Development Of Low-latency Multiple-retransmission Mechanism Based Beam Position Measurement And Control Processor For Storage Ring

Hefei Advanced Light Facility(HALF),the fourth-generation vacuum ultraviolet and soft x-ray diffraction limit storage ring that is now under preliminary research,requires sub-micron beam orbit stability to achieve ultra-high brightness and extremely low emittance.In addition to improving the magnet accuracy and enhancing the stability of the support device,real-time global orbit feedback control is an essential method to stabilize the beam orbit.The designed fast orbit feedback system of HALF needs to control 240 beam position monitors and 160 fast corrector magnet power supplies(80 for horizontal direction and 80 for vertical direction).This paper develops a beam position measurement and control processor(BPMC)based on low-latency multiple-retransmission mechanism.And the fast orbit feedback system architecture of HALF is designed with BPMC.Compared with the three-layer communication architecture of beam position monitor electronics,distributed controllers and corrector magnet power supplies in traditional fast orbit feedback system,BPMC processor integrates the functions of the beam position monitor electronics and the distributed controller,and controls the corrector magnet power supply point-to-point,which could simplify the complexity of hardware and connections,decrease the network layers of the communication topology,reduce the cost of the communication time,and improve the system bandwidth.BPMC is developed using domestic chips,which composes of the pilot tone module,the analog front-end module,the digital processing module and the embedded control module.The pilot tone module is placed near the beam position monitor.A low phase-noise pilot tone is generated in this module and added to the beam signals as a reference to eliminate the effect of gain drift of the individual channel on position calculation.The analog front-end module consists of four identical RF channels,each with several amplifiers,attenuators and filters to adjust the signals for ADC sampling.Digital signal processing module integrates analog-to-digital converters(ADCs),field programmable gate array(FPGA),and etc.,for band-pass sampling,digital signal processing,data transmission.The embedded control module communicates with accelerator control system to improve the scalability of the system.The digital signal processing algorithms,integrated in the BPMC,realize beam position calculation,data communication and orbit correction.In order to improve the long-term stability,pilot tone is used to compensate channel inconsistency between cables and analog circuits.Beam position measurement algorithms,including digital down conversion module,filtering and decimation module,CORDIC module,and pilot tone based difference-over-sum algorithm,are implemented to calculated the fast acquisition data(FA data)for fast orbit feedback system,and the slow acquisition data(SA data)for slow orbit feedback system.The BPMCs are connected by high speed optical fiber,and the low-latency multiple-retransmission mechanism is proposed to transmit the beam position information.Based on the physical design of the storage ring of HALF,the double loop communication topology of fast orbit feedback system with the lowest communication delay is designed.To achieve orbit correction,singular value decomposition(SVD)algorithm is adopted to decompose the orbital response matrix and realize independent PID(Proportional Integral Derivative)control for each eigenspace,and PID parameters can be updated flexibly.All digital signal processing algorithms are integrated in FPGA and implemented in modules.The resolution tests of beam position are carried out in this paper.The test results indicate that the resolutions of the FA data and SA data are better than 120 nm and 70 nm respectively(Kx=Ky=8 mm)with the input amplitudes ranging from-55 dBm to 5 dBm,which meet the design requirements.In addition,the beam tests based on the storage ring of Hefei Light Source(HLS-Ⅱ),further verify that the BPMC can monitor the beam orbit well.With two existing BPMCs,we test the communication reliability and the communication delay using debug tools.According to the test results,the communication delay of the whole fast orbit feedback system is calculated,about 18.688 μs,which meets the requirement of FOFB system with update rate of 10 kHz.The simulation results of the orbit correction module with Modelsim verify that the orbit correction module is correct functionally.A series of offline tests and beam tests verify that BPMC based on low-latency multiple-retransmission mechanism can complete long-time stable beam position measurement,reliable beam position sharing and orbit correction.