Design and desktop experiment of LLRF system for CYCIAE-230 superconducting cyclotron

The CYCIAE-230 cyclotron is a proton beam accelerator dedicated to cancer therapy. The low-level radiofrequency (LLRF) system of CYCIAE-230 cyclotron is designed based on the previous LLRF systems which have been successfully commissioned for CYCIAE-100 and CYCIAE-14 cyclotrons. This design includes four feedback control loops, namely, tuning loop, amplitude loop, phase loop, and balance loop. The LLRF system for CYCIAE-230 cyclotron have the same amplitude, phase, and tuning loops as the previous designs. Meanwhile, the balance loop makes the LLRF system stand out. It is enabled after the amplitude loop is closed and controls the homogeneity of the accelerating fields of the four cavities. From the bird view of the control system of the cyclotron, the LLRF system is an embedded control unit which communicates with the program logical controller with Profibus-DP (Distributed Peripherals). In this paper, the desktop experiment has been performed with a small-scale copper cavity with low power. This small-scale cavity has an unloaded quality factor ($Q_0$) of around 7800 and has push–pull and push–push resonate modes, which are the characteristic parameters of the cavities of CYCIAE-230 cyclotron. The design and performance of the LLRF system were discussed based on the desktop experiment results in this paper.