Abstract
Mode-converting longitudinal corrugations are used as a means of improving the selectivity properties of cavities for second-harmonic gyrotrons. As an example, 100-kW 0.3-THz second-harmonic gyrotron is considered. For the operating second-harmonic mode and most dangerous first-harmonic competing modes, the eigenvalues, ohmic losses and beam-wave coupling coefficients are investigated with respect to dimensions of a corrugated cavity. The most optimal parameters are found for a gyrotron cavity with mode-converting corrugations, which ensure the widest range of a single mode operation for the 0.3-THz second-harmonic gyrotron. It is shown that, in this range, the gyrotron output power can be increased up to 180 kW. It is found that output mode purity of the 0.3-THz second-harmonic gyrotron falls off due to mode-converting corrugations, which induce undesirable coupling of the operating mode with neighboring Bloch harmonics in the output section of the gyrotron cavity.
Highlights
A new method of improving mode selection in cavities of sub-terahertz second-harmonic gyrotrons is investigated
The gyrotron is designed for collective Thomson scattering (CTS) diagnostics of fusion plasmas and has a limited output power due to competition between the operating TE13,2 mode and firstharmonic modes
The full-wave method of coupled azimuthal harmonics is applied to investigate the influence of dimensions of the corrugated gyrotron cavity on eigenvalues, ohmic losses and beam-wave coupling coefficients for the operating TE13,2 mode and the most dangerous competing modes
Summary
A new method of improving mode selection in cavities of sub-terahertz second-harmonic gyrotrons is investigated. That is why second-harmonic gyrotrons with medium-field magnets are recognized as advantageous radiation sources in the sub-terahertz-to-terahertz frequency range In these gyrotrons, an additional constraint on output power emerges [5, 6]. According to [13, 14], in a cavity of a secondharmonic gyrotron, the depth of mode-converting corrugations should be selected close to half wavelength of the operating mode In this case, the operating mode is generally weakly affected by corrugations of the cavity wall, while characteristics of the first-harmonic competing modes undergo a material change. BASIC EQUATIONS In a gyrotron, an electromagnetic radiation is produced by the interaction between a helical beam of electrons gyrating in applied magnetic field B0 B0ez and TE mode ( Ez 0 ) excited at the s -th harmonic of cyclotron frequency c0 in a metal cylindrical cavity. In cylindrical coordinates r, , z , the field components of TE mode can be written as
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