Abstract
The Spallation Neutron Source ring employs doublet quadrupoles and dipole correctors in its straight sections. The electromagnetic quadrupoles have a large aperture, small aspect ratio, and relatively short iron-to-iron distance. The corrector is even closer to one of the quads. There have been concerns on the magnetic fringe field and interference in the doublet magnets and their assemblies. We have performed 3D computing simulations to study magnetic field distributions in the doublet magnets. Further, we have analyzed the particle optics based on the $z$-dependent focusing functions of the quads. The effect of the magnetic fringe field and interference, including the third-order aberrations, on the particle motion are investigated. The lens parameters and the first-order hard edge models are derived and compared with the parameters used in the ring lattice calculations.
Highlights
Beam from its linac is injected into an accumulator ring to produce a proton beam, which is accumulated to high intensity before it strikes a target for the generation of intense neutrons
The theory on magnetic fringe field in a quadrupole has been well developed, it appears that its effects on particle optics are often overlooked in practice
Quads with small aspect ratios are often represented by ‘‘conventional hard edge models’’ obtained by calculating its magnetic length from the integrated gradient divided by the gradient around the magnet center
Summary
In the Spallation Neutron Source (SNS) just completed at Oak Ridge National Laboratory (ORNL), a 1 GeV H beam from its linac is injected into an accumulator ring to produce a proton beam, which is accumulated to high intensity before it strikes a target for the generation of intense neutrons. The relatively small iron-to-iron distance within the assembly causes magnetic interference between the two quads, as well as between 30Q58 and its neighboring corrector 41CD30. We have been concerned with the effect of the magnetic fringe field and interference in the SNS ring. The magnetic fringe field in quadrupoles has been well investigated [4 –12], it appears that its effect is often overlooked in practice. During the SNS ring development and construction, there have been neither measurements of the magnetic fringe field of an individual quad nor magnetic measurements of the quadrupole doublet assemblies. Though the analyses of the magnetic fringe field and interference presented and developed in this paper are based on simulation data, the techniques apply to measured data in experiments. The effect of magnetic fringe field and interference on particle optics is analyzed.
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