紧凑型THz源的史密斯-帕塞尔辐射模拟

 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75 μm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。     

自由电子激光中稳定波长反馈引起的类极限环振荡

研究了自由电子激光实验观察到的由稳定波长反馈引起的新的振荡现象.数值模拟结果证实了实验中观测到的激光功率和电子束能量的周期性调制振荡,调制频率和调制深度与实验值符合较好.分析表明这种类极限环振荡主要起因于超辐射引起的频率蠕变.     

线电荷串产生的史密斯-帕塞尔辐射

采用积分方法分析了线电荷串,沿周期性理想金属光栅表面平行移动时产生的史密斯帕塞尔辐射。分别对短周期光栅、低能量线电荷串与长周期光栅、高能量线电荷串的辐射情况进行了数值计算。相对单个线电荷而言,N个线电荷与反射光栅作用产生的辐射场密度,在频率是线电荷串调制频率的整数倍处出现最大,辐射谱宽变窄;随电荷能量增高,辐射能量向高频方向移动,并具有强烈的定向性;可以通过调节线电荷调制频率等参量实现辐射电磁波频率锁定。研究结果表明,对高能量与低能量的电荷参量,适当选择光栅尺寸与线电荷串间隔周期,都可使辐射波工作于太赫兹波段。     

新型史密斯-帕塞尔互作用结构中的辐射特性

新型史密斯-帕塞尔互作用结构是一个同轴型系统,它包括倒置的曲面镜和柱状光栅,并且光栅的截面可以是任意形状。用粒子模拟软件MAGIC对新结构进行模拟分析和优化,由于新结构采用倒置曲面镜,改变了光栅位置,大大增加了环状电子注半径,在同样电子注密度和厚度情况下,可以获得比改进前结构高几倍的电流,且辐射功率可提高1个数量级。并给出了新结构及改进前两种互作用结构在3 mm甚至更短波段的粒子模拟分析和计算结果,其输出功率达到数百MW以上。     

上海深紫外自由电子激光的非线性谐波辐射研究

 自由电子激光中的非线性谐波辐射能达到较高的谐波功率，可以用来得到短波长辐射或者降低第4代先进光源对电子束团品质的严厉要求。基于3维自由电子激光软件，深入详细地研究了上海深紫外自由电子激光装置的非线性谐波辐射，并且提出了谐波辐射实验和测量建议。研究表明上海深紫外自由电子激光装置3次非线性谐波辐射的功率可以达到基波功率的2%水平。     

自由电子激光同步振荡边带超辐射的数值模拟研究

给出了描述自由电子激光同步振荡边带超辐射的方程组和它的解法,用编制的数值计算程序,对系统进行数值模拟研究,给出边带超辐射的频率和功率的定量结果,并证明在边带辐射中诱导辐射仍是主要的。     

双光栅绕射辐射器件谐振系统Q值分析与结构优化

对双光栅绕射辐射器件谐振系统的Q值特性进行了分析,研究了谐振系统结构参数对Q值的影响及其优化措施。结果表明,双光栅绕射辐射器件谐振结构的Q值受矩形槽宽度和球面开放腔长度的影响最为明显。与奥罗管相比,双光栅绕射辐射器件中光栅的引入对Q值的影响要小得多,且光栅齿宽和齿深的增加都会导致谐振系统Q值降低。     

基于史密斯-帕赛尔效应的太赫兹振荡器的原理与特性分析

提出了一种基于史密斯-帕赛尔效应的太赫兹振荡器,该器件由电子枪、调制腔、输出腔、收集极等部分构成。在器件调制腔中,电子注的速度调制通过史密斯-帕赛尔效应完成的,当群聚电子注通过输出腔时,群聚电子注会在输出腔的间隙处激励起高频电场,该高频电场会使电子注的一部分动能转换为高频能量,并完成太赫兹振荡.模拟发现:当金属光栅的空间周期为0.6mm、电子注能量为100keV时,利用史密斯-帕赛尔效应调制的太赫兹振荡器可以输出频率为349.017GHz和346.324GHz、功率谱密度接近4kW/GHz、最大输出峰值功率超过2kW的太赫兹波.     

GeSi/Si异质结光波导光栅耦合器的设计与模拟

GeSi/Si异质结光波导是硅基光电集成(OEIC)领域一种重要的连接器件,研究其光的输入耦合,提高耦合效率有着重要的意义.通过分析导模-辐射模的耦合理论求得光栅耦合器的辐射损耗系数,设计分析了波导层厚度为2500 nm,入射波长为1300 nm的单模Ge0.05Si0.95/Si异质结波导光栅耦合器的周期、长度和槽深,得出从空气中输入角为75°(从衬底中入射角为16°)时,周期为0.512 μm,槽宽为0.256 μm,光栅长度为2.3 mm,从空气侧输入时耦合效率为22.5%,从衬底输入时耦合效率为46.3%,并对其输入、输出光场进行了数值模拟.     

矩形槽光栅矢量模式理论的数值计算

本文根据矩形槽光栅的矢量模式理论进行了数值计算。对于任意入射方向及任意偏振态的入射场，计算了其衍射效率和偏振特性变化，并详细研究了光栅结构参数（周期、槽宽、槽深等）对衍射场的影响。     

Design of Smith-Purcell emitter in femtosecond accelerator

Based on the femtosecond accelerator device, we are planning to build a broad band and tunable THz source using the Smith-Purcell radiation mechanism. Coherent Smith-Purcell radiation could be achieved owing to the super-short bunch produced in the device. To shorten the distance between the beam and grating, we use Transport to match the beta function producing a sheet beam on the grating surface. The optimization of grating length, groove depth and groove width are given in the paper. Then the radiation power for the shallow and deep grating using these parameters are presented. The detection devices and methods are also briefly discussed.     

Design of Smith-Purcell emitter in femtosecond accelerator

Based on the femtosecond accelerator device,we are planning to build a broad band and tunable THz source using the Smith-Purcell tadiation mechanism.Coherent Smith-Purcell radiation could be achieved owing to the super-short bunch produced in the device.To shorten the distance between the beam and grating,we use Transport to match the beta function producing a sheet beam on the grating surface.The optimization of grating length,groove depth and groove width are given in the paper.Then the radiation power for the shallow and deep grating using these parameters are presented.The detection devices and methods are also briefly discussed.     

介质加载复合光栅结构的色散特性研究

提出了一种用于Smith-Purcell器件的介质加载金属光栅周期慢波结构.通过采用本征函数法和单模近似法求解了介质加载金属光栅的"热"色散方程,在同步点得到了注-波互作用的一阶和二阶增长率,分析了光栅槽宽和槽深对色散特性的影响,并研究了电子注参数及其与光栅表面距离等主要参数对增长率特性的影响.结果表明:通过介质加载金属光栅有利于减弱色散,随着介质相对介电常数、槽宽度以及深度的增大,色散曲线变平缓且向低频区移动;当电子注参数变化时,一阶增长率曲线从整体上粗略地描述增长率变化趋势,二阶曲线则更精细地描述增长率相应值的变化.利用软件MAGIC对该结构的色散特性进行了二维模拟,模拟结果与理论计算值符合良好.     

非相干光纤激光组束系统耦合效率的优化分析

单光束耦合效率是决定非相干组束系统输出功率的关键,它受到透镜焦距、光栅周期和光斑半径等参数的影响。为了寻求较为优化的系统参数,通过理论分析和仿真研究,结果表明对于中心波长1060nm的光纤激光,应当选择透镜焦距20cm,光栅周期5μm,并且需要将光斑半径控制在50μm左右。通过反解光栅频率及组束波长带宽的第一个零点来选择组束阵列宽度一定条件下较优的光栅参数。通过理论分析和数值计算,结果表明较小的光栅频率和光栅厚度对提高衍射效率是有利的。     

Tunable radiation source by coupling laser-plasma-generated electrons to a periodic structure

Near-infrared radiation around 1000 nm generated from the interaction of a high-density MeV electron beam, obtained by impinging an intense ultrashort laser pulse on a solid target, with a metal grating is observed experimentally. Theoretical modeling and particle-in-cell simulation suggest that the radiation is caused by the Smith-Purcell mechanism. The results here indicate that tunable terahertz radiation with tens GV/m field strength can be achieved by using appropriate grating parameters.     

Smith-Purcell radiation in the “pre-wave” zone

A model is proposed for calculating the angular distributions of the intensity of resonant diffraction radiation generated by a charged particle moving near a plane conducting periodic structure (Smith-Purcell radiation) with allowance for the effect of the so-called pre-wave zone. It is shown that the criterion of the location of a detector in the pre-wave zone for this kind of radiation in the relativistic case is determined by the geometric sizes of the grating and is almost independent of the Lorentz factor of the particle. The characteristics of the radiation in the pre-wave zone are calculated by numerical integration over the grating surface. It is shown that the line of Smith-Purcell radiation is broadened in this case, whereas the results of the calculations for the “wave” zone are in satisfactory agreement with the known analytical model of the resonant diffraction radiation.     

Grazing incidence off-plane lamellar grating as a beam splitter for a 1-Å free electron laser

A beam splitter based on the diffraction grating working in the grazing incidence conical diffraction is proposed for x-ray free-electron laser (XFEL) experiments with coherent beams in plasma and atomic physics. Such a beam splitter can provide undistorted wavefronts, high-power radiation scattering, and split beams with equal intensities, which propagate with delay along different paths to the target chamber of the XFEL end station. Using the PCGrate software based on rigorous electromagnetic theory and developed for the short-wavelength range, it is shown that the plane grating with lamellar groove profile of a certain depth, operating in grazing conical incidence mount (grooves are parallel to the incident beam), separates three beams in the ?1, 0, and +1 orders with close diffraction efficiency. Numerical simulation predicts 23–27% absolute efficiency for 0.1-nm incident radiation in each separated order of a bulk or multilayer grating, taking into account the atomic level roughnesses and interdiffusion. When using a multilayer coating based on Ru/C or Ru/B₄C pairs, the optimum grazing angle providing approximately equal efficiencies is ～1.038° which is four times higher than for the Pt-coated grating. Such optimization of radiation geometry, groove profile shape, and multilayer coating parameters can be performed for various XFEL wavelengths. The proposed grating, in addition to diffraction, technological, and design advantages over the beam splitter based on a set of perfect crystals, can be fabricated and tested using currently available methods.     

太赫兹波段Smith-Purcell辐射的介质加载光栅高频特性

提出了一种用于Smith-Purcell效应器件的介质加载光栅慢波结构,通过研究该结构的注-波互作用过程,推导出带电子注的色散方程,并数值求解出波的线性增长率.利用色散方程,结合电磁场传播的边界条件,推导出Smith-Purcell效应振荡器工作所需的起振电流.详细研究了高频结构长度、电子注主要参数和介质相对介电常数对起振电流的影响,并与普通金属光栅结构进行了比较.结果表明:保持其他参数不变时,高频结构长度越短,起振电流越大;保持高频结构参数不变时,起振电流随电子注厚度和注-栅距离的增大而增大,随电子注电压的增大而减小;与金属光栅相比,介质的引入提高了注-波互作用的增长率,有效减小了振荡器的起振电流.理论计算结果与软件CHIPIC的模拟结果比较符合.     

Enhancement of Smith—Purcell radiation with surface-plasmon excitation

With the aid of a three-dimensional particle-in-cell code simulation,the enhancement of Smith-Purcell radiation with a surface-plasmon mode excited by a single electron bunch and by a premodulated electron beam is considered in the paper.In the simulation,the model is a grating covered by Ag film.The results demonstrate that when the surface-plasmon mode is excited by a single electron bunch,the maximum radiation occurs at an observation angle depending on the surface-plasmon frequency,and the radiation power can be enhanced more than ten times.And for pre-bunched electron beam excitation,when one of the harmonics of the bunching frequency is resonant with that of the surface-plasmon mode,the radiation power is twenty times more than that from a perfectly conducting grating excited by the same premodulated electron beam.