Foundations For Microwave Engineering

This paper presents a comprehensive theory of the cyclotron resonance maser (CRM) interaction in a circular waveguide. The kinetic theory is used to derive the dispersion relationships for both TE and TM modes. The TE mode case has been investigated by several authors, but there has been comparatively little work on the TM mode case. However, the TM mode interaction competes effectively with the TE mode interaction at relativistic electron energies. The conditions for maximum temporal and spatial growth rates are shown. The TM mode growth rates are found to vanish when the RF wave group velocity equals the beam axial velocity (‘grazing incidence’). The single particle theory is used to derive a compact set of self-consistent non-linear equations for the TE and TM mode interactions. These equations are particularly appropriate for the cyclotron auto-resonance maser (CARM) regime but applicability extends to other regimes as well. The conditions for optimum efficiency are investigated for oscillator and amp...

Linear and nonlinear theory of the Doppler-shifted cyclotron resonance maser based on TE and TM waveguide modes

We present the theory of a new amplification regime in traveling wave tubes (TWTs) composed of a slow wave periodic structure that supports multiple electromagnetic modes that can all be synchronized with the electron beam. The interaction between the multimodal slow wave structure and the electron beam is quantified using a multi-transmission line (MTL) approach based on a generalized Pierce model and transfer matrix analysis, leading to the identification of modes with complex Bloch wavenumber. In particular, we address a new possible operation condition for TWTs based on the supersynchronism between an electron beam and four modes exhibiting a degeneracy condition near a band edge of the periodic slow wave MTL. We show a phenomenological change in the band structure of a periodic MTL, where we observe at least two growing modal cooperating solutions as opposed to a uniform MTL, interacting with an electron beam where there is rigorously only one growing modal solution. We discuss the advantage of using such a degeneracy condition in TWTs that leads to larger gain conditions in amplifier regimes and also to very low starting beam current in high-power oscillators.

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Theory and New Amplification Regime in Periodic Multimodal Slow Wave Structures With Degeneracy Interacting With an Electron Beam

In this paper, we describe the design and experimental results from the rebuild of a 250 GHz gyrotron used for Dynamic Nuclear Polarization enhanced Nuclear Magnetic Resonance spectroscopy on a 380 MHz spectrometer. Tuning bandwidth of approximately 2 GHz is easily achieved at a fixed magnetic field of 9.24 T and a beam current of 95 mA producing an average output power of >10 W over the entire tuning band. This tube incorporates a double disk output sapphire window in order to maximize the transmission at 250.58 GHz. DNP Signal enhancement of >125 is achieved on a 13C-Urea sample using this gyrotron.

Continuously Tunable 250 GHz Gyrotron with a Double Disk Window for DNP-NMR Spectroscopy

The effect of broadening the bandwidth with a dielectric load in a cylindrical gyrotron is investigated for a hollow electron beam. The linear dispersion relation for the azimuthally symmetric, transverse electric (TE) modes is obtained by the method of the wave impedance matching. It is found that the TE perturbations exhibit three unstable modes characterized by their phase velocities vph : one fast wave, the long wavelength mode (LWM, vph ≳c), and two slow waves, the intermediate (IWM, c≳vph ≳ce−1/2) and the short (SWM, vph <ce−1/2) wavelength modes. The optimum conditions for the wide band operation are obtained individually for each mode. Although the bandwidth in excess of 40% is possible for the slow waves (IWM, SWM) at a small axial momentum spread, it decreases rapidly as the spread increases. On the other hand, the LWM yields approximately 10% of the bandwidth insensitive to the spread. It is also shown that for a small spread (<5%), the slow wave (IWM) is preferable for the wide band operation,...

Analysis of the wide band gyrotron amplifier in a dielectric loaded waveguide

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 fleld 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 TE01-, TE02- and TE03-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 fleld 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.

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Analysis of a circular waveguide loaded with dielectric and metal discs

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.

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

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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Modelling of axially periodic circular waveguide with combined dielectric and metal loading

An all-metal disc-loaded cylindrical waveguide excited in TE modes was cold-analyzed in the fast-wave regime for dispersion characteristics, keeping in view of its potential application as an interaction structure for wide-band gyro-traveling-wave tubes (TWTs). The analysis was carried out considering the standing and propagating waves in the disc-occupied and disc-free regions, respectively, using three approaches which differ from one another with respect to how they process the boundary conditions at the interface between these two regions. One such approach is capable of including higher order harmonics in both the structure regions. An adequate number of harmonics with reference to the two regions was taken in the calculation to ensure the convergence of results. The results have been validated against available published results based on different other approaches. The passband and shape of the dispersion characteristics both depend on the disc-hole radius and periodicity, being more sensitive to the latter. The adjustment of disc parameters led to the widening of the straight-line portion of /spl omega/-/spl beta/ dispersion characteristics, for wide-band coalescence between the beam-mode and waveguide-mode dispersion characteristics of a gyro-TWT as required for wide-band device performance.

Analytical approaches to a disc-loaded cylindrical waveguide for potential application in wide-band gyro-TWTs

The interaction structures used in microwave tubes that continue to be important despite competitive incursion from solid-state devices can have the periodicity of both the axial and azimuthal types. The present Part I of the paper reviewed the analysis of helical slow-wave structures of traveling-wave tubes (TWTs), excluding and including the effects of axial space harmonics generated due to the axial periodicity of helix turns revealed by the sheath-helix and tape-helix models, respectively. The field and the equivalent circuit analyses yielded one and the same dispersion relation of the structure. The models for taking into account in the analysis the effect of azimuthal harmonics generated due to the azimuthal periodicity of discrete dielectric helix-support rods and metal vanes provided with the envelope of the structure were also presented. Rigor was added to the analysis by considering the effect of non-uniformity of radial propagation constant over the equivalent dielectric tube regions to...

Vishal Kesari

Papers

Analysis of a circular waveguide loaded with dielectric and metal discs

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

Modelling of axially periodic circular waveguide with combined dielectric and metal loading

Analytical approaches to a disc-loaded cylindrical waveguide for potential application in wide-band gyro-TWTs

Analysis of some periodic structures of microwave tubes: part I: analysis of helical slow-wave structures of traveling-wave tubes