LASING IN VISIBLE OF A STORAGE-RING FREE-ELECTRON LASER AT ETL

Lasing at 598 nm of a storage-ring free electron laser (FEL) has been observed at the Electrotechnical Laboratory (ETL), though the output power was weak because of the low gain due to the short straight section of the ring. The storage ring was operated in a 3-bunch mode at 230 MeV instead of a full 18-bunch mode. Coherent instabilities of the stored electron beam was suppressed by exploiting a Landau cavity with the 2nd harmonic of the ring frequency 171.62 MHz. The beam current during the oscillation ranged from 5 mA to 3 mA per bunch. A 1.47 m optical klystron consisting of NEOMAX-35 permanent-magnet blocks has been used to enhance the FEL gain. The number of periods (76 mm) in a usual-undulator section is eight. The round-trip cavity loss of dielectric multilayer mirrors, separated by 5.238 m, was measured to be about 2 x 10(-4). The laser linewidth was about 0.3 nm, and the peak FEL output power was deduced to be at least 10 mW from a rough measurement.