Novel design of a high-frequency radio frequency quadrupole accelerator for medical application

Proton therapy is an advanced particle radiotherapy technology. Lanzhou University recently designed a new linear accelerator system for proton therapy. In the system, a 750[Formula: see text]MHz radio frequency quadrupole (RFQ) accelerator was used to complete the initial acceleration of particles. The RFQ can accelerate a proton beam intensity of 1[Formula: see text]mA from 30[Formula: see text]keV to 3[Formula: see text]MeV kinetic energy within a length of 2[Formula: see text]m. This paper aims to complete the physical design of this high-frequency RFQ, including beam dynamics design, radio frequency (RF) design, and multiphysics analysis. In the dynamics design, a novel design strategy for controlling the longitudinal emittance was adopted to improve the output beam quality of the RFQ. The simulation results showed that the RFQ outlet longitudinal emittance was controlled below 0.1 [Formula: see text] mm mrad. In the RF design, the pi-mold rod structure was applied to RFQ at such high frequencies for the first time. After the whole cavity simulation, it was found that the separation between the operating frequency of the RFQ cavity and its closest dipole mode reached 58.3[Formula: see text]MHz, and its quality factor reached 6352. Finally, a multiphysics analysis was performed. The analysis showed that the maximum temperature rise of the RFQ cavity was less than [Formula: see text]C and the frequency drift due to heating was 18[Formula: see text]kHz. This paper presents the current status of the design of this new linear accelerator. Compared with previous RFQs, the new RFQ has a very high transmission efficiency and higher RF stability.