Saturation property of the terahertz FEL at ISIR, Osaka University

We have been developing a Terahertz FEL based on the 40 MeV, L-band electron linac at the Institute of Scientific and Industrial Research (ISIR), Osaka University. The linac was extensively remodelled in 2002 - 2004 to realize higher stability and reproducibility of operation. At this opportunity, we added a new operation mode for FEL, in which the pulse duration of the RF power is extended to 8 μs from 4 μs in the normal mode, so that the FEL power can reach saturation. After re-commissioning of the linac, we have conducted FEL experiments again and have observed lasing at the wavelength region from 70 μm to 120 μm (4.3 - 2.5 THz) with the high peak power. The peak power in the macropulse is roughly estimated to be over 80 W. We will report the recent progress of the FEL experiments conducted with the L-band linac. (author)