Recently, free electron laser teams, Shanghai Advanced Research Institute, Chinese Academy of Sciences and Shanghai Institute of Applied Physics, Chinese Academy of Sciences make important progress in external seed free electron laser, thus putting forward a self-amplification mechanism of coherent energy modulation in theory. The experimental verification has been completed based on Shanghai soft X-ray device free electron laser (SXFEL), indicating that this new mechanism can greatly reduce the power demand of the external seed free electron laser to the external seed electron laser, and solve the key problem of the external seed free electron laser running to high repetition frequency. The research results with the title "Self-Amplification of Coherent Energy Modulation in Seeded Free-Electron Laughers" was published in Physical Review Letters. The first author was Dr. Jia-Wei Yan. Researcher Hai-Xiao Deng and Academician Zhen-Tang Zhao were the co-corresponding authors.

Fig.1 Schematic diagram of self-amplification mechanism of coherent energy modulation.

Free electron lasers, which use relativistic electron beams to produce femtosecond to attosecond X-ray pulses of extremely high brightness, have now become a key tool in many fields, including atomic and molecular physics, materials science and life science, etc.. Externally seeded free electron laser (EFL) uses the external seeded laser to introduce coherent energy modulation to the electron beam cluster, thus producing highly stable and fully coherent free electron laser pulses. However, the current running mode for the performance demand of the seed laser, especially the peak power, has become the bottleneck for the external seed free electron laser running at a high repetition rate.

In this work, researchers propose to use the self-modulation of the electron beam cluster to amplify the initial coherent energy modulation, thus relaxing the peak power demand of the external seed laser by 1-2 orders of magnitude. At the same time, using the existing hardware conditions of Shanghai Soft X-ray Free Electron Laser Facility, we successfully demonstrated the relaxation of the peak power demand of seed laser by 10-25 times. In the experiment, the 266 nanometer seed laser only introduced the energy modulation of 1.8 times slice energy dispersion. Through self-amplification, the single-stage HGHG in the seed laser achieved 7 times harmonic amplification, and the two-stage cascade HGHG in the seed laser achieved 30 times harmonic amplification. At present, this is internationally the highest efficiency of external seeded free electron laser amplification with working harmonic/laser modulation. This result paves the way for the external seed free electron laser with the megahertz repetition frequency and is expected to bring new breakthroughs in time-resolved spectroscopy and extreme ultraviolet lithography. 

The successful experimental operation of the self-amplification mechanism in SXFEL has important practical significance and far-reaching influence on the development of the current and future free electron laser and other large scientific devices. At present, a group of free electron laser devices of high repetition rate driven by superconducting accelerators, such as European XFEL, LCLS-II, SHINE, are internationally being constructed and put into operation. The amplification mechanism can be the standard mode for the next generation of external seed light source driven by continuous wave superconducting accelerators. In addition, the amplification mechanism of coherent energy modulation has important potential application prospect in improving the stability of external seed free electron laser, strengthening cluster factor, extending working wavelength and super fast pulse and so on.

The research was supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, the Chinese Academy of Sciences and Shanghai Municipal Government.

The article links : https://journals.aps.org/prl/abstract/ 10.1103/ PhysRevLett. 126. 084801