Effect of Magnetic Fringe Field and Interference on Beam Matching in a Medium Energy Beam Transport Line of the Spallation Neutron Source Linac

Abstract

A Medium-Energy Beam Transport (MEBT) line is employed in the SNS linac to match the beam from an RFQ to a DTL and to perform other functions. The MEBT lattice consists of fourteen 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 simulations of the magnets, computed their optical properties, and compared their performance with what predicted by simple hard edge models. This paper reports our findings and a general solution to the problem.