Iron-free detector magnet options for the future circular collider

In this paper, several iron-free solenoid-based designs of a detector magnet for the future circular collider for hadron-hadron collisions (FCC-hh) are presented. The detector magnet designs for FCC-hh aim to provide bending power for particles over a wide pseudorapidity range ($0\le |\eta |\le 4$). To achieve this goal, the main solenoidal detector magnet is combined with a forward magnet system, such as the previously presented force-and-torque-neutral dipole. Here, a solenoid-based alternative, the so-called balanced forward solenoid, is presented which comprises a larger inner solenoid for providing bending power to particles at $|\eta |\ge 2.5$, in combination with a smaller balancing coil for ensuring that the net force and torque on each individual coil is minimized. The balanced forward solenoid is compared to the force-and-torque-neutral dipole and advantages and disadvantages are discussed. In addition, several conceptual solenoid-based detector magnet designs are shown, and quantitatively compared. The main difference between these designs is the amount of stray field reduction that is achieved. The main conclusion is that shielding coils can be used to dramatically reduce the stray field, but that this comes at the cost of increased complexity, magnet volume, and magnet weight and reduced affordability.

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Figure 1

Twin solenoid detector with forward dipoles detector layout. Also shown are the hadron calorimeters in dark green, the electromagnetic calorimeters in light green, the trackers in grey, and the muon chambers in yellow.

Figure 2

Three-dimensional representation of the cold mass of the force-and-torque-neutral dipole. The main dipole coils contribute most of the useful field integral for particle tracking and the lateral coils return the flux generated by the main coil, thus reducing the stray field and canceling the net force and torque on the cold mass. See [2] for more details.

Figure 3

Magnetic field map of forward dipoles in combination with twin solenoid. The dipoles are arranged in an up-down configuration. The red arrows indicate the solenoidal and dipole field orientations. It is clear that the dipoles are located very close to the twin solenoid, so that the stray field from the twin solenoid on the dipole conductor is as high at 1 T.

Figure 4

Three-dimensional representation of the superconducting coils of the twin solenoid and the two balanced forward solenoids. The color indicate the magnetic field on the surfaces of the cold masses, also see Fig. 5. The blue arrows illustrate the various possible misalignments of the cold mass as discussed in Sec. 2.

Figure 5

Two-dimensional axisymmetric representation showing half of the twin solenoid and a balanced forward solenoid. The arrows indicate the magnetic field orientation, while the colors indicate the field magnitude. The plus and minus symbols indicate the direction of current within the solenoids. The red dotted lines indicate the common cold masses.

Figure 6

Two-dimensional axisymmetric representation showing half of a very long twin solenoid. This magnet configuration provides the same field integral at $|\eta |=4$ as the shorter twin solenoid in combination with two balanced forward solenoids (Fig. 5), but the stored energy is more than twice as high.

Figure 7

First field integral $I_1$ of the twin solenoid and the balanced forward solenoid versus the forward dipole.

Figure 8

Second field integral $I_2$ of the twin solenoid and the balanced forward solenoid versus the forward dipole.

Figure 9

Three-dimensional representation of the spherical detector assembly. The color indicates the field on the surface of the cold mass (see the legend in Fig. 10).

Figure 10

Two-dimensional axisymmetric representation showing half of the main detector magnet and the forward solenoidal magnet. The arrows and colors indicate the magnetic field orientation and magnitude, respectively. The plus and minus signs in the superconducting coils indicate the direction of the current, while the red dotted lines indicate the common cold masses.

Figure 11

Three-dimensional representation of the unshielded solenoid in combination with two balanced forward solenoids. The color indicates the field on the surface of the cold mass (see the legend in Fig. 12).

Figure 12

Two-dimensional axisymmetric representation of half of the unshielded solenoid in combination with a balanced forward solenoid. Here the arrows and colors indicate the magnetic field orientation and magnitude, respectively. The plus and minus signs indicate the direction of the current inside the superconducting coils.

Figure 13

Magnetic field magnitude as a function of radial distance to the interaction point.

Figure 14

Magnetic field magnitude as a function of axial distance to the interaction point.