In order to develop a stable LWFA based accelerator and demonstrate FEL generation, the unique LWFA platform was constructed in the RIKEN SPring-8 center and systematic experiments have being conducted financially supported by ImPACT (2013-2018) and JST MIRAI (2018-) programs. Although undulator radiation in an XUV spectral range driven by LWFA electron beams was successfully demonstrated on the platform in 2019, the sufficient reproducibility was not obtained due to the poor electron pointing stability and large energy fluctuations. In order to solve the above problems, the accelerated electron beam quality has been improved by developing the Shock injection scheme enabling a precise injection control and a stable plasma condition. This development has dramatically improved the reproducibility and stability of the LWFA electron beam. The preliminary proof-of-concept experiment has recently demonstrated the clear amplification of the undulator radiation and the possibility of LWFA based FEL in XUV range. The talk will be presenting the outline of the LWFA platform, the setup of a proof-of-concept experiment focusing on key improvements and obtained results.

Our recent experiments achieved EUV range undulator radiation amplification based on the stable electron beam obtained from laser wakefield accelerator (LWFA). The experiments were conducted on the LWFA platform in RIKEN Spring-8 center supported by ImPACT and JST MIRAI project. By optimizing the driving laser system and gas target, the reproducibility of the acceleration process has been significantly improved. The electron beam with central energy of 380 MeV can be steadily generated with an energy spread less than 1% and a pointing instability less than 0.5 mrad in RMS. The typical electron beams with an average charge of 15 pC were focused by three permanent magnetic quadrupoles and four electromagnetic quadrupoles to the undulators located 6.5 meters downstream to the target. The amplified undulator radiation centered at 45 nm has been detected and the maximum gain of the radiation power is approximately 14-fold. Such the demonstration is not only the first time in Japan but also one of the world leading results. Based on our current achievements, we anticipate a navigable road from EUV to X-ray wavelengths.