摘要:
采用模拟和数值计算的方法,研究了THz波段的受激史密斯-帕塞尔辐射特性.实验装置以"上海电子束离子阱"为原型,采用紧凑型设计以便最终实现其可移动性.束流动力学模拟表明,此装置采用强磁场,可以得到平均流强为0.2 A、束流半径为75μm的高品质电子束,为电子束工作在自由电子激光模式下创造了条件.基于Andrews和Brau的理论,优化了光栅参数,保证了辐射角度在60°.其中消散场的计算频率为0.365 9 THz.采用particle-in-cell(PIC)程序模拟了光栅表面的辐射场以及电子的动力学特性.模拟结果表明电子有群聚效应,且二次谐波(0.723 THz,约为消散频率的2倍)得到增强.采用后处理方法计算了史密斯-帕塞尔辐射的功率空间分布.计算显示辐射角度与理论角度相一致,表明了方法的有效性.输出的功率约为2 mW.
Abstract:
The characteristics of stimulated Smith-Purcell radiation in THz region are simulated and nu-merically calculated. The experimental device is designed to be compact. The notion of the design for the experi-ment device is originated from the prototype of the Shanghai Electron Beam Ion Trap. Strong axial magnetic field is necessary to achieve high current and low emittance of electron beam. The simulation results of dynamic characteristics for the electron beam show that an average beam current can be as high as 0.2 A with an RMS beam radius of 75 μm, which is the precondition to work as free electron laser(FEL) while passing over the grating surface. Based on the theory of Andrews and Brau, we have optimized the parameters of grating to en-sure the emission angle is near 60°. And the calculated frequency of evanescent wave is 0.365 9 THz. The pa-rameters are then used in the particle-in-cell(PIC) simulation code. The simulation results show that beam bunc-hing occurs. Besides, the second harmonic is enhanced, whose frequency is 0.723 THz, about twice as that of evanescent wave. To calculate the exact emission power from simulation results corresponding to the Smith-Pur-cell(SP) radiation, we also present a new post processing method. With this method, we have obtained the space distribution of emission power. The calculation shows that the emission is centralized at the expected an-gle, which accounts for the validation of this method. And the extracted power is about 2 mW.
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