Study of Magnetic Shield for the STF Cryomodules

The Superconducting RF Test Facility (STF) is being constructed at KEK. Two types of high gradient accelerating cavities, with 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 field inside the cavities must be well shielded, to the level of several mG. The earth´s magnetic field in the STF tunnel, and the ambient magnetic fields inside the cryomodule and inside the magnetic shield were measured. The results are presented along with the permeability measurement data of several materials at 300 K, 77 K and 4 K, which will be used to estimate the shielding effect when the cryomodules are cooled down to the operating temperature in the future.