Solving the spectral gap problem with a four-mirror bow-tie cavity in waveguide free-electron laser oscillators.

The generation of intense coherent radiation pulses in the far-infrared and terahertz regimes is of considerable interest to the free-electron laser (FEL) radiation user community. At long wavelengths, the diffraction effect can be quite severe, therefore, an optical waveguide is required to confine the radiation field. However, it will also bring about some new phenomena, and the most noteworthy one is the spectral gap phenomenon: at some particular wavelengths, regardless of electron beam adjustments, the coupling efficiency and output power of waveguide FEL oscillators drop significantly. Such spectral gap has an adverse effect on experimental results since numerous experiments require continuous spectral scanning. In this paper, we propose to utilize a bow-tie cavity instead of conventional cavities to the waveguide FEL to solve the spectral gap problem. The simulation was carried out based on the parameters of FELiChEM, a newly built user facility in China. Numerical simulation code OPC combining with modified GENESIS is used to enable the modelling, for the first time, of a bow-tie cavity based FEL in the far-infrared wavelength regime. The simulation results indicate that this novel structure can effectively eliminate the spectral gaps and substantially enhance long-wavelength laser performance.

Generating three-color pulses in high-gain harmonic-generation free-electron lasers with a tilted electron bunch.

A multi-color light source is a significant tool for nonlinear optics experiments, pump-dump/repump-probe experiments and in other fields. Here, a novel method is proposed to create three-color pulses based on a high-gain harmonic-generation (HGHG) free-electron laser with a tilted electron bunch. In this method, the initial bunch tilt is created by transverse wakefields after the bunch passes through a corrugated structure with an off-axis orbit, and is further enlarged in a following drift section. Then the tilted bunch experiences the off-axis field of a quadrupole magnet to cool down the large transverse velocity induced before. After that, it enters an HGHG configuration adopting a transverse gradient undulator (TGU) as the radiator, where only three separated fractions of the tilted bunch will resonate at three adjacent harmonics of the seed wavelength and are enabled to emit three-color pulses simultaneously. In addition, the use of the natural transverse gradient of a normal planar undulator instead of the TGU radiator to emit three-color pulses is also studied in detail. Numerical simulations including the generation of the tilted bunch and the free-electron laser radiation confirm the validity and feasibility of this scheme both for the TGU radiator and the natural gradient in the extreme-ultraviolet waveband.

Phase-merging enhanced harmonic generation free-electron laser with a normal modulator.

A phase-merging enhanced harmonic generation free-electron laser (FEL) was proposed to increase the harmonic conversion efficiency of seeded FELs and promote the radiation wavelength towards the X-ray spectral region. However, this requires a specially designed transverse gradient undulator (TGU) as the modulator to couple the transverse and longitudinal phase space of the electron beam. In this paper, the generation of the phase-merging effect is explored using the natural field gradient of a normal planar undulator. In this method, a vertical dispersion on the electron beam is introduced and then the dispersed beam travels through a normal modulator in a vertical off-axis orbit where the vertical field gradient is selected properly in terms of the vertical dispersion strength and modulation amplitude. The phase-merging effect will be generated after passing through the dispersive chicane. Theoretical analysis and numerical simulations for a seeded soft X-ray FEL based on parameters of the Shanghai Soft X-ray FEL project are presented. Compared with a TGU modulator, using the natural gradient of a normal planar modulator has the distinct advantage that the gradient can be conveniently tuned in quite a large range by adjusting the beam orbit offset.