Research on online RF tuning of DTL accelerator for CSNS-II energy upgrade

Abstract With the energy upgrade of the China Spallation Neutron Source (CSNS-II) and the increasing demand for isotope production, the RF repetition frequency of the Drift Tube Linac (DTL) accelerator must be elevated from 25 Hz to 50 Hz. However, the frequency shifts induced by RF heating exceed the current tuning range. Traditional tuning methods cannot be applied during DTL operation, so online tuning technology is explored in this study. Multi-physics simulations were used to analyze the electromagnetic and temperature distribution, frequency shifts, and compensation strategies. It was found that adding tuning blocks on the side of the cavity is the optimal solution. Two types of tuning blocks were developed and installed during the summer maintenance, achieving a beam transmission rate of over 98%. Experimental results showed that the new tuning blocks effectively compensated for the frequency deviation without affecting the cavity's stability or field distribution. This solution improves the high-frequency performance of the DTL accelerator and meets the upgraded requirements. It is practical approach that offers valuable insights for the online tuning of DTL accelerators, as well as other high-frequency cavities.