Efficiency-enhanced TM-mode gyrotron with down-taper interaction structure

• Published in: Physics of plasmas 2024-12, Vol.31 (12)
• Main Authors: Yang, Tien-Fu, Nelson Anariba, Jean Edward, Yao, Hsin-Yu, Chang, Chia-Chuan, Chang, Tsun-Hsu
• Format: Article
• Language: English
• Subjects: Brittleness; Cyclotron resonance devices; Efficiency; Electron beams; Parameters; Real time; Transverse electric modes; Transverse electromagnetic modes
• ISSN: 1070-664X; 1089-7674
• DOI: 10.1063/5.0239852

Recent advancements have shown that transverse magnetic (TM)-mode gyrotrons are feasible under specific conditions, yet their capabilities remain insufficiently explored. This article systematically investigates a W-band TM11-mode gyrotron within the down-tapered structure(s) to uncover its limitations and underlying physics. 2D interaction-efficiency maps are scanned as functions of the tube's geometrical parameters or beam parameters under magnetic-field tuning. An oversized tube integrated with short two-stage down tapers enhances the output efficiency of the fundamental axial mode and effectively alleviates the axial-mode competition. The peak electron-beam efficiency of the TM11 mode exceeds 50% with an idealized cold beam. The 3D particle-in-cell simulations are utilized to validate the real-time scheme including multiple transverse modes. Incorporating realistic beam spread, the first-harmonic TM11 mode effectively suppresses the second-harmonic and third-harmonic transverse electric modes with a maximum steady output of 130 kW, corresponding to an interaction efficiency of 37%. Complex dynamics regarding the mode-competing and mode-forming processes are revealed and discussed. This study not only facilitates the exploration of TM-mode gyrotrons but also provides insights into the harmonic gyrotron using the axis-encircling electron beam, where TM modes have more chances to be excited and dominate oscillations.