Magnetic Shield Material Characterization for the Facility for Rare Isotope Beams' Cryomodules

The saturation fields of cylindrical cryogenic magnetic shields, for quarter-wave resonator cavities, of diameters ranging from 229 to 447 mm were measured at 300 and 10 K. A 0.86 T normal conducting solenoid and a 2.5 T superconducting solenoid were used at 300 and 10 K, respectively. The shielding of the high magnetic field region of the cavity, due to fringe fields entering the shield through the beam-port holes of the shield, was examined. The relative initial permeability of toroidal samples from two cryogenic shielding materials were measured at 0.358 A/m, at different frequencies, to extrapolate the static value. These samples were also exposed to external fields up to 0.86 T at 300 K, and the residual magnetic field was measured to check for magnetization of the shielding materials. Results indicate that the performances of the measured cryogenic shielding materials are dependent on the magnitude of the applied field. The performances are improved at cryogenic temperatures only when the shields are exposed to small magnetic fields. At greater fields, however, the performance is degraded at cryogenic temperatures. The shielding materials are not magnetized when exposed to fields up to 0.86 T.