The cold (beam-absent) analysis of an all-metal structure consisting of a circular waveguide loaded with axially periodic annular discs was developed in the fast-wave regime for potential application in wide-band gyro-traveling-wave tubes (TWTs) in the millimeter-wave frequency range. The analysis includes the effect of higher order standing-wave modes in the disc-occupied region, as well as higher order space harmonic propagating modes in the disc-free region of the structure. The analysis also takes into account the effect of the finite disc thickness. The dispersion characteristics of the structure obtained by the analysis have been validated against those reported elsewhere using an alternative coupled-integral-equation technique and also against those obtained using commercial simulation code HFSS. The dependence of the eigenvalue and dispersion characteristics of the structure on the disc parameters, namely, the disc hole radius, periodicity, and thickness was studied. The optimum disc parameters corresponding to the widest frequency band over which the dispersion curve can be straightened were suggested for widening the bandwidth of coalescence between the waveguide-mode and beam-mode dispersion characteristics for wide-band gyro-TWT performance. The optimum disc parameters for wide device bandwidths and high device gains were also predicted with the help of the small-signal gain equation of a gyro-TWT. Index Terms-Disc-loaded waveguide, gyrotron, millimeterwave amplifier, periodic electromagnetic structure, wide-band gyro-traveling-wave tube (gyro-TWT). I. INTRODUCTION T HE DEVELOPMENT of fast-wave gyro-amplifiers like gyro-klystrons and gyro-traveling-wave tubes (gyro-TWTs), which overcomes the high-power and high-frequency limitations of conventional klystrons and TWTs in the family of microwave tubes, opens up applications in high power, millimeter-wave radars of high resolution, and communication systems of high information density [1]. Clearly, the conventional slow-wave devices like TWTs, due to reduced sizes of their interaction structures, cannot compete with the fast-wave devices like gyro-TWTs in the high-power and high-frequency (millimeter-wave) regime. Consequently, this has aroused considerable interest in the development of gyro-devices like gyro-TWTs, despite the fact that in bremsstrahlung devices like gyro-devices, it is required to generate an electron beam of gyrating electrons using a magnetron injection gun for the Manuscript

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Abstract-A circular waveguide loaded with dielectric and metal discs was chosen to evaluate its dispersion characteristics and dispersion shaping with change of structure parameters for wideband coalescence of beam-and waveguide-mode dispersion characteristics for wideband gyro-TWT performance.The azimuthally symmetric TE-mode analysis of the structure was carried out in field matching technique by considering the propagating wave in cylindrical free-space region having radius equal to the hole-radius of metal disc, and the stationary waves in free-space and dielectric regions between two consecutive metal discs. The dispersion relation and, in accordance, a computer code were developed. Further, the roots of the dispersion relation for various sets of the structure parameters were obtained using the developed computer code; the dispersion characteristics were plotted; and the dispersion shaping was projected for typically chosen TE 01 -, TE 02 -and TE 03 -modes. The analytical results were validated against those obtained for the conventional and earlier published structures, and also those obtained using commercially available simulation tool. Finally, a study on azimuthal electric field available over the radial coordinate was carried out to show the control of structure parameter on the gyrating electron beam position for the chosen operating mode of a dielectric and metal discs loaded gyro-TWT.

A previously developed field matching technique for the analysis of a metal disc-loaded circular waveguide, excited in a non-azimuthally varying transverse electric (TE) mode, was used to explore the advantage of the presence of a dielectric in controlling its dispersion characteristics for widening the bandwidth of a gyro-travelling-wave tube (gyro-TWT). The modelled structure, consisting of an axial dielectric insert and dielectric discs alternately placed between metal discs, was analysed considering the propagating and stationary waves in the disc-free and disc-occupied regions, respectively. While the axial dielectric insert gave no specific advantage with respect to dispersion control, the dielectric disc axial thickness, permittivity and periodicity and disc-hole radius quite effectively shaped the structure dispersion. In controlling the structure dispersion, the disc-hole radius, which was not as effective as the disc-periodicity in a conventional metal disc-loaded waveguide, became more effective, though the disc periodicity did not enjoy any additional advantage. The thickness or permittivity of dielectric discs controlled the passband frequencies and hence helped attain operating frequencies of a gyro-TWT. The passband of the lower and higher order modes remaining unchanged by a suitable choice of the structure parameters, a higher order mode, for instance the TE02 mode, gave a better wideband potential than the TE01 mode.

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