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.     

对史密斯-帕塞尔自由电子激光光栅的研究

为了研究史密斯-帕塞尔自由电子激光的输出频率和光栅槽深、光栅槽长、光栅槽宽的关系,对于基于矩形光栅的史密斯-帕塞尔自由电子激光利用粒子模拟软件进行模拟和理论分析。首先,利用粒子模拟软件模拟对于基于矩形光栅的史密斯-帕塞尔自由电子激光进行了研究,发现史密斯-帕塞尔自由电子激光的输出频率随光栅槽深、光栅槽长、光栅槽宽的增大而减少。接着,对史密斯-帕塞尔自由电子激光的光栅槽进行了理论分析,发现每个光栅槽都可以等效为一个LC谐振电路,并发现在史密斯-帕塞尔自由电子激光中存在两种辐射,一种是史密斯-帕塞尔辐射,另一种是LC振荡辐射。最后,对光栅槽的LC振荡辐射进行了估算,发现史密斯-帕塞尔自由电子激光输出频率的模拟值与光栅槽的LC振荡辐射估算值的数量级均为102 GHz,且变化规律上一致。据此推测决定史密斯-帕塞尔自由电子激光输出频率的应该是光栅槽,而不是谐振腔。     

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.     

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.     

Design of a new compact THz source based on Smith-Purcell radiation

In recent years, people are dedicated to the research work of finding compact THz sources with high emission power. Smith-Purcell radiation is qualified for the possibility of coherent enhancement due to the effect of FEL mechanism. The compact experiment device is expected to produce hundreds mW level THz ray. The electron beam with good quality is provided under the optimized design of the electron gun. Besides, the grating is designed as an oscillator without any external feedbacks. While the beam passes through the grating surface, the beam bunching will be strong and the second harmonics enhancement will be evident, as is seen from the simulation results.     

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.     

Design of a new compact THz source based on Smith-Purcell radiation

In recent years, people are dedicated to the research work of finding compact THz sources with high emission power. Smith-Purcell radiation is qualified for the possibility of coherent enhancement due to the effect of FEL mechanism. The compact experiment device is expected to produce hundreds mW level THz ray. The electron beam with good quality is provided under the optimized design of the electron gun. Besides, the grating is designed as an oscillator without any external feedbacks. While the beam passes through the grating surface, the beam bunching will be strong and the second harmonics enhancement will be evident, as is seen from the simulation results.     

A millimeter wavelength radiation source using a dual gratingresonator

A novel means of producing coherent radiation by passing an electron through a dual-grating resonator is presented, The observed radiation is in accordance with the Smith-Purcell dispersion relation for a single grating. Feedback is provided by a second grating. Experiments carried out at beam energies from 30-55 keV produced radiation at wavelengths from 6 to 0.75 mm. Power measurement were used to clarify the grating beam interaction, Indications are that operation could be easily extended to shorter wavelengths to provide an inexpensive and compact radiation source in the far-infrared     

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。     

Monochromaticity of the Smith-Purcell optical radiation generated by a 75-keV electron beam

The monochromaticity of the Smith-Purcell optical radiation generated by a 75-keV electron beam with a final emittance of ε = 0.65 × 10^?4 mm rad that passes over a grating with a period of D = 0.833 μm has been analyzed. It has been shown that the monochromaticity of Smith-Purcell radiation is determined not only by the angular aperture of a monochromator but also by the divergence of the electron beam.     

Numerical analysis of radiation by a relativistic electron beampropagating parallel to a reflection grating

Two-dimensional radiation by a relativistic sheet electron beam propagating parallel to a reflection grating composed of a sinusoidally corrugated conducting surface is studied rigorously using the mode-matching method. Accurate dispersion curves for the eigenmodes that govern the Smith-Purcell radiation and the instability of the electromagnetic surface wave are presented. The imaginary parts of the eigenwavenumber and eigenfrequency give the leakage coefficient of space-charge waves and the growth rate of electromagnetic surface waves. Their dependences on the dimensions of the radiating structure are discussed. Optimum parameters for achieving the maximum leakage coefficient and the growth rate are determined. The method is general and can be applied to any two-dimensional system of electron beam coupled to an open periodic structure of arbitrary profile     

Smith-Purcell radiation improved by multi-grating structure

The photonic crystal structure has attracted much attention due to its ability to confine light. In this paper, we present our study on an improved Smith-Purcell radiation from a simple metal photonic crystal excited by moving electrons. Different from the wide-band Smith-Purcell radiation from a single metal grating, the results show that the injected electrons could induce more dipole oscillations inside the multi-grating structure, and it leads to the enhancement of the radiation intensity. In addition, there are strong resonances in metal multi-grating structure, and the resonance characteristics may narrow the radiation band, which leads to a radiation with an obvious peak in the spectrum. Therefore, the multi-grating structure has the ability to enhance the radiation intensity and shape the radiation frequency band. By optimizing the structure parameters, coherent and tunable Smith-Purcell radiation can be realized, and it provides a potential way to develop band-controllable light or THz radiation source.     

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.     

Investigation of a Novel Smith-Purcell Tunable Radiation Source

A novel Smith-Purcell tunable radiation source with relativistic electron beam of middle energy and a quasi-optical resonator composed of diffraction grating and three-mirror reflector is reported. Theoretical analyses and numerical simulation results indicate that coherent radiation with peak power of a few MW order can be achieved from this experimental model. The typical curve of PIC simulation and cool testing experiment is also given.     

A Novel Smith-Purcell FEL

A novel Smith-Purcell FEL with a relativistic electron beam of middle energy and a quasi-optical resonator composed of diffraction grating and three mirror reflector is described in this paper. Coherent radiation with peak power of tens of KW at 3 mm waveband is successfully detected from an experimental facility characterized by beam energy of 400-500 KeV, pulse length of 70 ns, pulse beam current of 0.2 KA, and pulse guide magnetic field of up to 1.2 T.     

Theoretical and numerical analysis of coherent Smith-Purcell radiation

Coherent enhancement of Smith-Purcell radiation has attracted people's attention not only in adopting a better source but also in beam diagnostics aspect. In this paper, we study the intrinsic mechanism of coherent Smith-Purcell radiation on the basis of the van den Berg model. The emitted power of Smith-Purcell radiation is determined by the bunch profile in transverse and longitudinal directions. For short bunch whose longitudinal pulse length is comparable with the radiation wavelength, it can be concluded approximately that the power is proportional to the square number of electrons per bunch.     

Enhancement of terahertz radiation in a Smith-Purcell backward-wave oscillator by an inverse wet-etched grating

A terahertz (THz) Smith-Purcell (SP) backward-wave oscillator with an inverse wet-etched grating based on silicon has been proposed to enhance radiation intensity. This grating strengthens the interactions between an electron beam and the evanescent wave due to the adjacent surface structure between gratings that improves the magnitude of the electric field up to 1.7 times compared to the conventional rectangular gratings. A two-dimensional particle-in-cell (PIC) simulation shows that the radiated power is increased up to 2.3 times higher at the radiated frequency of 0.66 THz for an electron-beam energy of 30 keV.     

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.     

The Forked Blazed Grating as a Quasioptical Element for Gyrotron Applications

In previous papers we have shown how beams of millimeter-wavelength electromagnetic radiation with phase singularities can be generated by diffraction from a specially-configured forked blazed grating. Here we consider how the hollow conical beam output of a gyrotron is diffracted by this kind of grating. Two applications are considered; using the grating (a) as a single-element quasioptical antenna to convert the output into a well-collimated gaussian-like beam and (b) as a device to enable the estimation of the fractions in oppositely-rotating modes in the gyrotron output.