Physical design of a high-intensity radio frequency quadrupole accelerator for Heavy-Ion Cancer Therapy

A radio frequency quadrupole (RFQ) accelerator for a heavy-ion cancer therapy has been developed by CAS Ion Medical Technology Co., Ltd. to advance domestic heavy-ion cancer therapy in China. The RFQ is designed to accelerate particles with a mass-to-charge ratio of 3 from 10 keV/u to 600 keV/u, operating at a frequency of 200 MHz. The detailed physical design process is systematically introduced in this paper, including the beam dynamics design, RF electromagnetic design, and multi-physics analysis. The beam dynamics design adopts a lower inter-vane voltage scheme to reduce power loss, while adopting a reduced average aperture to compensate for the weakened transverse focusing caused by the high frequency and low voltage. Beam simulations are also carried out to analyze the effects of beam errors on transmission efficiency and the output transverse and longitudinal emittance. The RF electromagnetic structure has been optimized for higher performance and achieving the field distribution required by the beam dynamics design. The beam transmission simulation verifies that the field obtained in the electromagnetic structure agrees well with that required by the beam dynamics design. The multi-physics analysis is also carried out for the high-duty operation in the future, and the beam dynamics transmission with deformed structure at a 100% duty-factor demonstrates that the high duty-factor operation is feasible for this RFQ. Additionally, the required coolant temperature ranges for achieving zero-frequency-shifting under different duty factor conditions are presented to provide a reference for the practical operation. At the end of the article, a brief introduction is given to the cold test results, high-power conditioning, and preliminary beam measurements of the RFQ. The measurements indicate that the RFQ meets the requirements of the design objectives.