Design of an electron gun for terahertz radiation source

An EC-ITC(External-Cathode Independently Tunable Cells) RF gun was employed with the aim of obtaining short-pulse bunches with high peak current for a terahertz radiation source. A gridded DC gun plays a key role as the external injecting electron source of the ITC RF gun, the performance of which determines the beam quality in the injector and transport line. In order to make the beam well compressed in the ITC RF gun, the energy of the electrons acquired from the gridded DC gun should be 15 keV at most. A proper structure of the gridded gun with double-anode is shown to overcome the strong space- charge force on the cathode, which is able to generate 6 μs beam with 4.5 A current successfully.     

Physical design of FEL injector based on the performance-enhanced EC-ITC RF gun

To meet the requirements of high performance THz-FEL （Free Electron Laser）, a compact scheme of FEL injector was proposed. A thermionic cathode was chosen to emit electrons instead of a photo-cathode with its complex structure and high cost. The effective bunch charge was improved to ~200 pC by adopting an enhanced EC-ITC （External Cathode Independently Tunable Cells） RF gun to extract micro-bunches; back bombardment effects were almost eliminated as well. Constant gradient accelerator structures were designed to improve energy to ~14 MeV, while the focusing system was applied for emittance suppressing and bunch state maintenance. The physical design and beam dynamics of the key components for the FEL injector were analyzed. Furthermore, start- to-end simulations with multi-pulses were performed using homemade MATLAB and Parmela. The results show that continual high brightness electron bunches with a low energy spread and emittance could be obtained stably.     

Beam tail effect of a performance-enhanced EC-ITC RF gun

The beam tail effect of multi-bunches will influence the electron beam performance in a high intensity thermionic RF gun. Beam dynamic calculations that illustrate the working states of single beam tail and multi-pulse feed-in of a performance-enhanced EC-ITC （external cathode independent tunable cavity） RF gun for an FEL （free electron laser） injector are performed to estimate the extracted bunch properties. By using both Parmela and homemade MATLAB codes, the effects of a single beam tail as well as interactions of multi-pulses are analyzed, where a ring-based electron algorithm is adopted to calculated RF fields and the space-charge field. Furthermore, the procedure of unexpected deviated-energy particles mixed with an effective bunch head is described by the MATLAB code as well. As a result, the performance-enhanced EC-ITC RF gun is proved to have the capability to extract continual stable bunches suitable for a high requirement THz-FEL.     

Beam dynamics simulations of the injector for a compact THz source

Terahertz radiation has broad application prospects due to its ability to penetrate deep into many organic materials without the damage caused by ionizing radiations. A free electron laser (FEL)-based THz source is the best choice to produce high-power radiation. In this paper, a 14 MeV injector is introduced for generating high-quality beam for FEL, is composed of an EC-ITC RF gun, compensating coils and a travelling-wave structure. Beam dynamics simulations have been done with ASTRA code to verify the design and to optimize parameters. Simulations of the operating mode at 6 MeV have also been executed.     

独立调谐微波电子枪耦合结构的仿真及测试（英）

精确的功率耦合结构设计是保证高品质束流的关键。为确保系统实际运行中的独立调谐微波电子枪耦合性能满足设计要求,对其进行了仿真与测试。首先应用电磁仿真软件CST对电子枪进行了频域和本征模式的仿真计算,两种方法计算结果吻合,谐振频率分别为2 856.10 MHz和2 856.07 MHz,耦合系数分别为1.624和1.620。同时,对已经安装在加速器上实际运行的电子枪分别采用反射系数法与腔体损耗法进行了耦合性能的测试。单点的反射系数法测得耦合系数为1.78,谐振频率为2 855.77 MHz。在此谐振点上,结合反射系数法和腔体损耗法进行多点拟合,测得耦合系数为1.785。结果表明,仿真计算结果与实际测试的结果基本相符。     

Optimization of the lattice function in the planar undulator applied for terahertz FEL oscillators

Since the beta function of the electron beam within the undulator has a great influence on the power gain of the free electron laser (FEL), optimization of the undulator lattice becomes important. In this paper, the transfer matrix of the planar undulator is obtained from differential equations of the electron motion. Based on this, the lattice function of the planar undulator in a terahertz FEL oscillator proposed by Huazhong University of Science and Technology (HUST-FEL) is optimized and the expressions of the average beta function are derived. The accuracy of the optimization result was confirmed well by the numerical method. The application range of this analytical method is given as well. At last, the emittance growth in the horizontal direction due to the attenuation of the magnetic field is discussed.     

A Novel Method for Rigorously Analyzing Beam Loading Effect Based on the Macro-Particle Model

The beam loading effect is a considerable issue for a high average current beam. We apply a novel method without utilizing the specious beam current value to rigorously analyze the beam loading effect in S-band linacs. By tracking every macro-particle＇s state and using the energy conservation law, power dissipated from the beam is calculated in each cell. In the new particle-based algorithm, the concept of equivalent beam current is proposed. Its value is not constant and it has a small variation through the accelerating structure. Also, we introduce the iteration algorithm as comparison and we find that the two algorithms coincide with each other very well.     

Physical design of FEL injector based on the performance-enhanced EC-ITC RF gun

To meet the requirements of high performance THz-FEL (Free Electron Laser), a compact scheme of FEL injector was proposed. A thermionic cathode was chosen to emit electrons instead of a photo-cathode with its complex structure and high cost. The effective bunch charge was improved to ～200 pC by adopting an enhanced EC-ITC (External Cathode Independently Tunable Cells) RF gun to extract micro-bunches; back bombardment effects were almost eliminated as well. Constant gradient accelerator structures were designed to improve energy to ～14 MeV, while the focusing system was applied for emittance suppressing and bunch state maintenance. The physical design and beam dynamics of the key components for the FEL injector were analyzed. Furthermore, start-to-end simulations with multi-pulses were performed using homemade MATLAB and Parmela. The results show that continual high brightness electron bunches with a low energy spread and emittance could be obtained stably.     

采用直接数字频率合成器混频信号的高频功率源设计

 提取直接数字频率合成器芯片AD9850输出的混频信号作为系统的频率源,介绍了高频功率源的结构和原理。系统的工作频率范围为(96±1) MHz,最大输出功率为10 kW。驱动级采用MOSFET晶体管功率放大模块,末级采用高频真空四极管4CW10000B。四极管采用阴地电路,工作在甲乙类。高频功率源采用压控可变衰减器进行幅度调制,为满足系统的频率和幅值稳定度要求,功率源采用带温度补偿的晶体振荡器,并让四极管工作在过压状态。在应用Ziegler-Nichols法整定控制器的PID参数时,为实现控制系统的在线自整定,系统采用了继电反馈法,由极限周期和增益得出PID参数。