Magnetic field measurement of large aperture FFAG magnet for muon-phase-rotation ring

The PRISM-FFAG ring, which consists of 10 scaling-radial-sector magnets, has been designed to store muons with large emittance and momentum spread. The FFAG magnets differ from conventional dipole magnets in that they have a magnetic field that varies strongly with radius and the vertical direction of the magnetic field alternates. Each magnet is composed of three sets of coils to produce a defocussing (outward bending)-focussing-defocussing triplet. After careful design, the PRISM-FFAG magnets were constructed and magnetic field measurements were made. The field maps in the fiducial volume show satisfactory agreement with the design fields based on OPERA 3D/TOSCA calculations.