Coils are cooled by thermal conduction of a cryocooler via heat
exchanger plates. The bulks are cooled by gas molecule conduction
of rarefied helium gas filled in the inner vessel. A pressure of the
gas should be carefully adjusted to an appropriate value at which
the gas molecule conduction is enough to cool bulks and a windage
loss of the rotor remains little. The value, which is dependent on a
gap, is typically about 1–102 Pa [10]. Note that the cryostat has two