The first lasing at around 590 nm of a free-electron laser (FEL) in the storage ring NIJI-IV was achieved on August 18, 1992, though just above threshold. The NIJI-IV is a racetrack-type ring dedicated to FEL, which was completed in December 1990. Though the ring is compact, it has two 7.25-m straight sections. The electron beam was stacked for the first time in February 1991. After a 6.3-m optical klystron was installed in one of the straight sections, FEL related experiments were started at the end of April this year, and oscillation experiments have been carried out since the beginning of August. The electron-beam energy during the lasing was about 240 MeV, and the current was 1.1−0.2 mA/bunch. The laser wavelength ranged from 594.5 to 588.7 nm, and the bandwidth was below 0.4 nm.

First lasing of the NIJI-IV storage-ring free-electron laser

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 2×10 −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.

Lasing in visible of a storage-ring free electron laser at ETL

A visible laser oscillation has been achieved on the electron storage ring TERAS at 598 nm using a 1.47 m optical klystron. TERAS was operated in a three-bunch mode at a beam energy of 231 MeV with a beam current ranging from 3 to 10 mA per bunch. Coherent phase oscillations of the beam bunches were successfully suppressed by controlling a second harmonic RF cavity (Landau cavity) whose driving frequency is 343.24 MHz. The laser linewidth was observed to be 0.26 nm, including the detector resolution (0.21 nm). The spectral intensity was about 104 times as large as that of the spontaneous emission. The peak laser power of the microtemporal pulse is roughly measured to be 10 mW at beam current 4 mA/bunch.

Visible oscillation of storage-ring free electron laser on TERAS

The macrotemporal structure of the storage ring free electron laser is studied in relation to deterministic chaos laws. Comparison between experimental and theoretical results shows that the laser obeys such laws. This can have some important consequences for the future operation of free electron lasers.

Yves Petroff

Papers

Free Electron Lasers II