3D-modeling for the LANL-APT RFQ

An 8 meter long four-vane Radio Frequency Quadrupole (RFQ) has been proposed by Los Alamos National Laboratory (LANL) for us in high-current proton accelerators. The RFQ is made up of four 2 meter long segments that are resonantly coupled. Disregarding the vane modulations, most of the structure can be modeled in 2D. However, the end regions and the segment joints need a full 3D treatment. Even using the four-fold symmetry of the structure, it is too complex to be modeled in one step. Here, the strategies for a piece-wise treatment of the RFQ and the results of this modelling are represented. The simulations have been done with the electromagnetic simulator MAFIA. Many of the features of such a RFQ have also been demonstrated in a laboratory cold model. Comparison between numerical results and laboratory measurements for this cold model gave us confidence in doing the detailed design for the final high current RFQ, using the same techniques. Depending on the beam dynamics requirements, the final RFQ has been laid out to yield either a flat, or a significantly ramped on-axis field at different locations along its length. The discrete model simultaneously took into account vane modifications for radial matching, vane undercuts to adjust the field flatness and tuning devices. Another simulation has been set up to determine appropriate vacuum slot configurations. 5 refs., 5 figs., 1 tab