Improvement of the RF system in the storage ring NIJI-IV for VUV free electron lasers

A new RF cavity was installed in place of the old one in the storage ring N IJI-IV and electron-beam qualities in the NIJI-IV were investigated. This c avity has a re-entrant-type design and its resonant frequency is about 162. 2 MHz, which is basically the same as that of the old one. However, its des ign was modified to realize more stable and higher peak-density electron-be am for free electron laser (FEL) oscillations in the VUV region. It has two plungers which are of the same size, and one of the electrodes can move al ong the beam axis. The electron beam is stabilized by adjusting the positio n of the plungers, and the beam current in the single-bunch operation is ov er 30 mA. Moreover, efficient cooling system of the cavity allows us to sup ply much higher RF input, which would lead the natural bunch length to be s horter. We will also improve the RF power supply to increase the maximum RF input from 2 to 10 kW next autumn. (C) 2000 Elsevier Science B.V. All righ ts reserved.