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 NIJI-IV and electron-beam qualities in the NIJI-IV were investigated. This cavity 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 design was modified to realize more stable and higher peak-density electron-beam 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 along the beam axis. The electron beam is stabilized by adjusting the position of the plungers, and the beam current in the single-bunch operation is over 30 mA. Moreover, efficient cooling system of the cavity allows us to supply much higher RF input, which would lead the natural bunch length to be shorter. We will also improve the RF power supply to increase the maximum RF input from 2 to 10 kW next autumn.