L波段行波聚束器结构研究

介绍了中国原子能科学研究院Ｌ波段高亮度注入器中两种模式的行波聚束器的设计和调配，指出了少腔聚束器的研制特点。     

ECR-SFC轴向注入系统聚束器效率的研究

为了减少目前SFC轴向注入系统中聚束器杂散电场对束流轴向注入效率的影响, 提高新系统中SFC的束流俘获效率, 参考原始设计, 对目前聚束器的电极结构进行了设计改进. 两种情况下的聚束器杂散场对注入效率影响的计算表明, 改进后0B02的聚束效率较目前有较大提高. 同时计算了束流空间电荷效应对聚束效率的影响, 据此对新SFC轴向注入系统中聚束器的位置进行了重新调整.     

BEPCⅡ未来预注入器的优化研究

BEPCⅡ未来的预注入器由热阴极电子枪、两个次谐波聚束器(SHB)、行波预聚束器以及行波聚束器组成.除电子枪以外,各个部分都位于螺线管磁场中.使用PARMELA和EGUN程序模拟和优化了预注入器的束流动力学.对SHB的聚束电压和漂移距离、预聚束器与聚束器的相位和加速梯度、螺线管磁场的形状等都进行了细致的研究和优化,并讨论了聚束器出口10ps束长内电荷量的极限值.     

HIRFL中的束流纵向运动

 对兰州重离子加速器的两台等时性回旋加速器中的束流纵向运动作了简单分析,主要介绍两条束运线上的几台聚束器的物理设计。超低能束运线SFC轴向注入线上的两台聚束器采用了线性聚束器,并提出了用半频聚束来提高两台回旋加速器的纵向匹配效率的新方法。低能束运线(BL1)上的聚束器采用多工作模式以解决加速粒子和能量范围宽的问题。     

BEPCII次谐波聚束系统物理公差要求的研究

 为了进一步提高束流品质并逐步采用双束加速技术, BEPCII直线加速器准备采用带有两个次谐波聚束器的双次谐波聚束系统。目前, 该次谐波聚束系统的物理设计和优化已经基本完成。由于新设计的双次谐波聚束系统很不同于BEPCII现有的聚束系统, 因此, 就要求对聚束系统中存在的各种抖动(jitter)效应重新进行研究。 使用多粒子模拟计算程序PARMELA对电子枪的高压抖动和定时抖动、次谐波聚束器的相位抖动以及功率抖动、聚束器中加速电场相位以及A0加速管(俘获加速节)中加速电场相位的抖动等效应进行了研究, 并最终确定了聚束系统中各部件的物理公差要求。     

70MHz连续波质子束脉冲化装置

为了进行强流回旋加速器关键技术研究, 中国原子能科学研究院建立了一个强流回旋加速器综合试验装置. 中国原子能研究院将在这个回旋加速器综合试验装置上建立强流脉冲化实验装置, 目标是实现几十至百keV量级的强流束的脉冲化. 具体是将70MHz连续波负氢束脉冲化为重复频率1—8MHz, 脉冲宽度约为10ns的脉冲质子束. 脉冲化装置将主要包括束流切割器和聚束器两大系统. 聚束器采用频率为70.487MHz的双间隙单漂移栅网结构, 可以将直流束压缩到±30°的回旋加速器高频接受相宽之内. 束流切割器将采用频率为2.2MHz的正弦波, 切割后的脉冲宽度将小于8ns, 最后得到的脉冲束的重复频率为4.4MHz.     

SFC的新的轴向注入束流线

为了满足HIRFL-CSR对注入器SFC的束流强度和品种的越来越高的要求, 兰州重离子加速器国家实验室在研制超导ECR离子源的同时, 设计了一个新的SFC的轴向注入束流线. 这个系统可以分别使用现有的常规ECR离子源和新建造的超导ECR离子源, 期望把从C到U的各种离子的能量和束流强度提高到一个新的水平. 这个系统由二极磁铁, 四极透镜, GLASSER透镜, 螺线管, 螺旋形静电偏转器和两台丝网型线性聚束器组成. 在总结现有系统运行经验的基础上,无论在横向还是纵向, 其性能结构都做了必要的改进. 文章给出了新系统的设计思想, 系统的布局结构和束流光学计算结果, 并对进一步提高聚束效率和聚束器的改进设计作了简要的描述. 目前, 系统正在安装中.     

光阴极RF腔注入器

 光阴极RF腔注入器是一种强流、窄脉冲(ps量级)、低发射度的电子束源。文章给出了CAEP光阴极RF腔注入器的结构和实验结果,结构主要包括驱动激光器、Cs₂Te阴极制备室、2.5个RF腔、高功率微波源和测量仪器等,其中驱动激光器的参数是影响注入器电子束质量的重要因素;实验上,该光阴极RF腔注入器可稳定输出能量2MeV、流强70A、发射度～4mm.mrad的高亮度电子束流。     

SFC轴向注入系统和中心区的研究和设计

介绍了在HIRFL注入器SFC上进行的中心区、聚束器系统和轴向注入束运线的设计、加工和调试结果.中心区的设计采用了两种注入半径及相应的两套螺旋式静电偏转镜,解决了高频电压在某些工作区域偏低及三次谐波加速时轴向注入线上空间电荷效应较为严重的问题.新的锯齿波聚束器系统不仅可以提高聚束效率,而且还提出了采用半频聚束模式以提高SFC与主加速器SSC?的纵向匹配效率.新设计的轴向注入束运线配备了两台在线ECR离子源,提高了电荷态分辨能力和注入相空间匹配能力,在提高注入效率的同时还改善了离子源及束运线的工作环境和调束手段.     

TRIUMF ARIEL 电子直线加速器设计

 TRIUMF 的三期升级工程(ARIEL) 计划建造一个 50 MeV 平均流强为10 mA的电子直线加速器作为注入器,通过光裂反应生成放射性核素。电子直线加速器包括两个主要部分：注入器和后加速器,注入器完成电子能量从100 keV到10 MeV的转换,随后的后加速器将电子能量从10 MeV加速到50 MeV。电子源拟采用重复频率为650 MHz的热电子枪提供初始能量为100 keV,束长为 171 ps的电子束。束流动力学模拟了几种不同的设计方案以获得最优化的设计,模拟显示通过对腔体以及聚焦元件的仔细设计以及电子枪出射电子的参数选择, 电子束能量在达到50 MeV时束长可以被聚焦到 11.75 ps (对应于1.3 GHz 频率下5.5°) ,并且可以使电子束在超导低温柜中的尺寸保持在1.26 cm以下。     

北京大学DC-SC光阴极注入器的升级设计

 基于直流电子枪-超导加速腔（DC-SC）光阴极注入器样机的初步实验结果，北京大学提出了新的注入器的改进设计。新注入器核心结构包括皮尔斯枪和3+1/2超导腔。文章给出了它们的详细结构参数，然后采用程序，对注入器的束流动力学进行了模拟。结果发现：新注入器可以提供具有高束流品质、高平均流强的电子束，束团的电荷量100 pC，横向发射度低于2 mm·mrad,脉宽5 ps,rms束斑可达0.5 mm，重复频率81.25 MHz；也可以提供电荷量为300 pC低重复频率的高峰值流强的电子束，其横向发射度小于3 mm·mrad,脉宽约为9 ps，以满足北京大学自由电子激光（PKU-FEL）实验平台的要求。     

Bunch length measurement using a traveling wave RF deflector

RF deflectors can be used for bunch length measurement with high resolution. This paper describes a completed S-band traveling wave RF deflector and the bunch length measurement of the electron beam produced by the photocathode RF gun of the Shanghai DUV-FEL facility. This is the first time that such a transverse RF deflector has been developed and used to measure the bunch length of picosecond order in China. The deflector's VSWR is 1.06, the whole attenuation 0.5 dB, and the bandwidth 4.77 MHz for VSWR less than 1.1. With a laser pulse width of 8.5 ps, beam energy of 4.2 MeV, and bunch charge of 0.64 nC, the bunch lengths for different RF input power into the deflector were measured, and an averaged rms bunch length of 5.25 ps was obtained. A YAG crystal is used as a screen downstream of the deflector, with the calibrated value of 1 pix = 136 μm.     

Compressing ultrafast electron pulse by radio frequency cavity

An ultrafast electron diffraction technique with both high temporal and spatial resolution has been shown to be a powerful tool to observe the material transient structural change on an atomic scale.The space charge forces in a multi-electron bunch will greatly broaden the electron pulse width,and therefore limit the temporal resolution of the high brightness electron pulse.Here in this work,we design an ultrafast electron diffraction system,and utilize a radio frequency cavity to realize the ultrafast electron pulse compression.We experimentally demonstrate that the stretched electron pulse width of14.98 ps with an electron energy of 40 keV and the electron number of 1.0 ×10~5 can be maximally compressed to about0.61 ps for single-pulse measurement and 2.48 ps for multi-pulse measurement by using a 3.2-GHz radiofrequency cavity.We also theoretically and experimentally analyze the parameters influencing the electron pulse compression efficiency for single-and multi-pulse measurements by considering radiofrequency field time jitter,electron pulse time jitter and their relative time jitter.We suggest that increasing the electron energy or shortening the distance between the compression cavity and the streak cavity can further improve the electron pulse compression efficiency.These experimental and theoretical results are very helpful for designing the ultrafast electron diffraction experiment equipment and compressing the ultrafast electron pulse width in a future study.     

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.     

Wakefield Effects on the BEPCII Injector Linac

The high current and short bunch of the electron beam in the BEPCII injector linac cause the wakefield effects on the beam performance dilution. These wakefield effects on longitudinal and transverse beam dynamics are systematically studied with analysis and numerical beam modeling, including the single bunch short-range wake effects and the multi-bunch long-range wake effects on beam energy, energy spread, emittance, orbit offset and primary electron beam spot size on the positron production target. The measures to effectively cure these wake effects are also studied.     

BEPCII直线注入器的尾场效应

BEPCII直线注入器中的强流、短束团的尾场效应将损害束流的性能.用分析解和数值模拟计算的方法,系统地研究了尾场对纵向和径向束流动力学的影响,包括单束团的短程尾场和多束团的长程尾场对束流能量、能散、发射度、轨道和初级电子束在正电子产生靶上束斑尺寸的影响等.研究了有效抑制这些尾场效应的措施     

Simulation study of a photo-injector for brightness improvement in Thomson scattering X-ray source via ballistic bunching

Increasing the peak brightness is beneficial to various applications of the Thomson scattering X-ray source. A higher peak brightness of the scattered X-ray pulse demands a shorter scattering electron beam realized by beam compression in the electron beam-line. In this article, we study the possibility of compressing the electron beam in a typical S-band normal conducting photo-injector via ballistic bunching, through just adding a short RF linac section right behind the RF gun, so as to improve the peak brightness of the scattered x-ray pulse. Numerical optimization by ASTRA demonstrates that the peak current can increase from 50 A to > 300 A for a 500 pC, 10 ps FWHM electron pulse, while normalized transverse RMS emittance and RMS energy spread increases very little. Correspondingly, the peak brightness of the Thomson scattering X-ray source is estimated to increase about three times.     

新型跑道腔式束流位置监测器

系统提出了一种新的腔式束流位置监测器(BPM),即跑道型腔式BPM,并使用微波工作室软件进行了理论研究和模拟计算,提出了通用设计方法。跑道型腔式BPM的金属腔部分具有跑道形横截面,引出束流位置偏移激励起的一对偶极模式作为束流位置信号。该方案的偶极模式极化方向固定,频率分离,可以解决随机误差产生的横向串扰问题,其理论位置分辨力可以达到15 nm。     

Electron beam extraction from a broad-beam vacuum-arc metal plasmasource

We describe experiments demonstrating the formation of a high current electron beam from a vacuum arc plasma. A preexisting vacuum arc ion source was used, with the extraction voltage reversed in polarity so as to form an electron beam rather than an ion beam; no other changes were required. The beam formed was of energy up to 33 keV, beam current up to 70 A, beam diameter about 10 cm, pulse width 500 μs, and energy density up to 25 J/cm². This kind of source can be used for material surface modification