Efficient and stable operation of Cerenkov devices relies on effective mode control, which means efficient generation of the operation mode and effective suppression of the competition modes. This paper explores the feasibility of suppressing asymmetric modes by loading a conductivity anisotropic material in Cerenkov devices. We theoretically study the dispersion characteristics of a slow-wave structure (SWS) loaded with a conductivity anisotropic material. The theoretical analyses indicate that asymmetric modes such as the HE11 mode have a low net temporal growth rate in a SWS coaxially loaded with the anisotropic material, of which the azimuthal conductivity is in the transition region from good to poor conductor. Accordingly, an anisotropic material with suitable azimuthal conductivity effectively suppresses asymmetric mode competition while maintaining the original characteristics of the symmetric TM modes. Furthermore, we numerically investigate the effectiveness of asymmetric mode suppression by anisotropic material loading using a 3D particle-in-cell CHIPIC code. A coaxially loaded conductivity anisotropic material effectively suppresses the asymmetric HE11 mode in a Ku-band Cerenkov device, and a pure quasi-TEM mode is obtained in the output waveguide. The output power of the generator is 2.9 GW, and the efficiency is 44%. This study provides a feasible method to eliminate asymmetric modes in Cerenkov devices, which may lead to a wide range of applications.

Field electron emission (FEE) is generally considered to be closely correlated with radio frequency (RF) breakdown in accelerating structures and high-power microwave (HPM) devices. In this study, the field emission characteristics of titanium are investigated by using a field emission test system. With the increase of the number of field emission tests and stability tests, the repeatability of the field emission characteristic curve of titanium tends to be better, and the amplitude of the current oscillation in the stability tests gradually decreases, indicating that the field emission performance of titanium gradually becomes stable. Subsequently, the field emission characteristic curve of titanium is analyzed by adopting the field emission theory of metal microprotrusions. Combined with the analysis of the electric field enhancement effects caused by the surface morphology, the dominant effect of the metal microprotrusions on the FEE of titanium is excluded from the two aspects of the electric field enhancement factor and local maximum emission current density. At last, the field emission theory of dielectric micropoints is introduced to analyze the field emission characteristics of titanium. The electric field enhancement factor of 102 ~ 103 are explained theoretically. Simultaneously, the reasonable effective emission area and local emission current density are given, which can better explain the field emission phenomena of titanium, such as the sharp decrease in emission current and repeatability of the field emission curve. Hence, it is revealed that the key factor that dominates the FEE of titanium is the dielectric impurities on the surface, rather than the metal microprotrusions.

Radio frequency (RF) breakdown can result in pulse shortening and severely limits the performance of the relativistic backward wave oscillator (RBWO). In this study, segmented slow-wave structure (SWS) rings, which can be observed directly using a scanning electron microscope without any post-processing, are used to study the breakdown in a RBWO. After undergoing high-power microwave experiments, the working surface of the SWS rings was found to exhibit corrugated morphologies at the sub-mm to mm scale, and a large number of craters in the vicinity of the corrugated morphologies and a mass of scattered droplets were visible on the surface. It was deduced that the unusual morphologies resulted from electron bombardment of the working surface. The observed structures and droplets provide new evidence that helps to explain the mechanism for RF breakdown in the RBWO.

In the study of breakdown in high power microwave (HPM) devices, researchers pay attention to the plasma effects caused by material ablation under electron bombardment. In this paper, the destruction of material under the bombardment of electrons with a range of energies around 100 keV is investigated. The simulation results reveal that low-energy electrons are more likely to cause ablation damage to the material, with the location of maximal energy deposition density close to the material’s surface. The experiment has a good agreement with the simulation results. Compared with high-energy electrons, the damage traces bombarded by low-energy electrons are more serious, possessing the characteristics of large size and shallow depth. This abnormal physics phenomenon provides a reference for the breakdown of HPM devices and the surface treatment of materials by pulsed electron beams.

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