Design of 4 Tm Forward Dipoles for the FCC-hh Detector Magnet System

The 4 T, 10 m free bore twin Solenoid with 4 Tm forward dipoles is discussed with an emphasis on the forward dipoles. This assembly, a possible detector magnet layout for the future circular collider for hadron-hadron physics, combines a 4 T axial magnetic field at the interaction point in the center of the twin Solenoid with off-axis magnetic fields generated by the two forward dipoles for the purpose of providing bending power to all particles emanating from the interaction point. The twin Solenoid provides 4 T over a free bore diameter of 10 m, resulting in a stored energy of 15.4 GJ, a cold mass of 1.25 kt, and a vacuum vessel mass of 1.4 kt. This configuration leads to acceptable quench protection and mechanical properties. The forward dipoles are located in the stray field of the twin Solenoid, resulting in large forces and torques on the coils. Two forward dipole options are presented. In the first option, the main dipole coils are combined with lateral coils for the purpose of bringing the net force and torque on the cold mass to zero. In the second option, the lateral coils are omitted and the net force and torque is handled through the vacuum vessel. The first option results in a stored energy of 240 MJ, a cold mass of 250 tons, and a vacuum vessel of 150 tons, whereas the second options gives 130 MJ, 100 tons, and 180 tons, respectively. The forward dipole conductor comprises copper and NbTi and is force-flow cooled with helium. The superconducting coils are placed in a stainless steel coil casing and precompressed. The coil casings are then fixed to the main body support structure, thus allowing transfa er of forces and torques between the coils. The analyses indicate that this leads to good quench protection and mechanical behavior.