  Recent discovery of a Higgs boson boosted interest in a low-energy
medium-luminosity Muon Collider as a Higgs Factory (HF). A preliminary design
of the HF storage ring (SR) is based on cos-theta Nb3Sn superconducting (SC)
magnets with the coil inner diameter ranging from 50 cm in the interaction
region to 16 cm in the arc. The coil cross-sections were chosen based on the
operation margin, field quality and quench protection considerations to provide
an adequate space for the beam pipe, helium channel and inner absorber (liner).
With the 62.5-GeV muon energy and 2 x 10^12 muons per bunch, the electrons from
muon decays deposit about 300 kW in the SC magnets, or unprecedented 1 kW/m
dynamic heat load, which corresponds to a multi-MW room temperature equivalent.
Based on the detailed MARS15 model built and intense simulations, a
sophisticated protection system was designed for the entire SR to bring the
peak power density in the SC coils safely below the quench limit and reduce the
dynamic heat load to the cold mass by a factor of 100. The system consists of
tight tungsten masks in the magnet interconnect regions and elliptical tungsten
liners optimized for each magnet.
