合肥光源直线加速器条带BPM系统的研制

【作者】 李吉浩；

【导师】 何多慧； 孙葆根；

【作者基本信息】 中国科学技术大学 ， 核技术及应用， 2006， 博士

【摘要】 合肥光源(HLS)200 MeV直线加速器建成于1989年，原先用于束流横向位置测量的荧光靶，是一种束流拦截型的检测器，精度不高，而且难以将测量结果数字化后用于进一步的控制和处理。针对上述情况，需要开发一种非拦截型，精度高，易于将测量结果数字化处理的束流位置测量(BPM)系统。本论文的选题，就是研制适用于合肥光源直线加速器的束流位置测量系统，同时考虑该系统在未来的软X射线自由电子激光(FEL)装置上的适用性。本论文的内容包括以下几点：直线加速器BPM系统概述、HLS直线加速器BPM系统方案的确定、条带BPM特性的理论分析、条带BPM探头结构的设计、BPM信号处理系统的研制、条带BPM系统的标定以及在线测量实验。 第一章简单介绍直线加速器及其束流测量手段的发展历史，BPM系统的构成，给出描述BPM系统特性的主要参数及其含义。另外通过日本KEKB和Spring—8直线加速器BPM系统两个实例，介绍了直线加速器BPM的系统组成以及系统应用情况。最后介绍了合肥光源直线加速器控制系统现状，并对其在束流位置测量功能的改造升级方面作了展望。 第二章对钮扣型BPM、条带型BPM和谐振腔BPM三种常用的BPM结构以及差比和处理、对数比处理、幅度相位转换处理，以及数字接收处理等BPM信号处理方案进行分析和对比，并综合考虑合肥光源直线加速器自身的环境和束流特点，确定合肥光源直线加速器BPM的探头类型为条带型，信号处理方案为对数比处理。条带BPM具有较高的感应信号幅度，在强干扰噪声环境中具有较高的信噪比，从而具有较高的空间分辨率。对数比处理方案，具有相对较好的偏移响应线性度、较大的动态范围和适中的成本。 第三章分析了作相对论性运动电荷的空间电磁场分布，介绍了适用于条带BPM电极结构的镜像电荷原理，推导了条带BPM的灵敏度公式，最后分析了束团尺寸和偏移响应非线性给差比和处理和对数比处理结果带来的影响以及条带BPM的电极耦合情况及其对灵敏度产生的影响。 第四章介绍了计算机辅助设计在加速器结构设计中的应用，基于Mafia进行条带BPM的仿真和设计，以及对加工完成的条带BPM的特征参数如特性阻抗、端口反射特性、时域响应、频域响应和电极耦合等进行测量和分析结果。结果表明，该条带BPM的特征阻抗为50±5Ω，2856MHz处的反射系数为-1.8dB，带宽大约为400MHz，2856MHz处的电极耦合度大于25dB。 第五章介绍了条带BPM信号处理系统的研制工作，包括前端模块、对数检波模块、信号采集模块和软件模块子系统的研制。系统工作频率为2856MHz，信号处理带宽为10MHz，对数检波动态范围为40dB，检波灵敏度为43.6mV／dB，信号采集的位数达到16位，具有外触发和4通道同步采集功能，可调触发延时为0～1μs，同步采样率大于100Hz。系统的离线误差测试表明，16位BPM系统的1σ误差不大于20μm，12位BPM系统的1σ误差不大于30μm。 第六章介绍了BPM标定方法的分类、合肥光源直线加速器条带BPM的标定平台和标定控制软件，给出了标定结果，并分析了该条带BPM的灵敏度、电中心与机械中心偏差。标定结果表明，该条带BPM的水平方向和垂直方向灵敏度分别为1.552 dB／mm和1.476dB／mm，电中心相对于机械中心的偏差为(0.212，0.450)mm。 第七章介绍了条带BPM电极信号的观测实验，条带BPM系统的在线误差测量实验，条带BPM系统读数与校正铁强度关系的实验，束流强度测量实验，以及束流位置的长期稳定性测量实验。条带BPM电极信号的观测实验表明，电极信号的宏脉冲宽度为1.1μs，微脉冲的频率为2856MHz。条带BPM系统的在线误差测量实验表明，12位BPM信号处理系统的在线测量的1σ误差在40μm的量级，16位BPM信号处理系统在线测量的1σ误差在30μm更多还原

【Abstract】 Hefei light source (HLS) LINAC was built in 1989. Beam transverse position was observed by fluorescent target, which is a beam-intercept device, has a poor precision and is difficult to digitalize results for further control and processing. So a new beam position monitor (BPM) is to be design for HLS LINAC, which is not beam-intercept, has a good precision and is easy to digitalize results for further processing. This paper is about developing BPM system for HLS LINAC. It is concerned that the BPM system developed can be used at further soft X-ray free electron laser (FEL) device. This paper includes outline of LINAC BPM systems, proposal for HLS HLS LINAC BPM system, theoretical analysis of strip line BPM, design of strip line BPM electrodes, developing of BPM signal processing system, mapping of strip line BPM system, and on-line measurement experiment of strip line BPM system.Chapter one introduces the history of LINAC and its beam measurement, structure of BPM system and the main parameters of BPM systems. Then LINAC BPM system applications are introduced based on two examples such as KEKB and Spring-8 LINAC BPM systems. At last we introduce the present control system of HLS LINAC and make a prospect of upgrade for HLS LINAC BPM system.Chapter two analyses three BPM structures such as button BPM, strip line BPM, cavity BPM and several BPM signal processing proposals such as difference sum ratio, logarithmic ratio, amplitude phase transformation, digitally receiving processing. Taking account of environment and beam characteristic of HLS LINAC, we determine strip line BPM and logarithmic ratio processing as HSL LINAC BPM system proposal. Strip line BPM has a higher signal amplitude, a higher signal noise ratio and a higher space resolution. Logarithmic ration proposal has a rather good response linearity, a big dynamic range and a low cost.Chapter three analyses electromagnetic field of charge with a relativity velocity, mirror charge theory and BPM sensitivity. Then we talk about influence of beam size and nonlinearity to BPM result, and coupling of strip line BPM electrodes.Chapter four introduces application of computer aid design (CAD) in LINAC structure design, design of strip line BPM based on Mafia and measurement of BPM characteristic parameters such as matching resistor, port reflection, time domain response, frequency domain response and strip line coupling. Results show that the strip line has a characteristic resistor of about 50Ω, a reflection of -1.8dB, a bandwidth of 400MHz and a coupling of better than 25dB.Chapter five introduces developing of BPM signal processing system, which consists of front end module, logarithmic detector module, signal acquisition module and software module. The signal processing system has a working frequency of 2856MHz, a processing bandwidth of 10MHz, a detector dynamic range of 40dB, a detector sensitivity of 43.6mV/dB, a trig delay time of 0~1μs and a sampling rate of more than 100Hz. Off-line error measurement shows that the 16-bit system has a 1σ error of not more than 20μm, and 12-bit system has a 1σ error of not more than 30μm.Chapter six introduces BPM mapping methods, mapping bench and control software for HLS LINAC BPM. Mapping results show that strip line BPM has a horizotal sensivity of 1.552 dB/mm and a vertical sensitivity of 1.476 dB/mm. The electric center of BPM has a offset (0.212, 0.450) mm to mechanical centor.更多还原