Improvement of the beam duality by chromaticity correction for wavelength shortening in the NIJI-IV FEL

Electron-beam qualities improved by chromaticity correction in the storage ring NIJI-IV were investigated at the beam energy of 309 MeV. Sextupole-qua drupole-sextupole (SQS) magnets, which were installed in all of the short-s traight sections in NIJI-IV, perfectly corrected a horizontal and a vertica l chromaticity. This improvement suppressed a head-tail instability, so tha t higher beam current (similar to 30 mA) and higher peak-electron density ( similar to 6 x 10(16) m(-3)) were available for FEL experiments. The maximu m FEL gain was estimated to be about 2.5% at a wavelength of 240 nm. The la sing of an FEL around 300 nm was achieved in March 1998, and the lasing of an FEL at around 240 nm was successfully observed in May 1998. The shortest wavelength of FELs with the NIJI-IV FEL system was 228 nm. (C) 1999 Elsevi er Science B.V. All rights reserved.