In this paper, we present an optimal design of TE0n nonuniform ripple-wall mode converter in circular waveguide. The research work is based on the general coupled wave theory and numerical optimization method. The results of numerical calculation are in good agreement with those of simulation by HFSS. Compared with traditional TE0n mode converter of periodic waveguide perturbations, the TE0n nonuniform ripple-wall mode converter can achieve a high mode conversion efficiency with less corrugated periods. The length of the converter is shortened nearly by a half and the operating bandwidth with over 95% mode conversion is increased by 150%. The research work provides an important theoretical reference and a physics model for designing high power gyrotron mode converter with small axial size, wide operating bandwidth and high conversion efficiency.

A relativistic Cherenkov source with slow wave system consisting of modified photonic band-gap cells is proposed and investigated. The high frequency structure simulator and three-dimensional particle-in-cell simulation code are used to study the dispersion characteristic of TM01-like mode and the interaction between electromagnetic wave and electron beam. The results show that, in the slow wave system with modified photonic band-gap cells, the TM01-like mode has much better azimuthal symmetry, the non-axisymmetry mode can be efficiently suppressed and the operation efficiency can be enhanced greatly.

Dispersion characteristics of two-dimensional dispersive and anisotropic-magnetized-plasma photonic crystals are studied using both the plane wave method and finite-difference time-domain method. When the wave vector lies in the periodic plane, due to the external magnetic field, two different regions of flat bands occur in the TE mode. Varying the values of external magnetic field will affect not only the location of flat bands, but also the position and size of band gaps. Increasing the background dielectric constant will make omnidirectional band gaps to be formed, and make the center of gaps reduced while making the width increase. When the wave vector lies in non-periodic plane, dispersion curves of magnetized plasma photonic crystal are no longer divided into TE and TM modes. With the increasing of the non-periodic wave vector, the location of gap shifts upward, and the gap width firstly increases and then varies little.

The discrete metallic-pole-planar slow wave structure (SWS) is introduced in this paper, and the high frequency characteristics are studied. And procedures based on three-dimensional finite-difference time-domain (3-D FDTD) arithmetic are used to calculate the dispersive characteristics of the new SWS, and HFSS simulation software is used to analyze the coupling impedance. Results show the high frequency characteristics of the pole structure not only have a general similarity in comparison with these of the grating, but also have itself advantages. For the electrons moving between multiple poles of the structure, the interaction impedances are symmetry; relatively thick electron beams can efficiently interact with the high-frequency field while it used as the high frequency system of vacuum electronic devices. This kind of SWS is promising to lower the starting current density and have better efficiency than the traditional grating SWS. According to the results, a sub-millimeter radiation source driven by the multiple beams can be designed at a low operating current density.

We study a two-stream backward-wave oscillator with a slot-hole structure at short millimeter waves with the help of a three-dimensional particle-in-cell simulation. In order to increase the interaction region of the electron beam, the efficiency and the output power, a slot-hole loaded rectangular waveguide structure used as the high-frequency system is proposed. Based on the mechanism of the backward-wave oscillator, a slow-wave oscillator with a frequency of 0.14 THz is designed. The simulations show that the output power and the efficiency of the oscillator can be enhanced due to the coupling between the two beams through the slot holes. The interaction efficiency is 5.18%, and the starting current density is below 5 A · cm−2 for the two beams. These attractive results indicate that, based on the two-stream backward-wave oscillator, we can get short millimeter wave sources with high power and low current density.