In this article, we present the promise of a new method generating double electron pulses in picosecondscale pulse length and tunable interpulse spacing at several picoseconds. This has witnessed an impressive potential of application in pump–probe techniques, two-color X-ray free electron laser, high-gradient witness bunch acceleration in a plasma, etc. Three-dimensional simulations are carried out to analyze the dynamic of the electron beam in a linear accelerator. Comparisons are made between the new method and existing ways.
In this paper, a division-of-amplitude photopolarimeter （DOAP） for measuring the polarization state of a free-electron laser （FEL） pulse is described. The incident FEL beam is divided into four separate beams, and four Stokes parameters can be measured in a single-shot. In the crossed-planar undulators experiment at the Shanghai deep ultraviolet FEL test facility, this DOAP instrument constructed in house responsed accurately and timely while the polarization-state of fully coherent FEL pulses were switched, which is helpful for confirming the crossed-planar undulators technique for short-wavelength FELs.
Coherent light with orbital angular momentum(OAM)is of great interest.Recently,OAM light generation by coupling a relativistic electron beam with a Gaussian mode laser pulse at the high harmonics of a helical undulator has been demonstrated experimentally.In this paper,the possibility of delivering coherent OAM light at the 3^(rd)harmonic of the Gaussian mode seed laser is discussed for the Shanghai deep ultraviolet freeelectron laser(SDUV-FEL).Considerations are given on the experiment setup,the expected performance and the possible measurement method.
Besides the original seeded undulator line,in the soft X-ray free-electron laser(SXFEL) user facility in Shanghai,a second undulator line based on self-amplified spontaneous emission is proposed to achieve 2-nm laser pulse with extremely high brightness.In this paper,the beam energy deviation induced by the undulator wakefields is numerically calculated,and 3D and 2D results agree well with each other.The beam energy loss along the undulator degrades the expected FEL output performances,i.e.,the pulse energy,radiation power and spectrum,which can be compensated with a proper taper in the undulator.Using the planned time-resolved diagnostic,a novel experiment is proposed to measure the SXFEL longitudinal wakefields.
Ming-Hao SongChao FengDa-Zhang HuangHai-Xiao DengBo LiuDong Wang
Dalian Coherent Light Source will use a 300 MeV LINAC to produce fully coherent photon pulses in the wavelength range between 150–50 nm by high gain harmonic generation free electron laser（FEL） scheme. To generate stable FEL pulses, a stringent tolerance budget is required for the LINAC output parameters, such as the mean beam energy stability, electron bunch arrival time jitter, peak current variation and the transverse beam position ofset. In order to provide guidance for the design of the Dalian Coherent Light Source, in this paper, the sensitivity of FEL pulse energy fluctuation to various error sources of the electron bunch was performed using intensive start-to-end FEL simulations.
Due to high reflectivity and high resolution of X-ray pulses, diamond is one of the most popular Bragg crystals serving as the reflecting mirror and mono-chromator in the next generation of free electron lasers（FELs）.The energy deposition of X-rays will result in thermal heating, and thus lattice expansion of the diamond crystal,which may degrade the performance of X-ray FELs. In this paper, the thermal loading effect of diamond crystal for X-ray FEL oscillators has been systematically studied by combined simulation with Geant4 and ANSYS, and its dependence on the environmental temperature, crystal size, X-ray pulse repetition rate and pulse energy are presented. Our results show that taking the thermal loading effects into account, X-ray FEL oscillators are still robust and promising with an optimized design.
电子束团纵向长度是束流诊断中的重要参数,电光频谱解码法(Electro-Optical Spectral Decoding,EOSD)是为数不多的可进行单发测量的高时间分辨率测量方法。本文基于上海深紫外自由电子激光装置(Shanghai Deep Ultraviolet Free Electron Laser,SDUV-FEL)设计了一套电光频谱解码法测量束团长度实验方案。通过采用针尖粗同步和可调的光栅展宽器可实现长度在1-10 ps内变化的束团无损测量。通过对电光调制过程及其后探测的仿真研究表明,选用0.5 mm厚的Zn Te晶体在距离束流中心3-5 mm处测量时,时间分辨率最高可达180 fs(Root Mean Square,RMS),信号展宽在3%以内,测量的电场形状无可观测畸变,满足SDUV-FEL的实验需求。
The polarization switch of a free-electron laser （FEL） is of great importance to the user scientific community. In this paper, we investigate the generation of controllable polarization FEL from two well-known approaches for Dalian coherent light source, i.e., crossed planar undulator and elliptical permanent undulator. In order to perform a fair comparative study, a one-dimensional time-dependent FEL code has been developed, in which the imperfection effects of an elliptical permanent undulator are taken into account. Comprehensive simulation results indicate that the residual beam energy chirp and the intrinsic FEL gain may contribute to the degradation of the polarization performance for the crossed planar undulator. Tile elliptical permanent undulator is not very sensitive to the undulator errors and beam imperfections. Meanwhile, with proper configurations of the main planar undulators and additional elliptical pernmuent undulator section, circular polarized FEL with pulse energy exceeding 100 bt.J could be achieved at Dalian coherent light source.
