Electromagnetic design of microwave cavities for side-coupled linear accelerators: A hybrid numerical/analytical approach

A homemade computer code for designing the coupled microwave cavities of a linear accelerator (linac) has been developed. A hybrid approach, based on both analytical investigation and numerical calculation, is exploited. A finite-element method (FEM) based on 2-D/3-D electromagnetic simulation software is used to find the eigenmodes and eigenfrequencies of the accelerator cavities as well as design typical figures of merit. The FEM investigation is integrated with a multiobjective particle swarm optimization approach in order to automatically optimize the geometry of the accelerating tanks. This approach seems very promising and general, allowing the optimization of a wide class of side-coupled resonant structures. The computer code is validated via measurements on a 27-MeV 3-GHz standing-wave side-coupled linac tank of five cavities closed with suitable end cells. The agreement between simulation and experiment is excellent; the displacement between the maxima of the simulated and measured longitudinal electric field modulus is close to 0.2%.