Starting Currents for Eigenmodes of a Gyrotron Cavity With Mode Conversion

J Infrared Milli Terahz Waves

Avramidis

Thumm

et al. 2021

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The influence of a coaxial dielectric rod on eigenvalues, ohmic losses, transverse field structure and beam-wave coupling coefficients is investigated for TE modes of a gyrotron cavity. It is shown that such dielectric insert, when made from a moderate-loss material, results in strong attenuation of all cavity modes, with the exception of those having caustic radii much larger than the insert radius. It is proposed to employ such dielectric loading for selective suppression of competing modes in cavities of second-harmonic gyrotrons. The high performance and flexibility of the proposed method of mode discrimination is demonstrated for the example of the cavity of the high-power 0.39-GHz second-harmonic gyrotron developed at the University of Fukui. In addition, some fascinating capabilities enabled by coaxial inserts made of ultralow-loss dielectrics are discussed.

Section:  mentioning

confidence: 99%

Mode Discrimination by Lossy Dielectric Rods in Cavities of Second-Harmonic Gyrotrons

J Infrared Milli Terahz Waves

Avramidis

Thumm

et al. 2021

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“…4). It is wellknown that in a complex cavity, any competing mode is reflected from the wall discontinuity [22]- [24]. This gives rise to two modes, which are mostly localized inside the left and right parts of the cavity volume.…”

Section: Coaxial Cavity With Dielectric-coated Insertmentioning

confidence: 99%

Improved Mode Selection in Coaxial Cavities for Subterahertz Second-Harmonic Gyrotrons

IEEE Trans. Electron Devices

Moskvitina

Avramidis

et al. 2020

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A coaxial metal rod with partial dielectric coating is considered as a means for efficient suppression of all volume competing modes in cavities for secondharmonic gyrotrons operated in whispering gallery modes. The rod radius is selected small enough to have only a slight effect on operating mode, which therefore remains insensitive to fabrication tolerances and a misalignment of the coaxial insert. By contrast, for the competing modes such a rod is shown to reduce the effective cavity length and thereby greatly increases the starting currents. Such a method of mode selection is demonstrated to be more versatile, as compared to that provided by a tapered coaxial conductor. The advantageous use of a dielectriccoated coaxial insert is illustrated by the example of a cavity for a 100-kW 300-GHz pulsed gyrotron operated in the second-harmonic mode.

“…Contrary to conventional cylindrical cavities, these cavities, however, can exhibit unwanted conversion between coupled radial modes due to abrupt change of the longitudinal (axial) structure. Even though such mode conversion is well-studied in cold cavities [26][27][28][29][30], its effect on the beam interaction with first-and second-harmonic modes still remains poorly explored [31] and therefore may present a hidden factor in designing the advanced cavities for secondharmonic gyrotrons.…”

mentioning

confidence: 99%

Starting currents of modes in cylindrical cavities with mode-converting corrugations for second-harmonic gyrotrons

Tkachova

J Infrared Milli Terahz Waves

Ткаченко

et al. 2021

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A self-consistent system of equations (known as single-mode gyrotron equations) is extended to describe the beam-wave interaction in a cylindrical gyrotron cavity with mode-converting longitudinal corrugations, which produce coupling of azimuthal basis modes. The system of equations is applied to investigate the effect of corrugations on starting currents of the cavity modes. For these modes, eigenvalues, ohmic losses, field structure and beam-wave coupling coefficients are investigated with respect to the corrugation parameters. It is shown that properly-sized mode-converting corrugations are capable of improving the selectivity properties of cylindrical cavities for second-harmonic gyrotrons.