Beam dynamics and Electromagnetic Structure Design of α RFQ

Abstract: This study addresses the demand for high-performance linear injectors in proton‑α particle integrated tumor therapy platforms and presents the beam dynamics design of a 220 MHz α particle Radio Frequency Quadrupole (RFQ) accelerator. To further minimize the longitudinal emittance obtained from the default ParmteqM design tool and satisfy the acceptance requirements of subsequent accelerating structures, an independent RFQ parameter design program is developed using Mathematica software. This program implements a "two-section shaper" strategy for the shaper section, achieving precise control over the bunching process through piecewise linear modulation of the evolution rates for the synchronous phase and the modulation factor. Dynamics simulation results demonstrate that the optimized design reaches a beam transmission efficiency as high as 98.1%. Notably, the exit longitudinal rms emittance is reduced by nearly 50% compared to the original scheme, significantly enhancing beam quality. A 3D model of cavity is constructed and simulated by using CST Microwave Studio. The results show the cavity exhibits Kp of 1.79 and Q0 of 13225. A total tuning range of tuners is -1.1 to 1.5 MHz. Through the collaborative regulation of end undercuts and tuners, the field unflatness across quadrants is strictly maintained within ±0.5%. Finally, multi-particle tracking is performed via TraceWin software. The verification results indicate that the beam transport performance based on the calculated 3D electromagnetic fields is in well agreement with the ParmteqM simulation results, confirming the self-consistency and reliability of the overall design.