NbTi/Nb/Cu multilayer shield for the superconducting shield (SuShi) septum

TL;DR

This study experimentally evaluates a NbTi/Nb/Cu multilayer superconducting shield for high-field applications, demonstrating effective attenuation up to 3.1 T at 4.2 K with minimal flux jumps and potential for use in collider septum magnets.

Contribution

It presents the first experimental results of a NbTi/Nb/Cu multilayer shield in a real magnet environment, showing promising attenuation and operational stability.

Findings

Shield attenuates magnetic field by over 99.995% at 4.2 K.

Leakage field of 12.5 mT at 3.1 T surface field.

Performance improves at 1.9 K but with flux jumps.

Abstract

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, a NbTi/Nb/Cu multilayer sheet. A cylindrical shield was constructed from two halves, each consisting of 4 layers with a total thickness of 3.2~mm, and inserted into the bore of a spare LHC dipole corrector magnet (MCBY). 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 mis-alignment 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 $\mathbf{4\times 10^{-5}}$, 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.