Design and performance of the digital LLRF control system for isotope production cyclotron

The project of a 14 MeV cyclotron used for isotope production was developed by ASIPP (Hefei, China). To generate stable radionuclides by the cyclotron, the beam parameters should be stabilized rigorously. Therefore, a digital low-level RF system (LLRF) was designed and built to stabilize acceleration voltage and control the resonance of the cyclotron. The system can be configured to use Non-IQ sampling techniques, resulting in a high performance RF control without the need of very high speed acquisition hardware. In this paper, the functional requirements of LLRF systems are analyzed, including the two feedback control loops, contain tuning loop, the amplitude loop, and the automatic conditioning process. Meanwhile, the hardware design of the FPGA-based core processing board is presented. In experiments, the automatic conditioning process was tested, which certifies the process can greatly shorten the conditioning time. The stability of the cavity voltage and control effect for the resonant frequency has also been measured, and the results show that the amplitude stability is better than 0.0092% under beam loading operation. Experimental results in different operating modes prove that the digital LLRF system can meet the demands of RF control systems for isotope production cyclotron.