Optical property of infrared-FELs at the FELI

First lasings at 5.5 and 1.88 μm were achieved in October 31, 1994 and February 27, 1995 at the FELI using a 2-m long undulator and a 3-m long one installed at 30 and 80-MeV beam lines of the FELI S-band linac with a thermionic gun, respectively. Lasing on the third harmonics at 0.63 μm was also observed in February 28. 24-μs stable rf pulses help FEL oscillations at short wavelengths. At 7.5, 1.88 and 0.63 μm, FEL peak powers are 3.5, 2, and 0.45 MW, respectively. The present maximum average power is 0.14 W around 7.5 μm at a 0.5-mm mirror aperture of the optical cavity for the 2-m long undulator for 4000 micropulses/s.