“…It has interesting features as it can couple two operating modes in the circular waveguide to generate new eigenmodes. It has attracted considerable interest and found its applications in gyrotron backward wave oscillators (gyro-BWOs) [15,16], gyrotron traveling wave amplifiers (gyro-TWAs) [17][18][19], mode converters [20,21], microwave pulse compressors [22,23], and as a microwave undulator [11]. The inner surface of an HCW can be expressed in cylindrical coordinates as ( , ) = 0 + 1 ( + 2 / ) (5) where R 0 is the mean radius of the circular waveguide, R 1 is the corrugation depth, m B is the fold number, and d is the axial period of the corrugation.…”