Present status of Kyoto University free electron laser

A vacuum duct was newly designed and manufactured to make the undulator gap narrower in order to increase the KU-FEL performance. The width of the undulator duct is 15 mm, but the minimum undulator gap is limited to 16.5 mm due to the small flexure of the duct. After replacing the old undulator duct whose minimum width was around 19.5 mm with new one, the tunable range of KU-FEL has been extended from 5-14.5 μm to 5-21.5 μm. The FEL gain higher than 60% has been observed at largely detuned optical cavity.