A Digital LLRF System Based on Intrapulse Phase Feedback for LINAC in Hefei Advanced Light Facility

The linear accelerator system (LINAC) in Hefei advanced light facility (HALF) under construction will accelerate the beam to 2.2 GeV to reach the required luminosity. To achieve stable amplitude and phase control of the radio frequency (RF) field inside the acceleration tubes, low-level RF (LLRF) systems should be adopted. As the solid-state amplifier (SSA) and klystron operate in the saturated state, the system requires amplitude open-loop control and stable phase close-loop control during large amplitude disturbance, proposing a demand for the online phase stability to be better than 0.2° rms with a klystron. In this article, we introduce a digital LLRF system designed for the LINAC of HALF. The LLRF system aims to achieve a fast intrapulse phase feedback control independent of the amplitude control, which is conducive to high phase-control stability. The system obtains digital phase data directly from a phase discriminator (PD) and an analog-to-digital converter (ADC). Besides, the independent output phase control is achieved through an analog time delay chip (ATD). As a result, the digital signal processing (DSP) algorithm is simplified, which is beneficial for achieving low closed-loop latency. Finally, we present the preliminary test results of the LLRF system, proving that the system could achieve fast and independent phase feedback control in one pulse duration.