Research

A low-level radiofrequency (LLRF) controller is an important piece of electronic equipment in a complex accelerator system. Particles are accelerated by radiofrequency electromagnetic waves inside resonators, also known as cavities, which can be either normal conducting or superconducting. The LLRF controller precisely controls the amplitude (strength) and phase (timing) of the electromagnetic waves so that the particles are accelerated in a controlled and predictable manner. If the amplitude and phase are not well controlled, the quality of the particle beams can deteriorate, and beam loss can occur.

LLRF control involves several areas within the electrical engineering field, including radiofrequency, embedded systems, and controls. Because radiofrequency signals change rapidly, high-speed analog-to-digital converters are usually required to measure them. After the signals are digitized, they are further processed to extract useful information. This information is used to generate control signals that correct variations in the amplitude or phase of the electromagnetic waves inside the cavities. Due to the high data rate, digital signal processing is typically implemented in hardware using a field-programmable gate array (FPGA) rather than in software running on a microprocessor.

Biography

I grew up in the southern part of China and went to college near the east coast. After college, I got a job in Beijing as an electrical engineer. I earned my master’s degree part-time before coming to the United States in 2008 for my doctoral degree. During my doctoral program, I focused on control theory and applications, particularly a control technique called active disturbance rejection control (ADRC).

In 2010, I had the opportunity to apply ADRC to superconducting cavity control at the National Superconducting Cyclotron Laboratory (NSCL) and was able to achieve good results. I joined FRIB in 2012 and participated in the design of the LLRF controller, giving me the opportunity to witness its full life cycle. Now, I want to pass on the knowledge and experience I have accumulated over the years to the next generation of young researchers.

How students can contribute as part of my research team

Students can perform simulation studies by comparing various control algorithms and trying out new strategies. Writing firmware to implement these algorithms on the FPGA and writing software to coordinate tasks are also important skills. Occasionally, new circuit boards will be designed and developed to add features and capabilities. The LLRF controller generates a large amount of information, so data processing and analysis are relevant as well.

The FRIB LLRF controller was fully designed in-house. Students will have the opportunity to learn the full technology stack behind it, from hardware and firmware to software, networks, and user interfaces. From a practical perspective, students will gain a better understanding of how the LLRF controller is operated to support scientific experiments.

Scientific publications

• BPM Feedback for LLRF Energy and Phase Regulation in Charge Stripping Beamlines, S. Zhao, S. Cogan, T. Kanemura, T. Maruta, D. McNanney, P. Ostroumov, A. Plastun, Q. Zhao, 15th International Particle Accelerator Conference, 2024.
• RF System Experience for FRIB Half Wave Resonators, S. Zhao, W. Chang, E. Daykin, E. Gutierrez, S. Kim, S. Kunjir, T. Larter, D. Morris, J. Popielarski, 20th International Conference on RF Superconductivity, 2021.
• The LLRF Control Design and Validation at FRIB, S. Zhao, W. Chang, S.H. Kim, H. Maniar, D.G. Morris, P.N. Ostroumov, J.T. Popielarski, H.T. Ren, N.R. Usher, North American Particle Accelerator Conference, 2019.
• On active disturbance rejection based control design for Superconducting RF cavities, J. Vincent, D. Morris, N. Usher, Z. Gao, S. Zhao, A. Nicoletti and Q. Zheng, Nuclear Instruments and Methods in Physics Research A 643, 11 (2011).