Enhancement of Smith-Purcel radiations by self-bunched electron beams in oversized backward wave oscillators

Summary form only given. Backward wave oscillators (BWOs) have a slow-wave structure (SWS) with periodically corrugated walls. To increase the operation frequency and the power handling capability, the SWS is oversized with the diameter larger than the free-space wavelength of output electromagnetic wave. Generally, the slow wave becomes a surface wave concentrated on the corrugation of oversized SWS. Excited surface waves are converted into radiating waves at the ends of SWS, leading to BWO radiations¹. There exists an alternative mechanism to radiate electromagnetic waves in periodic structures, i.e., Smith-Purcell (SP) mechanism which directly excites radiating waves². Original SP radiation was a spontaneous emission. SP radiation more intense than the original one was demonstrated based on the stimulated emission, in which self-bunching of electron beam due to the backward-wave interaction plays an essential role³. Our oversized BWOs are driven by weakly relativistic electron beams less than 100 keV, and their power P and frequency f scale as P_f² ~3.5×10⁵ [kW·GHz²] in K- and Q-bands. New element of this work is enhancement of SP radiation in oversized BWO using the bunching section. An annular beam is self-bunched in the K-band bunching section and then is injected into the oversized K-band BWO. SP radiations in the BWO are strongly enhanced in the U-band, which is about twice of the bunching frequency. Moreover, the oversized BWO is developed to G-band (~150 GHz) with radiation powers of kW level or higher. By using this strong G-band bunching, SP radiation is enhanced in H-band. Intense radiations in H- and G-bands based on oversized BWO are demonstrated for the first time and attractive to develop THz wave sources.