Development of a new tuning method for an radio frequency quadrupole accelerator

Abstract

Tuning is a critical step in the low-power test of the radio frequency quadrupole (RFQ) accelerator. A 163 MHz RFQ for boron neutron capture therapy (BNCT) at Lanzhou University has been fully assembled. Its designed output energy is 2.6 MeV, and the designed output current in continuous wave mode is 30 mA. Before the feeding of high power, a low-power tuning was performed. First, based on the analysis of the tuning algorithm, a new tuning code capable of tuning the RFQ longitudinal field distribution to any desired target field distribution has been developed using Matlab. The tuning code mainly consisted of two parts: the data processing module and the tuning module, including sub-functions such as smoothing processing function and Fourier expansion function. Among them, the smoothing function based on the Gaussian distribution can effectively eliminate random errors in the measured voltage distribution, thereby optimizing the tuning procedure. Then, a new bead-pull system which can directly track the cavity phase shift was built. The bead-pull system was mainly composed of a vector network analyzer, a stepper motor, etc. Finally, the field unflatness of the RFQ was controlled below 1% after 4 times of tuning. The experiment also measured the radio-frequency performance of the RFQ cavity after installing the fixed copper tuners. The results showed that the RFQ operation frequency was 162.937 MHz, the unflatness of the quadrupole voltage was ±0.74%, and the asymmetry of the dipole voltage was ±1.19%, all of which met the requirements of beam dynamics.