In this study, we examine absolute and global instabilities driven by the Cherenkov interaction between a magnetized electron beam and spoof surface plasmon polaritons (SPPs) with an azimuthal mode number m. The absolute and global instabilities are induced in long and short lengths of the cylindrical corrugated waveguides (CCWs), respectively. The temporal and spatial growth rates have different dominant modes of spoof SPPs that, respectively, affect the absolute and global instabilities. In the experiment, the G-band radiation, which corresponds to the dominant mode in the spatial growth rate, is observed with the short length CCW. In the long CCW, the G-band radiation vanishes because the dominant mode in the temporal growth rate has lower frequency than the G-band cutoff frequency of the detecting system. Our results demonstrate that the instability and the multimode radiation are changed by the length of the CCW.

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A surface-wave oscillator (SWO) is a high-power terahertz-wave source based on a slow-wave structure (SWS). A cylindrical corrugated waveguide is used as the SWS. The length of SWS is an important factor in the oscillation. Experiments of the SWO with various length structures are conducted. An instability responsible for the SWO changes depending on the SWS length. For the 200-period SWS, the radiation is due to the absolute instability and depends on the beam current like the temporal growth rate. By contrast, for the 80-period SWS, the radiation is due to the global instability having the current dependence like the spatial growth rate. Compared to the 80-period SWS, the 200-period SWS can improve the radiation power more than seven times in the backward wave oscillation (BWO) region. The maximum BWO power reaches up to about 10 kW and the maximum electric efficiency of 1.5% is obtained.

In this study, we demonstrate near-field measurement of a spoof surface plasmon (SSP) on a corrugated metal disk with concentric annular corrugation. The SSP is excited at the center of the disk and is probed by a needle antenna on the corrugated surface. In the measurement, SSP resonant modes are observed around the upper cut-off frequency. The dispersion relation and group velocity are extracted from the measured parameters and correspond well to the calculated dispersion relation. Our results help in designing SSP resonators to improve the device performance of an intense terahertz wave source.