High field septum magnet using a superconducting shield for the Future Circular Collider

A zero-field cooled superconducting shield is proposed to realize a high-field (3–4 T) septum magnet for the Future Circular Collider hadron-hadron (FCC-hh) ring. Three planned prototypes using different materials and technical solutions are presented, which will be used to evaluate the feasibility of this idea as a part of the FCC study. The numerical simulation methods are described to calculate the field patterns around such a shield. A specific excitation current configuration is presented that maintains a fairly homogeneous field outside of a rectangular shield in a wide range of field levels from 0 to 3 Tesla. It is shown that a massless septum configuration (with an opening in the shield) is also possible and gives satisfactory field quality with realistic superconducting material properties.

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Figure 1

(a) Layout of the beam extraction system of the FCC. (b) The principle of the superconducting shield. Blue arrows show the magnetic field in the transverse midplane; black arrows and color show the persistent shielding currents on the superconductor shield’s surface.

Figure 2

Possible longitudinal layout of the extraction system.

Figure 3

Possible shield configurations with different materials. See text for details.

Figure 4

(a) $J_c(B)$ curves used in the simulation for the $\mathrm{NbTi}/\mathrm{Nb}/\mathrm{Cu}$ multilayer sheet [18] and bulk ${\mathrm{MgB}}_2$ [19]. (b) Geometry of the shield, vacuum pipes and excitation coils. The thick red line indicates the required good field region.

Figure 5

Field pattern around a rectangular shield at a field strength of 0.5 and 3 T with a $\mathrm{NbTi}/\mathrm{Nb}/\mathrm{Cu}$ shield. Color indicates $|B|$ in Tesla.

Figure 6

Excitation and shielding current patterns assuming the $J_c(B)$ curve of the $\mathrm{NbTi}/\mathrm{Nb}/\mathrm{Cu}$ multilayer sheet and ${\mathrm{MgB}}_2$ at $T=20\mathrm{K}$, at a field strength of 3 T. Color indicates current density measured in $\mathrm{A}/{\mathrm{m}}^2$. For ${\mathrm{MgB}}_2$ the color scale was limited to ${10}^9\mathrm{A}/{\mathrm{m}}^2$ although the highest induced current exceeds this by about a factor of 4.

Figure 7

(a) $B_y$ in the horizontal midplane for the $\mathrm{NbTi}/\mathrm{Nb}/\mathrm{Cu}$ multilayer sheet and ${\mathrm{MgB}}_2$ at $T=20\mathrm{K}$ at two different field levels. (b),(c) Zoom into the surface region of the shield: $B_y$ from the 2D FEM simulation (black solid line, left axis) and $B(x)$ from a one-dimensional approximation (blue dashed line, left axis, see text for details), the shielding currents $J(x)$ (red solid line, right axis) and the critical current density at the actual level of magnetic field $J_c[B(x)]$ (red dashed line, right axis) for the two materials.

Figure 8

(a),(b) Magnetic field pattern in a superconducting shield septum with an 8 mm massless gap at 0.5 and 3.5 T. (c),(d) $B_y$ in the horizontal midplane on linear and logarithmic scale. (e) Homogeneity of the field in the good field region.