Tandem undulators for mid-Infrared FEL at the KEK compact energy recovery linac

Abstract At the KEK Compact Energy Recovery Linac (cERL), we have constructed two out-of-vacuum type undulators (U01 and U02) as a light source for the mid-infrared SASE FEL project. We designed the cERL-FEL undulators as adjustable phase undulators (APUs) with a fixed gap of 10 mm. The photon energy can be adjusted by moving the upper magnetic rows longitudinally relative to the lower rows. After configuring the magnetic settings to transport the 17.5 MeV electron beam through the undulators, we installed these undulators in the cERL. The operation of these tandem undulators began in June 2020. During their operation, we observed a strong vertical focusing force acting on the electron beam. In this paper, we describe the construction and operation of the cERL-FEL undulators, along with the characteristics of the focusing force produced by the APU.