Experiments to investigate copper surface fatigue caused by pulsed rf radiation were carried out using the 30 GHz free electron maser. The copper surface of a special test cavity was exposed to 15-20 MW=150-200 ns rf pulses with a repetition rate of 1 Hz, providing a temperature rise of up to 250 C in each pulse. An electron microscope was used to study the copper surface both before and after exposure to 10 4-10 5 rf pulses. An examination of the copper microstructure and cracks which developed during the experiment was made. Dramatic degradation of the copper surface and causes of very frequent breakdown were observed when the total number of rf pulses reaches 6 Â 10 4 .

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The frequency multiplication effects in high-power free-electron masers (FEM) with Bragg cavities were studied to provide the advance of the oscillators into short-wavelength bands. Theoretical analysis of frequency-multiplying FEMs was carried out within the framework of the averaged coupled-wave approach. Proof-of-principle experiments were performed based on a moderately relativistic induction linac LIU-3000 (JINR). As a result, an FEM multiplier operated with a megawatt power level in the 6-mm and 4-mm wavelength bands at the second and third harmonics, respectively, was realized. The possibility of using two-mode bichromatic FEMs for powering a double-frequency accelerating structure was discussed.