用于太赫兹源粒子模拟的有限电导率模块研制

针对THz频段微电真空器件波导壁面材料电导率及加工粗糙度引发损耗的模拟需求，研制了有限电导率模块，并将其添加进三维全电磁粒子模拟大规模并行程序NEPTUNE3D。介绍了有限电导率的时域有限差分显格式及时谐场近似解方法，针对上述方法的优缺点，提出了基于扩散方程隐格式的有限电导率模块算法，该算法具备无条件稳定、普适性好的优点。利用矩波导常见电磁波模传输损耗算例，测试了自编的有限电导率模块，测试结果与理论值及同类商业电磁软件计算结果进行了比对，验证了模块的可靠性。利用添加有限电导率的三维全电磁粒子模拟程序NEPTUNE3D，模拟了材料电导率以及表面粗糙度对0.22 THz折叠波导行波管性能的影响，模拟结果表明,材料电导率及表面粗糙度会显著降低器件输出功率和增益水平。综合色散关系、耦合阻抗、衰减常数等因素，给出了器件结构参数设计建议，并指出:通过增加电子束流、注入信号功率以及慢波结构周期数目等方式可一定程度上提高器件输出功率水平。

Development of Finite-Conductivity-Wall module in 3D fully electromagnetic and PIC code named NEPTUNE3D for THz source simulation

Based on a large-scale parallel code named NEPTUNE3D for 3D fully electromagnetic and PIC simulations programmed by our group, we design and develop a Finite Conductivity Wall (FCW) module in order to simulate the gain loss phenomenon in THz traveling-wave tube caused by metal conductivity and roughness. First, the advantage and disadvantage of FDTD scheme in metal material and harmonious conditions are introduced. Then, we put forward an implicit scheme of FCW and discuss its numerical flow. This method is of unconditional stability and good expansibility. Moreover, we test the FCW module with an example of electromagnetic wave mode transmission in a rectangular wave guide. Compared with theoretical results and commercial electromagnetic software simulation results, the FCW module is validated. By using the improved NEPTUNE3D code after adding FCW module, we simulate an 0.22 THz folded-waveguide traveling-wave tube (FWTWT) gain loss course caused by copper surface roughness. The simulation results indicate that metal surface roughness causes a serious gain loss effect on FWTWT device. Finally, the dispersion relation, interaction impedance and attenuation of device are analyzed, some structure design advices are presented. For increasing the output power, increasing signal power, beam current and structure period number is feasible in some ways.