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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We study, both theoretically and experimentally, a free-electron maser (FEM) with a two-mirror resonator that includes the advanced Bragg structure based on coupling of propagating and cutoff waves as an upstream reflector. Similar to gyrotrons, the presence of a cutoff wave in the feedback loop improves significantly the selectivity of the resonator at large values of the oversize factor and facilitates a stable single-mode oscillation regime in short wavelength bands. Proof-of-principle experiments conducted at the induction linac LIU-3000 (Dubna) demonstrate the operability of the FEM with the proposed modification of the Bragg resonator. At the transverse oversize factor of about 5, stable narrow-band generation was achieved with an output power of up to 7 MW and the oscillation frequency of about 80 GHz corresponding to the frequency of the cutoff mode excited in the advanced Bragg reflector. These investigations encourage the application of the advanced Bragg resonators for the development of high-power long-pulse FELs driven by induction linacs and operating in the subterahertz to terahertz frequency range.

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The regime of nonresonant trapping aimed at improving the operation of the amplifying scheme of free electron devices has been developed and experimentally demonstrated in the Ka-band. In this regime, the wiggler is profiled in such a way that the electron beam is out of resonance with the operating wave at the beginning of the wiggler, and different electron fractions gradually get into synchronism as the wiggler's period decreases. According to the simulations, such “nonresonant” trapping provides higher efficiency, a broader amplification band, and lower sensitivity to the beam quality as compared to conventional regimes of interaction. The conclusions of the theoretical analysis are confirmed by the results of joint experiments between the Joint Institute for Nuclear Research and the Institute of Applied Physics of the Russian Academy of Sciences, in which a free-electron maser utilizing this regime was implemented: Using a set of kilowatt magnetrons with frequencies in the range from 30 to 36 GHz, an output power of up to 25–28 MW with a gain of up to 36–37 dB was demonstrated. This makes the investigated regime attractive for shorter wavelength designs of free-electron devices whose efficiency is strongly affected by the electron beam quality.