Development Of Digital Low-level RF System For Linac Of HLS Ⅱ

HLS II is a dedicated synchrotron radiation light source operating in vacuum ultraviolet and soft X-ray bands.It uses an 800 MeV Linac as a full energy injector and directly injects beam into a storage ring.To accelerate electrons,the Linac utilizes an S-band(2856 MHz)RF field,whose stability affects the beam’s energy dispersion and emittance.The low-level RF system is employed to maintain the stability of power and phase,which is crucial for the stable operation and improved performance of HLS Ⅱ.This thesis develops an S-band digital low-level RF system based on HLS Ⅱ Linac and introduces the self-developed digital low-level RF system from three aspects:system architecture,software and hardware design,control algorithms and performance tests.By establishing the mathematical model for the normal conducting accelerating structure,this thesis examines how instability in the accelerating field impacts the beam.Additionally,this thesis investigates the origin of noise that causes the instability in the accelerating field.Afterward,a feedback control model using a PI controller is formulated.The continuous transfer function model suitable for intra-pulse feedback and the discrete transfer function model suitable for pulse-to-pulse feedback are both analyzed.Through the examination of the stability of the feedback system resulting from the noise in the driving and measurement chains,this thesis provides a theoretical foundation for system design,development,and parameter optimization.According to the requirements of HLS Ⅱ low-level RF control,the design scheme and hardware/software architecture of the self-developed low-level RF system are determined.The A/φ control strategy is adopted,and the intermediate frequency downconversion sampling scheme and the direct up-conversion scheme are selected to monitor the RF signal and generate the excitation signal.The hardware of the selfdeveloped low-level RF system is divided into three parts:signal source,frequency synthesis system,and IF signal processor.The signal source uses a low phase noise thermostatic crystal oscillator as the main oscillation source,and the reference signal is obtained through frequency multipliers,filters,and amplifiers.The frequency synthesis system obtains the LO and clock through the mixer and frequency divider,using the reference signal as input.The IF signal processor is designed with domestic chips and includes an ST optical port as a trigger port and an SFP port for data communication,which improves the anti-interference and data transmission ability of the system.The IOC and OPI of the low-level RF system are developed based on the MIPS-based domestic Loongson chip and the latest Phoebus,which enables the EPICS communication control of the processor.The bench tests and beam tests of the self-developed low-level RF system are carried out in this paper.The performance of each sub-module of the self-developed low-level RF system is separately tested,and an offline comparison test is conducted with the imported low-level RF system.The experimental results show that the shortterm amplitude stability and phase stability of the self-developed low-level RF system are both better than 0.01%and 0.01°,respectively,and the overall performance of the self-developed system is comparable to that of the imported one.An online beam experiment is conducted on the photocathode RF gun experimental platform,achieving a closed-loop phase stability of 0.1°(RMS)in the acceleration cavity,which verifies the feedback algorithm.Online verification of the feedforward algorithm and the nonlinear compensation algorithm of the amplifier is completed in the FELiChEM.The feedforward algorithm reduces intra-pulse fluctuations to one sixth of the original value.The nonlinear compensation algorithm has a good compensation effect on the nonlinearity of the amplifier amplitude,and with the same feedback coefficient,it improves the feedback efficiency by 42%.To solve the problem of slow phase drift in HLS Ⅱ,a dual-pulse algorithm is proposed for compensating phase drift.In offline tests at room temperature,the peak-to-peak value of phase drift is reduced from 1.7° to 0.5°,with a root mean square value of 0.062°,which has a good suppression effect on the phase drift of radio frequency signals in long cables.In May 2022,the self-developed low-level RF system partially replaced the imported low-level RF system of the HLSⅡ linear injector as an online device,achieving long-term stable operation.In summary,this thesis has achieved the design and production of a digital lowlevel RF system based on domestic chips for HLS Ⅱ Linac.The system has been tested both offline and online,and has demonstrated good performance,comparable to imported low-level RF systems.The successful development of this system provides valuable practical experience and a reference for the localization of domestic low-level RF system,as well as for the development of other domestic Linac low-level RF systems in the future.