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 novel concentric ring-based meta-surface assisted dielectric window is considered in the present study for examining its broadband response. The tolerable return loss in the Ka-Q band is taken as 20 dB for calculation of the bandwidth. The proposed window supports a bandwidth of 20.30 GHz (54.06%) for the TE0,1 mode. While comparing with a plane disk window, the bandwidth is increased by 17.55 GHz and 46.55%. On the other hand, compared with previously designed meta-surface assisted dielectric windows with square cross-sectional pillars without and with cylindrical tips, the bandwidth is increased by 1.1 GHz and 190 MHz, respectively.

Ultra-wideband TE0,1-mode dielectric window assisted with ring- based meta-surface

In [1] 
, the first name of the third author in the byline under the title of the paper [1, p. 3257] as well as in the biography of the third author [1, p. 3261] should read “Rengaswamy” instead of “Ramaswamy.”

Correction to “Boron Nitride and Sapphire Windows for 95-GHz Gaussian RF Beam” [Aug 16 3257-3261]

A general analytical approach was proposed to estimate the ohmic quality factor of circumferentially corrugated cavities having a finite-single-valued and continuous radius-profile in circular symmetry within the boundary of its length. In the present paper, two field profiles, namely, the sinusoidal and the Gaussian one, and four radius profiles, namely, the sinusoidal, the cosinusoidal, the triangular-sinusoidal and the triangular-cosinusoidal, of the cavity were considered and the results were presented in order to demonstrate the helpfulness of the proposed approach. Out of the eight cavity versions considered, the minimum and the maximum ohmic quality factors were obtained for the cavity with sinusoidal field profile and triangular-sin radius profile and for the cavities with Gaussian field profile with sinusoidal and cosinusoidal radius profiles. It has been found that for higher values of the cavity-length to corrugation-period ratio (≥3), the ohmic quality factor performances of the cavities in sinusoidal and cosinusoidal radius profiles overlap.

Analysis of Ohmic Quality Factor of Circumferentially Corrugated Circular Cavities

Thermal management of a quasi-optical mirror in a millimetre-wave gyrotron

Field-matching technique was used to analyse a disc-loaded-coaxial waveguide in the azimuthally symmetric TE modes for its dispersion relation that includes space-harmonics in the disc-free region as well as modal-harmonics in the disc-occupied region of the structure. The phase propagation constant, obtainable from the dispersion relation, was fed back in the small-signal gain-equation of the gyro-TWT and the gain-frequency response of a disc-loaded-coaxial gyro-TWT was plotted. Further, the effect of the radius of coaxial rod insert on gain-frequency response has been studied, and the parameter has been optimised for high gain and wideband of the device.

Vishal Kesari

Papers

Ultra-wideband TE0,1-mode dielectric window assisted with ring- based meta-surface

Correction to “Boron Nitride and Sapphire Windows for 95-GHz Gaussian RF Beam” [Aug 16 3257-3261]

Analysis of Ohmic Quality Factor of Circumferentially Corrugated Circular Cavities

Thermal management of a quasi-optical mirror in a millimetre-wave gyrotron

Analysis of a Disc-Loaded-Coaxial Waveguide for Wideband and High-Gain Gyro-TWTs