The Los Alamos free-electron laser (FEL) oscillator has been successfully operated over the wavelength range from 9 to 11 μm with a peak output power of 1 MW and an average output power of 1 kW over the 70-μsec pulse length. This FEL is driven by a conventional rf linear accelerator operating at 1.3 GHz with a nominal energy of 20 MeV. Sufficient rf power is supplied to the accelerator to produce a burst of 2000 uniform current pulses, each ~30 psec in width and spaced by 46 nsec, the round-trip time of light in the 6.9-m optical cavity. Each pulse had a peak current of ~25 A, an energy spread of 1–2%, and an emittance of 1-2π mm-mrad. For the present experiments, we used a 1-m long undulator composed of SmCo5 permanent magnets in a uniform-period plane-polarized configuration. Even with such a short undulator, the high-peak-current electron pulses resulted in a single-pass gain of up to 20%, which is in accord with theoretical predictions using a 1-D code. Multilayer dielectric reflectors, centered at 10 μm, on ZnSe substrates served as the cavity mirrors with output coupling of 3% from one end. The bandwidth of the mirrors restricted the continuous tuna- bility of the oscillation to the 9–1spectral range.
© 1984 Optical Society of AmericaPDF Article