A metal cylindrical waveguide coated with an inside layer of lossy dielectric which affects the propagation characteristics of a guided electromagnetic mode is investigated for gyrotron-traveling-wave tube (gyro-TWT) amplifier applications. This paper reveals a series of novel phenomena. The dispersion curve of a higher order mode has a turning point during its evolvement from the fast wave region to the slow wave region. An electromagnetic mode in the lossy dielectric-coated waveguide exhibits a transverse partial-standing-wave distribution. The dielectric loss induces modal transition which results in the dispersion curves of a pair of nearby modes crossing each other and interchanging mode structures. Modal reduction caused by strong dielectric loss merges a pair of nearby modes into one. In this one merged mode, the dielectric-coated waveguide is equivalent to a conventional cylindrical waveguide with imperfect conducting wall. This improved understanding of lossy dielectric-coated metal cylindrical waveguide is of value and usefulness for application toward gyro-TWTs capable of high-power and wide bandwidth.

A dielectric-loaded (DL) waveguide is an attractive possibility for interaction circuits with high-power sources in the millimeter-wave regime down to tenths of millimeters, particularly for gyrotron-traveling-wave-tube amplifiers (gyro-TWTs). We present results on a systematic investigation of the influence of the periodically loaded lossy dielectric on the propagation characteristics of the operating modes, which reveals that a complex mode in the periodic system can be mapped to a corresponding mode in an empty waveguide or a uniform DL waveguide. Dielectric losses not only induce modal transitions between different modes with similar field structures and close phase velocities in the uniform system but also unify the discrete mode spectrum into a continuous spectrum in the periodic system. Since the lossy dielectric functions as a power sink, the higher order Bloch harmonic components arising from the structural periodicity are suppressed, and the mode spectrum of the lossy periodic system degenerates into that of an empty waveguide. This alleviates the potential danger of spurious oscillations induced by the higher order harmonic components, making the periodic lossy DL waveguide promising in a high-power millimeter-wave gyro-TWT