The FCC-hh collider demands accelerator-grade, industrially reproducible 14 T dipoles and 375 T/m quadrupoles based on Nb₃Sn technology. Building on the experience from the 11 T dipole, HL-LHC MQXF, and MKQXF demonstrator, the study identifies a static collared-coil / pole-loaded architecture as the optimal structural concept for both magnet families.
The collared-coil design provides a locked, deterministic pre-stress, low part count, and simple load path. It offers stable coil geometry through cooldown and excitation, precise alignment, and proven compatibility with large-series industrial production, as demonstrated by the LHC and RHIC programs. In contrast, the bladder-and-key structure used in MQXF—while successful for limited HL-LHC volumes—relies on a non-static preload, numerous precision parts, and complex tolerance management, making it less suited to FCC-hh’s scale.
Lessons from LHC, RHIC, and HERA show that reproducibility, integrated metrology, and streamlined assembly outweigh marginal field-performance gains. The recommended FCC-hh baseline, therefore, combines:
- Static collared coils with stainless or aluminium collars and a welded stainless-steel outer shell acting as both helium vessel and structural skin.
- A fixed-datum interface strategy (coil-collar-yoke-shell) ensuring mechanical repeatability.
- Hydraulic press pre-load locking verified through FEM and strain-gauge validation.
The proposed R&D program will:
- Finalise the 14 T/375 T/m collared architecture;
- Validate cable and insulation windows using 11 T/MQXF heritage;
- Scale quench protection and mechanical validation to full length;
- Conduct pilot industrialisation with European industry partners.
This concept offers the best balance between mechanical rigidity, field quality, and manufacturability, positioning the static collared-coil design as the baseline cold-mass structure for the FCC-hh era.