scheme.
Highlights
Free-electron lasers (FELs) are devices that use the relativistic electron beams passing through a transverse periodic magnetic field in order to generate coherent electromagnetic radiation ranging from the infrared to the hard x-ray regions
Study demonstrates that the short pulse length (SPL) seed laser seriously and significantly influences the high-gain harmonic generation (HGHG) performances, and these influences have to be taken into account in the design and optimization of the seeded FEL scheme
In recent years, taking self-amplified spontaneous emission (SASE) [1] and high-gain harmonic generation (HGHG) [2] as two leading candidates for approaching the deep ultraviolet to hard x-ray spectral region, people are increasingly interested in the highgain, short-wavelength FEL
Summary
Free-electron lasers (FELs) are devices that use the relativistic electron beams passing through a transverse periodic magnetic field in order to generate coherent electromagnetic radiation ranging from the infrared to the hard x-ray regions. Benefited from the high quality seed laser, HGHG provides radiations with a high degree of stability whereas the central wavelength, bandwidth, and pulse duration can be controlled. These theoretical predictions have been demonstrated in the first HGHG proof of principle experiment [3]. In hard x-ray FEL generated by the cascading HGHG technique, the practicable pulse length of the initial seed laser is usually in tens of femtoseconds order. On the basis of multifrequency FEL equations, the SPL seed laser’s effects in HGHG are theoretically estimated and numerically simulated.
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