Design and verification of digital low-level RF control algorithms for an ultra-compact cyclotron
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摘要: 为实现PET医用小型回旋加速器的自主研发，中国原子能科学研究院开展了9.5 MeV超紧凑型回旋加速器的设计与研究。为满足对加速器束流的稳定加速，研制了基于全数字硬件平台的高频低电平控制算法，为提升控制精度，增加反馈速率，设计了高速DDC下变频解调系统，针对于数字下变频后IQ序列分布于任意象限的问题，设计了一种创新的象限预处理模块，以拓展其在全平面坐标系的适用性。为实现自动频率补偿，设计并实现基于位置式PID的调谐环，集成了自适应限位保护机制和实时调谐检测功能，并支持一键启动调谐。利用构建的高可靠性的跨时钟域数据通路，为幅度环调节激励信号幅值提供精度与稳定性保障。通过自闭环测试，验证了解调算法的可靠性，在与加速器联合调试中，稳定引出内靶束流100 μA，腔压幅度稳定度为0.047%（RMSE），失谐角保持在0.46°（RMSE），充分验证了系统的稳定性和可靠性，满足了加速器对低电平控制系统的需求。Abstract:
Background The China Institute of Atomic Energy has designed of a 9.5 MeV ultra-compact cyclotron to support the independent of Positron Emission Tomography (PET) cyclotrons. A high-performance control system is critical for the equipment, as the stability of the acceleration field directly impacts beam quality.Purpose In order to ensure the stable acceleration of the accelerator beam, this study aims to develop a Low-Level Radio Frequency (LLRF) control algorithm based on a fully digital hardware platform.Methods To enhance control precision and increase the feedback rate, a high-speed Digital Down-Conversion(DDC) demodulation system was designed. Addressing the issue where the IQ sequence after digital down-conversion may be distributed in arbitrary quadrants, an innovative quadrant preprocessing module was developed to extend applicability across the Cartesian plane. A position-type Proportion-Integral-Derivative (PID) tuning loop was implemented for automatic frequency compensation, integrating adaptive protection, timed detection, and one-click startup. Furthermore,a robust cross-clock-domain data path is constructed to ensure accurate and stable amplitude regulation.Results Closed-loop tests verified the reliability of the demodulation system. During the joint commissioning with the accelerator, a stable internal target beam current of 100 μA was successfully extracted. The system achieved a cavity voltage amplitude stability of 0.047% (RMSE) and maintained a detuning angle of 0.46°(RMSE).Conclusion The experimental results demonstrate that the proposed LLRF system fully meets the control requirements of the accelerator. The design ensures high stability and precision, providing reliable technical support for the operation of the 9.5 MeV ultra-compact cyclotron.-
Key words:
- cyclotron /
- LLRF /
- FPGA /
- closed-loop control /
- cross-clock domain
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图 1 9.5 MeV全数字低电平系统框图
Figure 1. Block diagram of the 9.5 MeV fully digital lLow-level RF system
图 2 实时解调系统频域分析
Figure 2. Frequency-domain analysis of the real-time demodulation system
表 1 9.5 MeV低电平解调参数
Table 1. Demodulation parameters of the 9.5 MeV low-level RF system
RF frequency/MHz sampling frequency/MHz low-pass fliter CIC decimation rate LO frequency/MHz 41.5 250 CIC、FIR 16 41.5 
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表 2 几何等效变换
Table 2. Geometric Equivalent Transformation
quadrant transformed coordinates (X0,Y0) geometric equivalence Ⅰ (x, y) identity transformation Ⅱ (y, −x) 90° counter-clockwise rotation Ⅲ (−x, −y) origin symmetry Ⅳ (−y, x) 270° counter-clockwise rotation
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