Zhimin Dai

Proposal for an X-Ray Free Electron Laser Oscillator with Intermediate Energy Electron Beam

Harmonic lasing of low-gain free electron laser oscillators has been experimentally demonstrated in the terahertz and infrared regions. Recently, the low-gain oscillator has been reconsidered as a promising candidate for hard x-ray free electron lasers, through the use of high reflectivity, high-resolution x-ray crystals. In this Letter, it is proposed to utilize a crystal-based cavity resonant at a higher harmonic of the undulator radiation, together with phase shifting, to enable harmonic lasing of the x-ray free electron laser oscillator, and hence allow the generation of hard x-ray radiation at a reduced electron beam energy. Results show that fully coherent free electron laser radiation with megawatt peak power, in the spectral region of 10-25 keV, can be generated with a 3.5 GeV electron beam.

Compensating the electron beam energy spread by the natural transverse gradient of laser undulator in all-optical x-ray light sources

All-optical ideas provide a potential to dramatically cut off the size and cost of x-ray light sources to the university-laboratory scale, with the combination of the laser-plasma accelerator and the laser undulator. However, the large longitudinal energy spread of the electron beam from laser-plasma accelerator may hinder the way to high brightness of these all-optical light sources. In this paper, the beam energy spread effect is proposed to be significantly compensated by the natural transverse gradient of a laser undulator when properly transverse-dispersing the electron beam. Theoretical analysis and numerical simulations on conventional laser-Compton scattering sources and high-gain all-optical x-ray free-electron lasers with the electron beams from laser-plasma accelerators are presented.

Numerical modeling of a high power terahertz source in Shanghai

On the basis of an energy-recovery linac, a terahertz source with a potential for kilowatts of average power is proposed in Shanghai, which will serve as an effective tool for material and biological sciences. In this paper, the physical design of two free electron laser (FEL) oscillators, in a frequency range of 2-10 THz and 0.5-2 THz respectively, are presented. By using three-dimensional, time-dependent numerical modeling of GENESIS in combination with a paraxial optical propagation code, the THz oscillator performance, the detuning effects, and the tolerance requirements on the electron beam, the undulator field and the cavity alignment are given.

Polarization control proposal for Shanghai deep ultraviolet free electron laser

In this paper, a fully coherent radiation option with controllable polarization is proposed for the Shanghai deep ultraviolet free electron laser (FEL) test facility. Intensive start-to-end simulation suggests that the two crossed planar undulators which generate the horizontal and vertical linear polarized FEL, should be placed as close as possible for avoiding the polarization performance degradation of the final combined FEL radiation. With the existence of the phase-shifter between the two crossed radiators. Fourier-Transform-Limited output radiation with 100 nJ order pulse energy, 5 Ps full pulse length and circular polarization degree above 90% could be achieved

Design a multi-port THz Fourier transform spectrometer

In this paper, a multi-port THz Fourier transform spectrometer is designed to measure Terahertz spectrum. Multi-port optical design make spectrometer more flexible used on pot light source and collimated light source directly, which eliminating the need of external conversion optical path for different light source, and also allows using a variety of external detectors flexibly, extending the spectrometer’s measured range. The Fourier transform spectrometer is described.

Coherent photon beam based diagnostics for a seeded extreme ultraviolet free-electron laser

Independently from conventional electron beam based procedures, the photon beam based diagnostic is an alternative way for electron trajectory alignment and commissioning of numerous undulator cells in high-gain short-wavelength free-electron lasers (FELs). In this paper, using the microbunched electron beam and the undulator fine-tuning technique, a novel method based on the spatial profile analysis of coherent harmonic radiation from individual or two consecutive undulator segments, the so-called coherent photon beam based diagnostic, is proposed for undulator alignment and commissioning in seeded FELs. The preliminary experimental results at the Shanghai deep ultraviolet FEL test facility are presented. It shows that the proposed method is effective and convenient in the electron beam trajectory control, the undulator magnetic gap verification, and the phase match between two undulator segments.