A Novel FDTD-PIC Scheme for Accurate Transient Analysis of Terahertz Gyrotrons

As the resonant frequencies advance into the terahertz regime, the mode competition in the gyrotron becomes increasingly intense, exhibiting atypical and varied transient processes. Consequently, rigorous time-domain simulation is crucial for developing terahertz gyrotrons. Traditional dedicated codes typically utilize the gyroaveraged method to address fast time-scale behaviors. However, this approach may lead to misinterpretation when analyzing transient processes involving modes with different response properties and multiscale behaviors, such as harmonic and gyro-backward-wave oscillator (BWO) operations. To address this challenge, this article directly analyzes the dynamic behavior of various modes based on fast-varying field variables without any time-scale assumption, ensuring a precise depiction of beam-wave interactions. The proposed numerical framework integrates a scalar 1-D finite difference time domain (FDTD) method with a 3-D particle-in-cell algorithm, providing a comprehensive description of full-wave physics. The simplified 1-D FDTD model, along with the preselection of relevant modes, significantly reduces data storage requirements. A thorough validation of the proposed numerical framework demonstrates that the results align well with those obtained from established dedicated codes, showcasing superior accuracy in transient analysis.