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Magnetic Shielding Effect at Cryogenic Temperature Evaluated for the STF Cryomodule

The Superconducting RF Test Facility (STF) is a project at KEK to build and operate a test linac with high-gradient superconducting cavities. Two types of high gradient accelerating cavities, with design accelerating gradients of 35 MV/m and 45 MV/m, will be tested along with other essential technologies needed for the International Linear Collider main linac systems at the STF. In order to achieve the design accelerating gradients, the ambient magnetic fields inside the cavities must be well shielded, to a level of mG . The permeability of the shielding material shows a strong temperature dependence, which often results in a degradation of the shielding effect in a cryomodule when cooled down to the operating temperature. The magnetic field inside the magnetic shield for the STF cavities is measured along the STF beam axis at cryogenic temperature and compared with measurements at room temperature. The measurements at both temperatures are compared with simulation where the permeability curves of the shielding material obtained at room temperature, liquid nitrogen temperature and liquid helium temperature are used. The shielding effect dependence on temperature of the magnetic shield inside the STF cryomodule is presented along with a comparison between measurements and simulation.