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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.

A new technique for frequency-upshifting electromagnetic radiation is demonstrated. By ionizing azulene vapor contained in a resonant cavity using a laser pulse, the frequency of the incident RF wave at 33.3 GHz is upshifted by 5% with greater than 10% efficiency. Maximum frequency upshift of 2.3 times the source frequency is observed. There are two mechanisms thought to be operative in producing the observed frequency upshift: The time-dependent dielectric constant due to increasing plasma density, and rapid "Q-switching" of the cavity. This technique has the potential of being able to generate tunable and chirped radiation over a very broad (A f/f 2 1) frequency range.

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