Abstract
The electron-beam qualities with and without free electron laser (FEL) oscillations were investigated in the compact storage ring NIJI-IV. The peak-electron density in a bunch was suppressed by beam instabilies, so that it was limited to about 1.0 ×1017 m-3. The maximum FEL gain estimated for 215 and 300 nm using a well-known one-dimension theory was about 2.8% and 4.6%. The cavity loss at 300 nm was evaluated to be about 2% from the threshold beam current. The bunch length and energy spread with FEL oscillations increased by 1.3 times or more due to bunch heating. The ratio of the FEL gain to a cavity loss estimated from the beam qualities with and without FEL oscillations was almost in accord with the ratio evaluated directly with the measured data of the FEL gain and the cavity loss.