RFQ accelerators are usually designed and modeled with standard codes based on electrostatic field approximations. There are recent examples when this approach fails to predict the RFQ performance accurately: for 4-rod RFQs 3D effects near the vane ends can noticeably influence the beam dynamics. The same applies to any RFQ where the quadrupole symmetry is broken, e.g., 4-vane RFQ with windows. We analyzed two 201.25-MHz 4-rod RFQs - one recently commissioned at FNAL and a new design for LANL - using 3D modeling with CST Studio. In both cases the manufacturer CAD RFQ model was imported into CST. The EM analysis with MicroWave Studio (MWS) was followed by beam dynamics modeling with Particle Studio (PS). For the LANL RFQ with duty factor up to 15%, a thermal-stress analysis with ANSYS was also performed. The simulation results for FNAL RFQ helped our Fermilab colleagues fix the low output beam energy. The LANL RFQ design was modified after CST simulations indicated insufficient tuning range and incorrect output energy; the modified version satisfies the design requirements. Our PS results were confirmed by multi-particle beam-dynamics codes that used the MWS-calculated RF fields.
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