The high-brightness synchrotron radiation facility “NanoTerasu” provides users with extremely stable synchrotron radiation by top-up injecting an electron beam from a 3-GeV linear accelerator into a diffraction-limited storage ring. NanoTerasu plans to develop a soft X-ray free-electron laser (SX-FEL) using the linear accelerator. To realize SX-FEL with practical gain length, it is required that the linear accelerator generates an electron beam with a peak current of 2 kA and a normalized emittance of 2 mm mrad or less. We have already developed a low-emittance electron gun system with a gridded thermionic cathode and have begun designing a low-emittance injector system using the electron gun system. The linear accelerator based on the electron gun system is planned to serve as both the storage ring’s injector and SX-FEL’s driver. This presentation describes the configuration of a compact SX-FEL linear accelerator that can be installed within existing facilities. It also presents a bunch compression scheme while maintaining slice emittance, along with particle tracking simulation results of the new injector system.

NanoTerasu is a newly constructed 3 GeV light source located in Sendai, Japan. The circumference of storage ring (SR) is 349 m and the natural emittance is 1.1 nm rad which is realized by the double-double-bend achromat lattice. The compact normal-conducting RF cavity operating in the TM020 mode with higher-order mode (HOM) dampers is employed for beam acceleration. The commissioning of the SR started in June 2023. However, the longitudinal instability was observed during the SR commissioning in 2023 at a beam current of approximately 150 mA. Although partial suppression was achieved by adjusting the RF cavity temperature, the stored beam current during user operation in 2024 remained limited to 200 mA. To overcome this limitation, a compact waveguide-overloaded kicker cavity and longitudinal bunch-by-bunch feedback (LBBF) system were developed and installed. The commissioning of the LBBF system in July 2025 enabled stable beam storage at the design current of 400 mA. The stored beam current for user operation was subsequently increased, and routine user operation at 400 mA with high reliability began in November 2025. The suppression of longitudinal instability will be presented.