Magnetic Properties of Shielding Materials for Superconducting Cavities

Magnetic shielding is a key technology for superconducting RF cavities. The tolerance of the ambient magnetic field depends on factors such as the operating RF frequency and acceleration gradient, but can be lower than 1 μT. Conventional permalloy is often used for shielding low magnetic fields such as the earth's magnetic field. However, it is not well known how much the shielding effects degrade at cryogenic temperatures where superconducting cavities are operated. Some high Ni-content alloys, which are reported to maintain high permeability at cryogenic temperatures, are commercially available at present and are used for magnetic shielding of superconducting cavities at many laboratories. In this study, the magnetic properties of these materials are measured at room and cryogenic temperatures. The permeability dependence on the heat treatment conditions is studied by using one of the high Ni-content alloys. The effects of mechanical strain are also examined at room temperature. Evaluation of the magnetic shielding for the superconducting cavities of cERL is also discussed in this paper.