Beam quality in storage ring NIJI-IV

Several remedies to achieve a high peak current with storage ring NIJI-IV are presented. We observe a bunch lengthening and a limitation on the single-bunch current, which reduces the FEL gain remarkably. To suppress with the coupled-bunch instability, we install a longitudinal feedback system which consists of the phase detector and phase shifter. This feedback system effectively suppresses the coupled-bunch instability and shortens the electron bunch. However, we need a solution to the limitation on the single-bunch current originated from a head-tail instability. Quadrupole and 6-pole magnets have been designed to be installed in a restricted space between bending magnets. Calculations on the injection orbit and dynamical aperture show that there should be a large aperture in the ring with the small bore, 45 mm. The installation of these small bore magnets will be started coming Autumn.