LLRF system design for HUST ultrafast electron diffraction

An ultrafast electron diffraction (UED) facility is under development at Huazhong University of Science and Technology (HUST). Minimizing time-of-flight (TOF) jitter is critical for improving the temporal resolution of UED, which depends on achieving high stability in the amplitude and phase of the accelerating electric field. To meet this requirement, a low-level radio frequency (LLRF) system has been designed and implemented, enabling precise closed-loop control of the radio frequency (RF) power. This paper presents the design and implementation of the LLRF system, including synchronization triggering for RF sampling, hardware architecture, and signal processing algorithms. The LLRF system employs in-phase and quadrature (I/Q) modulation to process input signals and utilizes the Coordinate Rotation Digital Computer (CORDIC) algorithm for rapid and efficient computation, which promises to adjust the output signal's amplitude and phase precisely and quickly. Simulink simulations reveal that the closed-loop control achieves root mean square (RMS) stability levels of better than 0.05% in amplitude and 0.04 deg in phase for the electric field, and the RMS TOF jitter is better than 46 fs. These results highlight the system's capability to significantly enhance the stability and performance of the UED facility.