NbTi/Nb/Cu Multilayer Shield for the Superconducting Shield (SuShi) Septum

A passive superconducting shield was proposed earlier to realize a high-field (3–4 T) septum magnet for the Future Circular Collider. This paper presents the experimental results of a potential shield material, an NbTi/Nb/Cu multilayer sheet. A cylindrical shield was constructed from two halves, each consisting of four layers with a total thickness of 3.2 mm, and inserted into the bore of a spare dipole corrector magnet of the Large Hadron Collider. At 4.2 K, up to about 3.1 T at the shield's surface, only a leakage field of 12.5 mT was measured inside the shield. This can be attributed to the misalignment of the two half cylinders, as confirmed by finite element simulations. With a better configuration, we estimate the shield's attenuation to be better than $\mathbf{{\text{4}}\times {\text{10}}^{-5}}$ , which is acceptable for the intended application. Above 3.1 T, the field penetrated smoothly. Below that limit, no flux jumps were observed even at the highest achievable ramp rate of more than 50 mT/s at the shield's surface. A “degaussing” cycle was used to eliminate the effects of the field trapped in the thick wall of the shield, which could otherwise distort the homogeneous field pattern at the extracted beam's position. At 1.9 K, the shield's performance was superior to that at 4.2 K, but it suffered from flux jumps.