A full-scale prototype of a superconducting sector magnet is under construction at RIKEN to assess the magnet design conceived and to gain operational experiences of the magnet system. In this paper some design features of the prototype and viable alternatives are investigated chie¯y through quench analysis. The coils are designed to be cryostable. The cryostability, however, may degrade when a helium vapor is trapped inside the coil vessel. Such an event is likely because the vessel is largely packed with supporting materials to elevate the coil stiness, and because the vessel is con®gured horizontally ¯at. Assuming the existence of an uncooled conductor, critical lengths of a normal region and minimum quench energies were numerically studied. Another salient feature of the prototype is the use of iron pole pieces at 4.5 K to eectively enhance the magnetic ®elds and to reinforce the mechanical structure. However, due to close coupling of the pole pieces with the coils, AC losses are rather substantial. They were computed to quantify the energy shift during a quench and to look into the possibility of rapid spread of a normal region. The proposed high-®eld cyclotron comprises six superconducting separated-sector magnets in which the total stored energy amounts to 400 MJ at maximum excitation. An optimal arrangement of coil connection has been pursued in the design of the quench protection system.