OPTICAL-PERFORMANCE OF THE UT-FEL AT 1ST LASING

First laser oscillation has been obtained at a wavelength of 42.8 mum on June 25, 1993, using a 15-MeV S-band linac, in collaboration with the University of Tokyo. The optical resonator with a 40 period vertical undulator (K = 0.95) is 3.78 m long and uses Au coated Cu mirrors. The optical axis of the mirrors is adjusted by CPU controlled actuators in order to coincide with the electron beam, which has been adjusted to pass through the center of the magnetic field as close as possible in advance. The power is about 10(4) times higher than that of the spontaneous emission. The FWHM of the FEL spectrum is less than 0.51 mum, which corresponds to DELTAlambda/lambda = 1.2%. The tuning range of the cavity is about DELTAZ = 20 mum.