Structure optimization of RF electric focused drift tube linac

BackgroundThe radio frequency electric focusing crossbar H-mode drift tube linac (RFC) accelerating structure combines the features of the crossbar H-mode drift tube linac (CH-DTL) and the four-vane radio frequency quadrupole (RFQ), both based on the TE210 mode, into a unified cavity design. It leverages the high shunt impedance of the CH-DTL to perform longitudinal acceleration and bunching, while utilizing an electrical focusing method that is more effective in the low-energy region to ensure transverse beam control. The RFC accelerating structure is not only suitable as a transitional structure connecting the injection section and the main acceleration section in large linear accelerators, but its compact and efficient characteristics also meet the demand for equipment miniaturization in the medical and industrial fields.PurposeThis study aims to enhance the radio frequency (RF) efficiency and operational stability of the RFC while improving its process feasibility through systematic optimization of its RF structure.MethodsFirstly, parametric simulations were conducted using the CST electromagnetic simulation software, and the equivalent circuit method was employed to analyze the RF characteristics of three types of accelerating cells, namely the CH-DTL cell, the RFQ cell, and the transition cell. Then, the RF structures of each cell type were sequentially optimized through parametric scanning, targeting improved shunt impedance and reduced peak surface field. Finally, the overall cavity tuning was performed in a step-wise manner: global field flatness was first adjusted by tilting the end-cell stems, followed by fine tuning of the gap-to-cell-length ratio to achieve uniform field distribution.ResultsThe optimization results show that the completed RF design of a 162.5 MHz RFC power cavity achieves an effective accelerating gradient of 1.6 MV·m-1 and an effective shunt impedance of 46 MΩ·m-1. The peak surface electric field is constrained within 1.60 times the Kilpatrick limit, and the maximum thermal power density is limited to 20.2 W·cm-2, ensuring stable continuous-wave operation.ConclusionsThe optimized 162.5 MHz RFC power cavity designed in this study demonstrates the effectiveness of the proposed optimization strategy, promoting the development of RFC accelerating structures and providing a reference for the design of similar hybrid RF cavities.