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

Abstract

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.

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

“…L L  and the ratio 2 1 Q Q of total quality factors can approach the peak value of 4 [5], provided that the ohmic losses in metal cavity walls are negligible. Despite this, the minimum starting currents of the first-harmonic modes are usually several times lower than those of the second-harmonic modes [3,[6][7][8][9][10][11]. The main reason lies in the constraint 2 0 1    for conventional weakly-relativistic gyrotrons [3,4].…”

“…Better performance can be theoretically achieved in coaxial gyrotron cavities with resistive inserts [19,23], which make it possible to increase the ohmic losses of competing modes. However, their chief practical drawback is the very low conductivity required for resistive (conducting) rods to ensure efficient suppression of high-order axial modes, which often brings the major threat to operation of second-harmonic gyrotrons [6][7][8][9][10][11] and feature high diffraction losses. Such conductivity can be beyond the range of the available metallic materials, especially in the terahertz range [11].…”

“…However, their chief practical drawback is the very low conductivity required for resistive (conducting) rods to ensure efficient suppression of high-order axial modes, which often brings the major threat to operation of second-harmonic gyrotrons [6][7][8][9][10][11] and feature high diffraction losses. Such conductivity can be beyond the range of the available metallic materials, especially in the terahertz range [11]. For mode selection by ohmic losses, dielectric inserts made from available lossy ceramics can be used as alternative to resistive rods in cavities of sub-terahertz second-harmonic gyrotrons [11].…”

Mode-Converting Corrugations for Cavities of Second-Harmonic Gyrotrons with Improved Performance

“…To discriminate against the first-harmonic competing modes advanced gyrotron cavities with improved mode selection are required. Among them are coaxial cavities [8][9][10][11][12]. In a coaxial cavity, the modes are discriminated by a coaxial insert.…”

“…Unlike the operating mode, competing modes, which have smaller caustic radii, are suppressed by losses induced by the coaxial insert. The first-harmonic whispering-gallery (WG) modes are usually the remaining competitors, which can hinder high-performance operation of second-harmonic gyrotrons [11,12].…”