Beam dynamics design and RF parameter optimization in SCDTL cavities: Theoretical analysis, prototype development and cold-test results

Institute of Modern Physics (IMP) have developed a compact 3 GHz SCDTL biperiodic chain structure for proton therapy linacs. This paper investigates the dispersion characteristics and electromagnetic properties of a biperiodic chain structure accelerating cavity, aiming to analyze its frequency stability and optimize its design for practical applications. By deriving the dispersion relation of an infinite periodic chain and considering nearest-neighbor and second nearest-neighbor coupling effects, this study explores the dependence of dispersion curves on circuit parameters, including the frequency and coupling coefficient between the accelerating and coupling cavities. Numerical simulations using CST Studio Suite are employed to determine the circuit parameters and validate the dispersion relation of the smallest unit in the biperiodic chain structure. The results demonstrate that the biperiodic chain structure exhibits enhanced frequency stability in the [Formula: see text]-mode, with two dispersion curves merging under specific conditions. The beam dynamics design incorporates three SCDTL modules that accelerate proton beams from 10 MeV to 30 MeV. A comprehensive analysis was performed to evaluate key RF parameters, with particular focus on their effects on operating frequency and field flatness. A prototype cavity was fabricated with strict attention to manufacturing tolerances, coaxiality and surface roughness. Cold tests validated the prototype cavity performance, with measured field distribution and quality factor satisfying all design criteria.