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

Abstract

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.

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

“…For the 0.3-THz second-harmonic gyrotron equipped with original smooth-walled cavity, the starting currents of the operating second-harmonic ТЕ 13,2 mode and competing first-harmonic ТЕ 8,1 mode are depicted in Fig. 5a by the dashed lines (see also [11]). One can see that the operating mode is the only oscillating mode in the wide range of magnetic fields from 5.78 T to 6.08 T, provided that the beam current equals 10 A.…”

“…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].…”

“…The aim of this paper is to provide the design of the gyrotron cavity with mode-converting corrugations, which aid in suppressing the first-harmonic WG modes and improving the performance of a second-harmonic gyrotron. As an example, we consider the 0.3-THz second-harmonic gyrotron designed for CTS diagnostics of fusion plasmas [11]. In…”

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

“…Among them, to name just a few, are iris-loaded cavities [6][7][8], complex-cavities [9,10], cavities with phase correctors [11] or output reflector [12], dielectric-coated [13] and corrugated [14,15] cavities. А special type of improved gyrotron cavities involves coaxial cavities for second-harmonic gyrotrons [16][17][18][19]. In these cavities, mode suppression by losses is applied, relying on dissimilar sensitivity of different cavity modes to a coaxial insert.…”