Nonlinear and Self-Consistent Single-mode Formulation for TM-mode Gyrotrons

Hsin Yu Yao^1*, Cheng Hsiung Wei¹, Tsun Hsu Chang¹

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

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

We built up the nonlinear and self-consistent theoretical framework for TM-mode gyrotrons. Unlike TE modes, a new nonlinear wave equation for TM modes was derived with consideration of additional axial modulation on the gyrating beam interacting with the axial electric field. Together with the equations of motion of electrons, particle tracing simulation was conducted to model the oscillation of TM modes. For a uniform structure, the nonlinear efficiency of TM₁₁ mode at W band can achieve 30% with a tunable bandwidth around 1.6 GHz. The simulation result further shows that the forward wave contributes the peak efficiency, which is relatively sensitive to magnetic field tuning due to bunching competition. On the contrary, the backward-wave oscillation is stable and allows broadband tuning due to the cooperation of two bunching mechanisms. Furthermore, we found that TM11-mode gyrotrons allow low-pitch factor and low-voltage operation and show great resistance against the velocity spread of the electron beam. Those features open the possibility of a high-power, frequency-tunable, and compact gyrotron system.

Keywords: TM-mode gyrotrons, gyro backward-wave oscillators, nonlinear beam-wave interaction