Wide-aperture magnets for an isocentric gantry for light-ion cancer therapy

A design for wide-aperture bending magnets is presented suitable for the last stage of a light-ion gantry beam-delivery system. Isocentric gantries for carbon beams require the use of wide-aperture magnets with a highly uniform field distributions over a wide range of flux densities. Since these magnets must be located at the outermost position of a rotating system, the magnet weights define the mechanical properties of the whole gantry. We here propose to use for such magnets “almost window-frame” magnets with air slots inside the magnet poles. Such low weight magnets allow flux densities from 0.3–2.0 T within a cross section of 200×200 mm2 with a homogeneity of better than ±0.03%.