| Shanghai Synchrotron Radiation Facility (SSRF) is the third generation lightsource, which has been built successfully and was opened to users in2009. Thefunctions of the low level RF (LLRF) controller in the booster RF system at SSRF arethe two5-cell cavities voltages’ ramping, the amplitude and phase stabilizing, and thetuning and the field flatness controlling. Its performance plays an important role inbeam quality and injection efficiency of the booster ring.The former LLRF controller system at SSRF booster is an analogy one whichwas imported together with the two5-cell copper cavities from a Germany company.Although it can work well to achieve the required function, it is inconvenient tohandle the fault and to maintain. Considering the fact that digital LLRF controllerbased on FPGA technology has been developed in the storage ring of SSRF, anupdated LLRF controller adopting digital technology for booster RF system at SSRFwas proposed.The equivalent circuit model of5-celll cavity and booster RF system have beenbuilt to obtain the transfer function of5-cell cavity and proportional-integrated (PI)regulator and to discuss the stability of closed loop operation mode. The optimumvalues of proportional and integral gains were acquired in this paper. The influence onthe time response with different PI settings has been analyzed with closed loopoperation.One of main tasks of my research is to develop the new digital LLRF controllerhardware environment. The hardware of the controller includes three new printedcircuit boards (PCBs) which are the digital signal processing board integrated theclock distribution, the RF front-end board and the motor control board. Finally, as thelack of ADC channels on the digital signal processing board, the digital LLRFcontroller system has to be divided into two controllers, the one for the amplitude andphase loop controlling, and the other one for the frequency and the field flatness loopcontrolling. It is important to obtain the cavity displacement during operation so thatthe displacement transducer W-DCD70has been selected taking into account theabilities of long distance and strong anti-interference. The local distribution in thebooster RF system and commissioning of the digital LLRF controller have beencompleted.Another task of this thesis is to develop the programs of the controller whichincludes the related control algorithms for hardware control in amplitude-phase loop,frequency tuning loop and field flatness loop together with interlocks for protection.The algorithms of the ramping curve, the vector sum of digital IQ and the PI regulatorhave been designed in the amplitude and phase controller, which can be ramped thetwo5-cell cavities voltages from0.4MV to1.8MV with a repetition rate of2Hz.The phase difference between the reference signal and the pickup signal of cell-2wasused to tuning cavity frequency. The CORDIC algorithm was employed in thefrequency tuning loop and the introduced phase error was less than0.007degree whenthe iteration times are14, which could be ignored. The influence on the field flatness when tuning the cavity frequency was discussed. It is succeeded in synchronouslydriving the two stepper motors installed on each cavity with the tuning loop algorithm.Three interlock control logic algorithm was programmed regarding to the two digitalLLRF controllers and four motor controllers which was carried out to meet therequired performance in test bench.Finally， the developed digital LLRF controller system has been put intooperation with beam at SSRF booster RF system since August,2014. The long periodoperation resulted in a phase stability better than±0.15o(RMS) in closed loop atramping mode. The tuning loop had a phase difference better than±4oand the fieldflatness was controlled to be better than±1.5%. The performances have achieved orbetter than SSRF booster RF system requirements with the digital LLRF controller,which means that the development of the digital LLRF controller for booster RF issuccessful. |
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