Trapping and Time Behavior of Pre-Tapered Gyro-BWOs

Chia-Chuan Chang^1*, Tien-Fu Yang¹, Zih-Cian Liou¹, Hsin-Yu Yao², Shih-Hung Chen³, Tsun-Hsu Chang¹

¹Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
²Department of Physics, National Chung Cheng University, Chiayi, Taiwan
³Department of Physics, National Central University, Jhongli, Taiwan

* Presenter:Chia-Chuan Chang, email:s111022803@m111.nthu.edu.tw

Gyrotron backward wave oscillators (gyro-BWOs) are a class of gyrotron devices whose oscillations are self-excited, eliminating the need for an external input signal. The internal feedback mechanism initiates oscillation, and the beam–wave interaction gives rise to unique phenomena such as field contraction and non-stationary oscillations. The latter typically occurs when the beam current exceeds the threshold value, which is nearly an order of magnitude higher than the starting current. Previous studies have mainly attributed the emergence of non-stationary oscillations to excessive interaction length, and some have suggested that applying a taper at the upstream section may help alleviate this behavior. However, the physical mechanism behind this stabilization remains unclear. In this study, we examine in detail the beam–wave interaction and electron bunch formation in a pre-tapered gyro-BWO. Our analysis reveals that an oversized pre-taper introduces strong detuning, which leads to an electron trapping effect. This trapping enhances the accumulation of electrons in the energy-losing phase, forming compact low-energy bunches that emit coherent high-frequency radiation. As a result, non-stationary oscillations are significantly mitigated. Further evidence indicates that this stabilization arises primarily from the trapping mechanism rather than the reduced effective interaction length due to detuning.

Keywords: Gyrotron, Gyro-BWO, ECM, Nonstationary