The upper figure shows the concept of FEL-HHG, the goal of this project.

Ultrashort and Ultra-intense MIR-FEL pulse is generated from the electron beam provided by an electron accelerator. The MIR-FEL pulse is focused on the gas target and generate attosecond hard X-ray by high harmonic generation (HHG) of the FEL pulse.

The intensity and cut-off energy of HHG X-ray is sensitive to the CEP of the driving laser pulse. To stabilize the CEP of the MIR-FEL pulse, injection of CEP-stable laser is being planned.

In this development stage, MIR-FELs driven by normal conducting accelerators, which can only provide short macro-pulse duration, are currently being used for performing basic researches. For the real user facility, a superconducting accelerator will be used for driving MIR-FEL. Then attosecond hard X-ray pulses with several tens of MHz repetition rate could be generated. This is one of dream light sources.

Up to now, driving the HHG by MIR-FEL has not been achieved due to insufficient peak intensity of the MIR-FEL.

Therefore, in this project, KU-FEL in Institute of Advanced Energy, Kyoto University and LEBRA-FEL in Institute of Quantum Science, Nihon University are used to develop the methods to increase the peak intensity of MIR-FEL.

In KU-FEL, the electron bunch charge is increased for increasing the conversion efficiency from electron to light and for shortening the pulse duration. And then the intensity of MIR-FEL could be increased.

In LEBRA-FEL, utilization of external pulse stacking cavity and cavity dumping scheme will be studied for increasing the peak intensity of MIR-FEL.

Before starting this project, the maximum conversion efficiency of KU-FEL was 5.5%. After the 1st upgrade of the facility, we have achieved increase of the conversion efficiency of KU-FEL up to 9.4%. This is the world highest conversion efficiency ever realized by a oscillator-type FEL.

Further increase of electron bunch charge is being planned as the 2nd upgrade. The 2nd upgrade will double the conversion efficiency and realize dramatic increase of peak intensity of KU-FEL.