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Electronic efficiency close to the value calculated for a perfect electron beam in a type of free-electron maser based on the stimulated electron cyclotron radiation has been obtained for the first time. The cyclotron-autoresonance-maser oscillator utilizing a 500 kV, 100 A electron beam with rotational velocity $0.55c$ yielded coherent radiation at 7.9 mm wavelength with a power of 13 MW. The corresponding electronic efficiency was 26%.
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Bratman et al. (1995) studied this question.
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