Supported by the Special Fund for Research on National Major Research Instruments (Grant No. 11127901), the research team from the State Key Laboratory of Strong Field Physics, Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences (CAS), has made important progress in the development of a table-top free-electron laser (FEL)——The experimental proof-of-principle of table-top FEL was realized for the first time in the world. The result entitled "free-electron lasing at 27 nanometres based on a laser wakefield accelerator", was published as a cover paper in Nature on July 22, 2021. The paper’s link is as follow: http://www.nature.com/articles/s41586-021-03678-x.

At present, FEL is the best way to produce high-brightness coherent light source in X-ray range. X-ray FELs (XFELs) can be used to detect the internal dynamic structure of matter and study the interaction process of light with atoms, molecules and condensed matter, greatly promoting the development of condensed matter physics, chemistry, structural biology, medicine, materials, energy and environment, among other disciplines. The development of compact and low-cost XFEL is very important for application expansion and technological change.

The LWFA driven by ultra-intense ultra-short lasers provides a new way for the development of miniaturized electron accelerators. In recent years, a lot of important progress has been made in LWFA, but there are still many problems and challenges for driving table-top FEL due to the insufficient quality and stability of the electron beam. The related research is still in its pioneering time. Since 2012, the United States, Germany, France, Japan, Italy and other countries have set up projects to carry out the development of table-top FEL.

In order to achieve the scientific goal of the national major scientific research instrument development project and keep ahead in the international competition, the research team from SIOM has pushed this research for 8 years with the spirit of "10 years for sword". They focused on improving the quality and stability of electron beams driven by lasers. A special plasma density distribution was designed to optimize the injection process and acceleration process of the electron beam, so that the comprehensive quality of the electron beam (including energy spread, emittance, and charge) could be effectively improved. By controlling and optimizing the phase space evolution of the electron beam, the smooth transfer of the electron beam from plasma to vacuum is realized, and the electron beam transport and undulators are designed to realize the long distance transport of the electron beam and effectively coupling them into the undulators. For the first time, the team has experimentally observed radiation in the extreme ultraviolet range, with a center wavelength of 27 nanometers and a single pulse of energy up to 150 nanojoules. It is proved that the energy gain in the last undulator is up to 100-fold by means of orbit kick and spontaneous radiation calibration. This is the first time in the world to realize the spontaneous emission amplification based on LWFA, which is of great significance for the development of table-top, compact and low-cost XFEL.

The work was reviewed in Nature, "... On page 516, Wang et al. report a milestone in the development of compact X-ray FELs...." ; " The authors’ experiment paves the way for FELs driven by extremely compact accelerators, which could be managed in university-scale facilities. One of the requirements for a new tool to facilitate discoveries is its availability, and this work is expected to increase the availability of FEL in the world."

The research team will further enhance the output power and photon energy of the FEL. As an important part of the ultra-fast chemistry and macromolecular dynamics research platform of the "XiHe" Laser Facility in Shanghai, this instrument is available for open sharing in future.

Figure1. The cover of Nature, the title is“Compact Source”

Figure 2. Device schematic of table-top FEL based on LWFA