Quadrupole magnets for the 20MeV FFAG, 'EMMA'

EMMA is a 20 MeV non-scaling fixed field alternating gradient accelerator (nsFFAG) proof-of- principle prototype, to be built at the Daresbury Laboratory as an accelerator physics experiment to explore the behaviour of such machines. Non-scaling FFAGs have potential applications in charged particle cancer therapy and also for particle physics; however, to date, no such accelerator has been constructed. The magnet designs present major challenges - the lattice is made up of 84 quadrupoles, with different horizontal offsets from the magnet centres in the focusing and defocusing quads. These offsets alone provide the necessary bending fields in the ring. The magnets are also very thin (55 mm and 65 mm yoke lengths) and end field effects therefore dominate. Careful design, followed by prototype construction and measurement, is essential. The magnets have been designed in 3D from the outset, using the CST EM studio software. The paper will present the results of the design, showing how the magnets have been optimised to improve the integrated good gradient region, and will report on the progress of the prototyping work.