The Design And Implementation Of The Beam Phase And Energy Measurement System For DTL In The Proton Accelerator Of CSNS

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.