A W-band (75-110 GHz) oversized surface wave oscillator driven by weakly relativistic electron beams with energy in the range of 10-80 keV is studied. Rectangular corrugations are used as slow-wave structures (SWS) having surface waves with an upper cutoff frequency of approximately 100 GHz (W-band). Uniformly distributed annular electron beams are generated by a disk-type cold cathode and then are injected into the W-band oscillator. A longer SWS length causes the oscillator to function in both backward wave oscillator (BWO) and travelling wave tube (TWT) operations, and no meaningful oscillation occurs at the π-point or the Bragg condition. When the SWS length is short enough, oscillation occurs in all regions: BWO, π-point and TWT. The operations of the oscillator are strongly affected by the structure length. The maximum radiation power is estimated to be approximately 20 kW with the figure of merit of about 2 × 10 2 MW.GHz 2 .

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We experimentally studied intense terahertz-wave generation based on a W-band (0.1 THz) surface-wave oscillator (SWO) using annular electron beams with different beam forms. In order to generate different beam forms, we prepare two-type anodes with open beam limiter and close beam limiter. The electron beam is shaped by the close beam limiter, and the thickness becomes thinner than that of a non-shaped beam with the open limiter. The electronic efficiency of SWO with the shaped beam is improved in comparison to that with non-shaped beam, and maximum efficiency is about 1% for 0.1 THz radiation.

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