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A medium-power broadband-tuning terahertz (THz) gyrotron operating at low voltages is especially attractive for applications in biomedicine. This article tends to elaborate on the challenges related to startup oscillation and ohmic loss suppression of an extremely low-voltage THz gyrotron. High interaction efficiency is achievable by applying long transit time of electrons to compensate the unfavorable decline of the beam-wave coupling strength for low-energy electron beams. A cavity structure which favors backward-wave interaction is proposed to extend the frequency tuning range. The theoretical study predicts that the optimal cavity contributes to the robust frequency tunability at low voltages. The results indicate that a continuous tuning range of 10.7 GHz around the 330-GHz band with output power higher than 1 W can be expected by utilizing a 0.3 kV, 0.5 A helical electron beam.

Spoof surface plasmons (SSP) has become an active research topic in microwave and terahertz (THz) spectrum since its extraordinary optical and physical properties. The strong near field of SSP mode on the corrugated metal surfaces makes it especially attractive for developing a THz electronic source. A THz electronic source based on the efficient generation of SSP modes on the doubly corrugated metallic waveguide is proposed and studied in this paper. The analytical dispersion relations of SSP modes are obtained based on a modal expansion method and the field profiles of SSP modes are also presented by the finite integration method. Besides, the interaction between SSP and injected electron beam is modeled and implemented by particle-in-cell (PIC) simulation based on finite difference time domain algorithm. The gap size between the doubly corrugated metal surfaces can significantly influence the output power and PIC simulation results reveal that output power can be increased from 272 mW to 36.5 W when the gap size decreases from 90 to $40~\mu \text{m}$ at the frequency near 1 THz by the 19.55 kV, 1 A injected electron beam within 4.5-mm interaction length. The dependencies of the output performance on electron beam parameters are also investigated and we find that there is an optimized beam voltage for the given operation frequency. Various electron beams of pulse and direct current electron beam are studied and we find that half pulsewidth of periodical electron beam is more preferable than other emissive shape of injected electron beam for the given structure. Our studies on the efficient generation of SSP modes on the doubly corrugated metallic waveguide may provide a new way to develop THz electronic sources.

Theoretical investigation of a broadband quasi-optical mode converter for 330-GHz TE62 mode gyrotron application is presented. The converter consists of a Vlasov launcher and three reflector mirrors. Special considerations, including a Vlasov launcher with reasonably large aperture radius and optimized combination of two elliptic reflectors and a bifocal parabolic reflector, are the keys to achieve broadband mode converting. The optimized internal converter is well compatible with the gyrotron electron-optical system and generates Gaussian beam with efficiency higher than 80% in an extraordinary broadband range between 310 and 340 GHz. The principle of the broadband converter can also be applied to gyrotron amplifiers.

A terahertz electronic source based on the spoof surface plasmon (SSP) with 2-D subwavelength metallic grating is presented. The SSP dispersion relation of plasmonic grating is derived by a simplified modal expansion method, and the coupling and interaction between the SSP and the electron beam is studied by particle-in-cell simulation. The results reveal that the output performance highly depends on the location of electron beam from grating surface. For an injected electron beam with 19.15 kV and 0.5 A, the SSP output power can reach 22.7 W for the optimized distance of the beam from the grating surface at a frequency near 1 THz for the given structure. Besides, the influence of different electron beam parameters on output power is also investigated and we find that pulse electron beam is preferable than continuous electron beam for good performance. There is an optimized operation frequency for the given beam voltage. Furthermore, output performance can be improved by changing grating structure parameters. By decreasing the grating groove filling factor from 0.8 to 0.2, the SSP output power can be increased from 17.2 to 23.6 W. The SSP power can also be significantly enhanced from 14 to 28.6 W using shallow grating with a groove depth changing from 76 to 56 $\mu \text{m}$ for the optimized operation frequency with the same electron beam. The present work may provide a new avenue to obtain powerful THz electronic sources.

In order to efficiently study the complex nonlinear beam-wave interaction in high-power klystrons, we have developed a 2.5-D code, named, KLY2D, based on a theoretical model combining particle-in-cell (PIC) method and finite-difference time-domain (FDTD) algorithm together. In the model, the particle charge and the beam current are properly assigned onto the grids based on the PIC method; the FDTD algorithm is introduced to solve Maxwell's equations so that the space charge of the electron beam can be accurately calculated, and the port-approximation method is employed to simulate high-frequency cavity fields; finally, the Lorentz motion equation is solved to further advance particle motion. This 2.5-D model is more accurate than the simple 1-D nonlinear code. For the cylindrical structure of a normal klystron, the port-approximation cavity model is accurate enough to model field-to-beam effect and saves much more computer resource than the full 3-D PIC model. This code is benchmarked on an S-band 50-MW high-peak-power klystron. The good consistency between the theoretical results and the experimental data indicates the reliability of the theoretical model and the simulation code, which is of importance for further promoting the design and the development of high-power klystrons.