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The application of a TM31 mode scheme in extended interaction radiation sources permits the use of higher beam power and larger power capacity for power enhancement as compared with that of the conventional fundamental mode in the millimeter-wave band. To support this point, the effect of coupling cavities on the electromagnetic characteristics of the TM31 and fundamental modes is studied. This study shows that the TM31 mode can build up (1) a weaker TM11-like field in the gaps which has the same typical distribution pattern as that of a conventional fundamental mode at the same frequency and (2) two extra parts of field energy to support larger coupling cavities. To demonstrate this feature, we observe the field distribution and strength by injecting a certain wave power in two cavities, which are predicted to resonate in the TM31 and fundamental modes at the same frequency. The mode analysis and the beam-wave interaction are discussed to analyze the feasibility of the TM31 mode scheme. Through particle-in-cell simulations, the maximum output power over 26.3 kW is obtained at the desired frequency when the beam voltage and current are 61 kV and 3 A, respectively. The TM31 mode scheme could be a promising candidate for achieving high power in millimeter-wave extended interaction radiation sources.

This paper presents a theoretical study on a start current of a 0.378 THz sheet beam extended interaction oscillator (SB-EIO), and the design of this device driven by thermionic cathode stands a chance to realize a compact and powerful THz coherent continuous radiation sources. After theoretical study, the SB-EIO with eleven gaps is designed to operate at 2π mode. The coupling and interaction abilities are optimized to meet the synchronization conditions and thus improve the output power and efficiency. The start current analysis is preliminarily accomplished, and the advantages are: (1) At the operation frequency of 0.378 THz, the start current and current density of the device are optimized to 20 A/cm2 and 8.4 mA, and the output power is 14 W. (2) It works in the condition of smooth copper plane in simulation, and the output power is still about 20 W when an electron beam with 252 mA and 41 kV is injected. The research gives an estimation of up to 3.5 times decline in output power when the surface roughness emerges. (3) The results of the simulation show that a relatively high efficiency of ∼13% in particle-in-cell (PIC) was observed, and a stable output power range from 98 W to 234 W is found when the injected electron beam (0.35 mm × 0.12 mm) is 16.8–42 mA and 42 kV. Moreover, the reliability of PIC results is verified in this paper.

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