The M15 beamline is dedicated to surface µ+ taken from the 1AT1 proton target, which is usually 1cm graphite or beryllium. Beam is collected by a permanent quadrupole doublet located very close to 1AT1. This gives good muon collection, but limits the momentum range of the channel from about 19 MeV/c to 40 MeV/c. Although designed primarily for µ+, M15 will work with µ- also (but M9B would be a better choice).
After transporting the beam up above the level of most other beamlines, M15 produces a high-luminosity focus in the M15 experimental area, which is not actually in the meson hall.
M15 is equipped with dual spin-rotators (Wein filters) which serve both to separate positrons from the beam and to rotate the muon spin to be perpendicular to the beam direction. They may be operated at low voltage to remove positron contamination without rotating the spins much. These dual rotators retain more beam flux than the single rotator in M20 because the dispersion from the first rotator is partly counteracted by the second.
M15 has slits for selecting the beam momentum acceptance and adjusting the final spot size.