A direct-current pion focusing magnet for low energy in-flight muon-neutrino beams

A cylindrically-symmetric, direct-current magnet has been designed for use in the focusing of pions to improve the muon-neutrino flux obtained from decays in-flight at low energy accelerators. The device uses conventional magnet technology with specially designed field-shaping windings to achieve optimized focusing for positive pions and defocusing for negative pions. A prototype magnet has been constructed and its magnetic field map used in a ray tracing program from which it has been calculated that a factor of four improvement in the neutrino flux at a downstream detector can be realized when compared to an unfocused pion source. The design approach taken in this paper to select the appropriate coil distribution to optimize focusing may be applicable in other types of particle beam lines as well as in high energy collider detectors where toroidal magnet geometries have been proposed.