3rd-harmonic Lasing at JAERI-FEL

(MO-1-03) Presenting Author: E.J. Minehara

R.Hajima, R.Nagai, N.Nishimori, N.Kikuzawa, E.J.Minehara  

Japan Atomic Energy Research Institute

We have demonstrated FEL lasing at 3rd harmonic in the JAERI FEL driven by a superconducting linac. The lasing was achieved with a super enhanced gold coated mirror, which is a gold-coated cupper mirror with Zn-Se multi-layers deposited on the gold surface. The enhanced mirror has quite high reflectivity, 99.9%, at the 3rd harmonic, 7 $\mu$m, but rather low reflectivity around the fundamental wavelength, 22 $\mu$m. Lasing at the fundamental wavelength, therefore, is suppressed and the 3rd harmonic only grows in the optical cavity. Lasing at the 3rd harmonic is single-supermode regime with average power of 15W so far, while the fundamental lasing has very large single-pass gain and reaches superradiance regime with FEL output over 1kW averaged within macro-pulse duration. We consider that the low output power of 3rd harmonic is due partly to energy spread of electron bunch introduced by CSR at the 180-degree arc.