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A high-gain, high-extraction-efficiency, linearly polarized free-electron laser amplifier has been operated at 34.6 GHz. At low signal levels, exponential gain of 13.4 dB/m has been measured. With a 30-kW input signal, saturation was observed with an 80-MW output and a 5% extraction efficiency. The results are in good agreement with linear models at small signal levels and nonlinear models at large signal levels.PACS numbers 42.60.By, 41.70.+t, 42.52. +xThe free-electron laser (FEL) is capable of producing coherent radiation from the ultraviolet to the microwave region of the electromagnetic spectrum. Several recent experiments have demonstrated lowgain, low-efficiency FEL operation in the visible 1 and infrared 2 regions while other experiments have demonstrated high-gain FEL operation in the millimeter-wave regime. 3,4 We have designed an experiment, the Electron Laser Facility (ELF), which can serve as a test of the physical models used to predict high-gain and high-efficiency FEL operation in the visible spectral region. The ELF consists of an amplifier with well-defined initial conditions on the radiation and the electron beam and with no axial magnetic field.

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An ultraviolet-laser-ionized channel in low-pressure benzene has been successfully used to guide and focus a 7-kA relativistic electron beam over distances up to 4 m. In addition, phase-mix damping of coherent, transverse beam motion has been demonstrated. A simple analytical model of the equilibrium beam profile is presented which is in reasonable agreement with the data.

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