Linear and Nonlinear Behaviours of TM-mode Gyrotrons

Hsin Yu Yao^1*, Tsun Hsu Chang¹

¹Department of Physics, National Tsing Hua University, Hsinchu, Taiwan

* Presenter:Hsin Yu Yao, email:s5te633v@hotmail.com

This work systematically studies the performance of TM-mode gyrotrons in linear and nonlinear regions. The linear theory suggests that the beam-wave interaction of TM modes depends on wave propagating direction; the azimuthal and axial bunching would cooperate with each other for the backward wave, indicating the possibility of broadband tuning toward far-cutoff region. To verify the predictions, the starting behaviors of TM modes are analyzed with the relativistic Vlasov equations. The calculations reveal that the starting current of the TM₁₁ mode is more stable as compared to the TE₀₁ mode under both magnetic-field or beam-voltage tuning. A nonlinear and self-consistent theory for TM-mode gyrotrons is also developed. For a uniform interaction tube operated at W band, high nonlinear efficiency (> 30%) of TM₁₁ mode with broad tunable bandwidth (> 2 GHz) is observed. The analysis confirms that the broad tuning bandwidth is contributed by the stable backward-wave oscillation, which exhibits high resistance against velocity spread and mode competition. On the other hand, the high beam efficiency results from the hybridization of forward and backward oscillations, in which the forward wave is responsible for beam-energy extraction while the backward wave provides the required feedback loop. With proper optimization, three important applications can be individually realized, including broadband tuning (tuning bandwidth > 6 GHz), high-efficiency operation (peak efficiency > 50%), as well as low-voltage operation (at 1 kV). Those designs are validated by the finite-difference time-domain simulation with Computer Simulation Technology (CST).

Keywords: gyrotrons, backward-wave oscillators, nonlinear beam-wave interaction, FDTD simulation