Effect of magnetic fringe field and interference on beam matching in a Medium Energy Beam Transport line of the Spallation Neutron Source linac

A Medium Energy Beam Transport (MEBT) line is employed in the Spallation Neutron Source linear accelerator (linac) to match the beam from a radio frequency quadrupole to a drift tube linac and to perform other functions. The MEBT lattice consists of 14 electromagnetic quadrupoles and other devices. The quads have very small aspect ratios (steel length over aperture diameter), and they are densely packed in the lattice. Significant fringe fields and magnetic interference cause difficulties in beam matching. We have performed 3D modeling of the magnets, expanded simulation data to 3D multipoles, computed their linear optical properties, and compared their performance with that predicted by simple hard-edge models. This paper reports our findings and a proposed general solution to the problem.