Azimuthally asymmetric oscillations in the dielectric Cherenkov maser

Theoretical and experimental evidences are presented showing that azimuthally asymmetric modes of a dielectric-loaded circular waveguide can also be generated due to the Cherenkov interaction mechanism with symmetric TM modes. A Cherenkov maser configuration with a thin annular relativistic beam was analyzed. Calculations for parameters of recent experiments [W. Main et al., IEEE Trans. Plasma Sci. PS-18 (1990) 507] and other experiments we carried out demonstrate the proximity of growth rates and radiation frequencies. In our experiments we investigated the far field structure, the angular distribution and the spectrum of the output radiation, and we observed qualitatively different far field patterns at various electron beam radii and, moreover, at various dielectric liner lengths. Also, the possibility of high-order modes generation in the experiments of Main et al. is discussed.