A proof-of-principle experiment is presented demonstrating the suppression of multipactor breakdown in a coaxial multipactor device with three-dimensional periodic wavy surfaces. By changing the power and pulse width of the microwave source, threshold behavior near breakdown was obtained for this wavy-surface structure and a smooth-surface structure used for comparison. With a wide pulse width at a suitable power, the coefficient of reflection for the smooth-surface structure was found to increase, whereas the coefficient of transmission decreased. For the wavy-surface structure, a similar behavior appeared, only when the microwave pulse had a width of order of a few seconds. Accompanied by changes in transmission power characteristics, distinct increases in the second and third harmonic components were evident for the smooth-surface structure. These experimental results demonstrate that the wavy-surface structure effectively suppresses multipactor breakdown with the suppression increasing with the pulse width.

Asymmetric modes competition in Cerenkov devices were studied with a view to developing a novel method to suppress modes competition, based on loading of the slow-wave structure (SWS) with an anisotropic medium. Due to differences in the field distribution between the asymmetric and symmetric modes, the asymmetric modes are susceptible to significant attenuation in anisotropic media with an appropriate imaginary part for the azimuthal permittivity. In contrast, the symmetric modes, such as the TM01 mode, incur no such losses. Using the EH11 mode as an example, the quality factor obtained from experiments involving the SWS loaded with an anisotropic medium was reduced by approximately 90% compared with that for a copper SWS. The attenuation was sufficiently high that the electromagnetic fields for the asymmetric modes were barely established, indicating that the suppression technique was highly effective. The approach provides a feasible way to eliminate asymmetric modes in high-power microwave (HPM) sources and may lead to a range of new applications in HPM systems.

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.