Analysis of a circular waveguide loaded with thick annular metal discs for wide-band gyro-TWTs

Abstract

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

Propagation Characteristics of a Variant of Disc-Loaded Circular Waveguide

“…Clearly, in the said model, one may consider, the region I (disc free region) supporting propagating and the regions II and III (disc occupied regions) supporting standing waves. The axial magnetic and azimuthal electric field intensities in various considered regions are, respectively [27][28][29][30][31][32][33][34][35]: In region I:…”

“…and even in the vacuum electronic devices, such as, linear/particle accelerators [11,12], backward-wave oscillators (BWOs) [13], magnetrons [14], coupledcavity and helix travelling-wave tubes (TWTs) [15][16][17][18][19], cyclotron masers [20,21], gyrotron sources [21][22][23][24] and amplifiers [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39], etc.. In these devices, one may see the periodic beam-wave interaction structures holding either azimuthal or axial periodicity [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36]…”

“…The azimuthal periodicity in interaction structure may be seen, for example: in travelling-wave magnetrons [14] for π-mode operation, in gyrotrons [21][22][23][24] for mode rarefaction, in conventional helixTWTs [17] for broadbanding, in gyro-travelling wave tubes (gyroTWTs) [25,26] for higher interaction impedance and, in turn, for higher device-gain. The axial periodicity in interaction structure may be seen, for example, in conventional helix-TWT [17] for higher device-gain, in coupled-cavity TWT [17] for getting fundamentalmode backward-wave characteristics, and in gyro-TWT [27][28][29][30][31][32][33][34][35][36][37][38][39] for broadbanding.…”

“…The axial-periodicity in the interaction structure of a gyro-TWT promises the broadbanding through dispersion shaping that allows wideband coalescence between the beam-and waveguide-mode dispersion characteristics and, in turn, the broadband gain-frequency response [27][28][29][30][31][32][33][34][35][36][37]. Choe and Uhm [36] analyzed the infinitesimally thin disc-loaded circular waveguide ignoring the higher order stationaryand propagating-wave modes in the field matching technique and presented the dispersion shaping by changing the structure parameters.…”

“…Choe and Uhm [36] analyzed the infinitesimally thin disc-loaded circular waveguide ignoring the higher order stationaryand propagating-wave modes in the field matching technique and presented the dispersion shaping by changing the structure parameters. Kesari et al [27][28][29][30] improved the analysis of Choe and Uhm [26] considering higher order stationary-and propagating-wave modes, first for infinitesimally thin disc-loaded waveguide [27,28] and then for thick disc-loaded waveguide [28][29][30]. Kesari [31,32] also proposed the coaxial-disc-loaded circular waveguide interaction structure, such that in one configuration metal discs radially projecting inward from the metallic wall, and in second configuration metal discs radially projecting outward from the coaxial insert, for the beamabsent [31] and the beam-present [32] cases, and demonstrated the second configuration a better interaction structure.…”