“…The main attention has been focused on the study of Smith-Purcell radiation when the wavelengths are comparable to the structure period (e.g., [6]). However, it is also interesting to study long wave radiation with wavelengths which are much greater than the structure period [1,4,5,7,8]. The generation of radiation in the terahertz frequency range via ultrarelativistic electron bunch moving through a cylindrical periodic structure with small corrugations is described in [1,4].…”