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We report on experimental studies of an ultraviolet (UV) free-electron laser (FEL) oscillator driven by low-energy electrons from a radio-frequency linear accelerator. Previous UV FELs have been driven by 350-800-MeV electrons from storage rings. We verify the concept of driving an UV FEL with a low-energy, but high-current, low-emittance electron beam. This and other innovations allowed the FEL to lase at wavelengths from 369 to 380 nm using 45.9-45.2-MeV electrons, and to achieve a peak optical output power of 270 kW. The experimental results are in good agreement with simulations.

We report the first high-gain self-amplified spontaneous emission experiment at 15 mm driven by a high-brightness 17-MeV electron beam. A change of two decades in the beam current yields an increase of four decades in the measured infrared power. By fitting the measured infrared pulse energy to an analytic model, we estimate that eight power gain lengths, corresponding to a gain of 300, exist in the wiggler at 279 A. [S0031-9007 (98)06692-7] PACS numbers: 41.60. -m