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.