Characteristics of the NIJI-IV FEL at the electron-beam energy of 263 MeV

Electron-beam qualities and characteristics of the FEL oscillation with the NIJI-IV FEL system were investigated at the low electron-beam energy of 263 MeV. The peak electron density at 263 MeV was suppressed due to instabilities in high electron-beam current, but it was about 40% higher than that at the normal electron-beam energy of 310 MeV below 4 mA. The maximum gain estimated from the measurements of the electron-beam qualities was about 1.5% at 4 mA. The measurement of the detuning curve suggests that the practical gain was about 0.8 times as low as the estimated value. Moreover, variation of the average FEL power versus the electron-beam current and the transverse profile was observed with a CCD camera system. The typical average power of the FEL was about 53 μW at 3.2 mA and we were able to observe up to the TEM03 mode.