0.1 kW operation of the JAERI superconducting RF linac-based FEL

First and stable laser oscillation has been obtained around a wavelength of 24 μm using the Japan Atomic Energy Research Institute, Tokai (JAERI) superconducting RF-linac-based FEL driver and far infrared FEL device. The electron beam energy and spread are 15.8 MeV and 0.6%, respectively. The beam current varies between 2 and 4 mA, and the pulse width between 0.1 and 0.9 ms, respectively. The near-concentric optical resonator is 14.4 m long, and uses gold-coated copper mirrors 120 mm in diameter. The hybrid, planar undulator has 52 periods, 33 mm in length and K = 0.7. Remote-controlled actuators precisely adjust the optical axes and distance of the mirrors in order to coincide with the electron beam and micro pulse repetition rate, respectively, before oscillation. The power has been measured and is scattered from 10⁷ to 10⁸ times higher than that of the spontaneous emission. During the first successful operation, the highest average FEL power was measured to be about a hundred watts. The FWHM of the FEL spectrum is around the Fourier-transform limited value, and less than 0.09 μm, which corresponds to Δλ/λ = 0.37%. The detuning curve of the cavity is asymmetric, and spans about 15 μm.