The X-ray free-electron lasers (FELs) have stimulated the growing interest of researchers in different fields. This gives rise to an increasing simulation work of design and optimization of FEL facilities and demonstrations of novel FEL ideas. Most of the multi-dimensional simulation codes in use require large computational resources, while one-dimensional simulation codes can merely give an acceptable description of the FEL amplification process. This paper presents the development of a super-fast time-dependent FEL simulation code, which is mainly designed for seeded FEL and its application on a seed FEL facility. The diffraction factors have been introduced in order to model the effects of radiation field diffraction and drastically simplified the working equations. Meanwhile, a specially designed module for seeded FEL has been added in order to study the laser-electron beam interactions with arbitrarily parameters. The code can also provide a fairly accurate, quasi-real-time assistant tool for online optimization.

Terahertz (THz) imaging holds increasing importance across various scientific fields and practical applications due to its unique characteristics. However, achieving high sensitivity and high frame rates simultaneously remains a major challenge for most current THz imaging techniques. In this paper, we demonstrate an improved THz imaging system based on Rydberg-atom vapor, which converts THz waves into visible fluorescence. A high-sensitivity camera and a high-speed camera are used simultaneously to capture the visible fluorescence. Specifically, for a 0.55 THz source, a sensor with a minimum detectable power of 41.7 aW μm⁻² at 100 fps and 43 fW μm⁻² at 6000 fps has been achieved simultaneously, with an effective imaging area larger than 100 mm². To demonstrate the spectral properties, the measured spectrum of the emitted fluorescence is presented with a high resolution of 0.1 nm. The demonstrated system can promote the development of the Rydberg-atom vapor based THz imaging technique to practical THz imaging frontier applications.

Precisely synchronized X-ray and strong-field coherent terahertz (THz) enable the coherent THz excitation of many fundamental modes (THz pump) and the capturing of X-ray dynamic images of matter (X-ray probe), while the generation of such a light source is still a challenge for most existing techniques. In this paper, a novel X-ray free-electron laser based light source is proposed to produce a synchronized high-powered X-ray pulse and strong field, widely frequency tunable coherent THz pulse simultaneously. The technique adopts a frequency beating laser modulated electron bunch with a Giga-electron-volt beam energy to generate an X-ray pulse and a THz pulse sequentially by passing two individual undulator sections with different magnetic periods. Theoretical analysis and numerical simulations are carried out using the beam parameters of the Shanghai soft X-ray free-electron laser facility. The results show that the technique can generate synchronized 4 nm X-ray radiation with a peak power of 1.89 GW, and narrow-band THz radiation with a pulse energy of 1.62 mJ, and the frequency of THz radiation can be continuously tuned from 0.1 to 40 THz. The proposed technique can be used for THz pump and X-ray probe experiments for dynamic research on the interaction between THz pulse and matter at a femtosecond time scale.

External seeded free-electron lasers (FELs) hold promising prospects for producing intense coherent radiation at high harmonics of a conventional laser. The practical harmonic up-conversion efficiencies of current seeding techniques are limited by various three-dimensional effects on the electron beam. In this paper, a novel method is proposed to extend the wavelength coverage of a seeded FEL by combining the reverse taper undulator with the echo-enabled harmonic generation. The proposed technique can significantly enhance the bunching at ultra-high harmonics and preserve the electron beam qualities from degradation by deleterious effects. Theoretical analysis and numerical simulation are performed, and the results demonstrate that stable, intense, nearly fully coherent FEL pulses with photon energy up to 1 keV can be generated. The proposed technique may open up new opportunities to obtain laser-like pulses at sub-nanometer wavelength.

Terahertz (THz) radiation is a powerful tool for exploring various scientific frontiers through THz pump–probe experiments. However, different experiments may require THz radiation with different spectral properties, such as broad-band or narrow-band, which are difficult to be generated by a single light source. In this paper, we propose a THz light source that can produce both types of THz radiation by manipulating the longitudinal profile of an electron beam and exploiting coherent transition radiation. We perform theoretical analysis and numerical simulations based on the parameters of the Shanghai soft X-ray Free-Electron Laser facility, and the results show that the proposed light source can generate broad-band THz radiation with a pulse energy of 342 µJ and narrow-band THz radiation with a pulse energy of 91 µJ. The proposed light source can offer more flexibility and versatility for free-electron laser (FEL) users to conduct THz pump–probe experiments.