【作者】 赵雷；

【导师】 安琪；

【作者基本信息】 中国科学技术大学 ， 物理电子学， 2009， 博士

【摘要】 中子散射具有非常广阔的应用前景,被认为是21世纪物质结构和动力学研究的极为重要的手段。为满足我国前沿重要领域的基础研究和高新技术发展的需要,提升我国的创新研究工作的水平,迫切需要建设中国散裂中子源(CSNS,China Spallation Neutron Source)这一重大科技基础设施。CSNS一期总体设计目标为:打靶质子束流功率100kW,脉冲重复频率25Hz,每脉冲质子数1.56×10~3,质子动能1.6GeV。CSNS建成出束时,将与英国,美国,日本的散裂中子源相并列,称为世界上四大主要脉冲散裂中子源科学中心之一。CSNS主要由离子源,强流质子直线加速器,快循环同步加速器(RCS,Rapid CyclingSynchrotron),靶站,中子谱仪和科学实验系统等部分组成。从离子源(IS,Ion Source)产生的负氢离子束流,通过射频四极加速器(RFQ,Radio-Frequency Quadruple)聚束和加速后,由漂移管直线加速器(DTL,Drift Tube Linac)将能量进一步提高,经剥离后注入到快循环同步加速器中,使束流达到1.6GeV的能量。从RCS引出的质子束流经传输线打向钨靶,靶上产生的散裂中子经过慢化后供用户开展实验研究。ADS(Accelerator Driven Sub-critical System)涉及到强流加速器研制的核心问题,对控制束流损失进行了深入的研究,相关技术及硬件设施可以方便的用于散裂中子源加速器部分的建设,从某种意义上来说,ADS相关技术的研究可以看作CSNS的部分技术方案的预研。本DTL束流相位及能量测试系统首先用于ADS项目中,并在未来进一步用于CSNS的DTL中。从直线加速器部分的DTL中通过FCT(Fast Current Transformer)取出束流感应信号,测试其相位及能量,用以实时监控和进一步的反馈。FCT输出的信号为重复频率352.2MHz(ADS,CSNS中324MHz),前沿200-300ps的经调制的射频脉冲,FCT响应信号的动态范围为5mV-900mV(peak-to-peak),要求测量系统在该信号范围内经过100米times LMR-400线缆传输能够工作,并且在25mV-900mV(peak-to-peak)范围内达到相位分辨的要求。系统指标要求整个系统输入信号动态范围大于45dB,相位分辨小于±0.5°(33dB动态范围内,1ms的更新时间)。本论文第一章回顾了加速器的发展历史,分类,及最新进展,并由此引出对中国散裂中子源,ADS系统及其与CSNS的联系进行了介绍。第二章介绍了束流测试的主要内容,着重讨论了束流相位和能量测试的基本原理与方法。CSNS中使用的对DTL束流调谐方案是KEK采用的“相位扫描法”(phase-scan method)。束流能量测试的方法使用的是飞行时间法(TOF,Time of Flight),即通过相位差值反算出飞行时间,进而求出粒子飞行速度。因此,本系统的核心测试内容是相位测试。而就束流相位测试方法而言,主要分为正交解调和直接中频正交采样两种方法。正交解调又分为模拟正交解调和数字正交解调两种方案。三种方法的原理技术,主要电路结构及相关的仿真结果都在此章进行了论述。第三章介绍了目前国际上比较有名的束流相位测试系统,通过对它们的系统结构和技术原理的调研,对相关的核心技术进行了归类和对比。BEPCⅡ中直线加速器相控系统相位测试模块采用的基本技术是模拟正交解调技术,在20dB的输入信号的动态范围内,其相位分辨好于0.2度。LEDA束流相位测试系统则采用了直接中频正交采样技术获取束流的相位及能量信息,其相位分辨在输入信号为10dBm至-30dBm的范围内小于0.1°,在-46dBm时则增大到了0.12°。SNS LLRF控制系统中的相位测试系统采用的是直接中频欠采样技术,核心模块是Bergoz公司生产的AFE模块。最后还介绍了Libera束流测试系统中的相关技术。在以上介绍的基础上对比了三种技术方案的特点:模拟正交解调模拟电路复杂,且器件不平衡性易导致相位值的偏差,修正复杂;数字正交解调技术对于ADC的带宽,高频输入信号下的性能,时钟系统等很高,且算法复杂;直接中频正交解调技术由模拟下变频电路和正交采样两大部分组成,模拟部分相对模拟正交解调部分简洁,而且对ADC的性能要求较低,同时算法简洁,因此便于系统的集成和用于高电磁干扰的环境中。此系统基于VME 6u的构架,系统设计采用了直接中频正交采样的技术路线。第四章首先介绍了系统的方案设计和相关的仿真结果。因为实际系统输入的是调制RF信号,因此频谱成分相当复杂,对此问题的讨论和仿真结果在此章进行了介绍,以确认传统的直接中频正交采样技术是否仍旧适用。同时,关键的信号处理模块-基频抽取,自动增益放大,下边频,正交采样及相关的数字信号处理算法都在本章进行了介绍。第五章基于第四章的论述,详细介绍了各功能模块的具体设计与实现。为正确评估系统指标和性能,正确而高效的测试是必不可少的。第六章介绍了整个系统的测试指标,主要的测试手段与整个测试平台的构建。主要测试内容包括输入信号大动态范围内的相位分辨测试,幅度分辨测试,自动增益控制功能测试和调制信号自动鉴别功能测试等。基本测试手段包括使用示波器进行信号时域性能测试,使用频谱仪进行频域性能测试及系统联合测试进行系统总体性能测试。测试结果表明,在367kHz的更新率下,此系统在-50dBm至7dBm的输入信号动态范围内的相位分辨小于0.1°,在-40dBm至7dBm的范围内相位分辨约为0.06°,因此整个系统充分满足了实际相位测试的需求。更多还原

【Abstract】 Neutron scattering is considered one of the most important technologies in the study of material structure and dynamics.To satisfy the needs of the science and technology development in many domains,it’s an urgent task to construct China Spallation Neutron Source.In the first phase of the CSNS construction,the beam power is up to 100kW with the repetition frequency of 25Hz;in every pulse there exist 1.56×10~3 protons with the energy of 1.6GeV.CSNS will be one of the 4 major pulsed spallation neutron sources in the world.CSNS consists of Ion Source(IS),High Intensity Proton LINAC(Linear Accelerator),Rapid Cycling Synchrotron(RCS),Target,Spectrometer,and corresponding experiment instruments. The H beam from IS is bunched and accelerated by RFQ(Radio-Frequency Quadruple),the energy of which is further increased by the DTL(Drift Tube LINAC).Then the beam is striped of electrons and injected into the RCS to be accelerated to 1.6GeV.The proton pulses from RCS impinge on the tungsten target,and the resulting spallation process produces large amount of fast neutrons which are slowed down for different applications.The study on ADS(Accelerator Driven Sub-critical System) contains the research on the key issues about the design of the high intensity LINAC,the beam loss control,and so on.It’s easy to transplant corresponding technique and hardware instruments to the construction of CSNS.This beam phase and energy measurement system is to be initially used in ADS,and then function in CSNS.This system imports the output signal of FCT(Fast Current Transformer) in DTL,measures the beam phase and energy which is feedback to tune the beam.The FCT output signal is modulated RF signal(repetition frequency is 352.2MHz,with the leading edge of 200～300ps);its amplitude varies from 5mv to 900mv(peak-to-peak) before transmitted through the LMR-400 RF cable(100 meter long).This system aims to achieve phase resolution better than±0.5°over the dynamic range of 45dB.Chapter 1 includes the review of the particle accelerator’s history,its classification,and recent development.CSNS and ADS,as well as their relationship are also discussed.Chapter 2 starts with the main contents of beam diagnostics and measurements,focuses on the basic theory and methods of beam phase and energy measurement.The beam tuning scheme is based on "phase-scan method" adopted in KEK."Time of Flight" is the basic method to calculate beam energy,which requires the difference phase of two channels;thus phase measurement is the kernel task for this system.There exist two ways to measure beam phase - IQ demodulation and direct IF signal IQ sampling.The former include analog IQ demodulation and digital IQ demodulation. These methods and corresponding simulations are presented in this chapter.The investigation on beam phase measurement systems is introduced in Chapter 3.Their basic structures and theories of function are discussed and compared.The phase measurement in RF phasing system for BEPCII LINAC is based on the analog IQ demodulation technique;the IQ demodulator has a phase resolution better than 0.2°in the dynamical range of about 20dB.The beam phase and energy measurement system in LEDA uses the direct IF signal IQ sampling technique;the phase resolution of the system is better than 0.1°over the input amplitude of 10dBm to -30dBm,and deteriorates to about 0.12°when the input signal amplitude drops to -45dBm.The basic method for the phase measurement in SNS Low Level RF control system is direct IF signal IQ Sub-Nyquist sampling;the kernel module is AFE(Analog Front End) manufactured by Bergoz Corporation.Some techniques in Libera Brilliance & Electron Beam Position Processor are also included.Based on the investigation and study,these three techniques are analyzed and compared.The implementation of analog IQ demodulation requires complex analog electronics circuits,and the phase errors are inevitable due to imbalance of different parts.The digital IQ demodulation depends on excellent performance of high input bandwidth ADCs and its clock system,meanwhile the DSP algorithms are rather complicated;the analog part for direct IF signal IQ sampling technique is comparably simple and the requirement on ADCs is reduced,and another advantage is its simplified algorithm.This beam phase measurement system is based on VME-6U crate,so high density and resistance to EMI(Electron-Magnetic Interference) is essential,which means the direct IF signal IQ sampling is the optimum choice.Chapter 4 presents the structure of the system and basic design techniques.The input signal is modulated RF signal,whose frequency spectrum is much complicated than continuous pulses. Corresponding simulation based on Matlab platform is conducted to ensure that the traditional method is still applicable.The key functional modules,e.g.sine wave abstraction,automatic gain control(AGC),down conversion,IQ sampling and DSP algorithms are introduced.The design details of each part are included in Chapter 5.To evaluate the system’s quality correctly and accurately,effective and precise test is prerequisite.Chapter 6 introduces the basic items of system test,main methods and the structure of the test platform.The contents of the system test include the test of phase resolution over large input signal amplitude range,the amplitude resolution,the function of the AGC and modulation signal discrimination.High quality telescope is used for time domain test,while frequency spectrometer is used for frequency domain test.The system quality is estimated through the test results when all parts are connected and function as one instrument.The test results indicates that the phase resolution is better than 0.1°,over the input signal amplitude range of-50dBm to 7dBm (367kHz update rate);when the input signal amplitude is restricted to -40dBm to 7dBm,the phase resolution is about 0.06°.This means the performance of this beam phase and energy measurement system meets the beam measurement requirement of the DTL in proton accelerator of CSNS.更多还原