Quench Protection of the Main Quadrupole Magnet for the FCC-hh

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Abstract
A high-gradient (361 T/m) Nb3Sn lattice quadrupole magnet (MQ) is being developed for the proposed Future Circular Collider hadron-hadron (FCC-hh) for operation at 1.9 K. The very high winding energy density of this magnet (61 MJ/m3) makes quench protection especially challenging. Limiting the hot-spot temperature and peak voltage-to-ground requires a system that rapidly transitions most of the windings into the normal state. This paper evaluates three protection methods: Quench Heaters (QH), the Coupling-Loss Induced Quench (CLIQ) system, and a novel method, Energy Shift with Coupling (ESC). ESC enables fast and effective quench initiation without direct electrical connection to the magnet coils. The scope includes analyzing hardware requirements, such as CLIQ and ESC unit energy, and ESC coil geometry. Failure cases for ESC were also studied to assess redundancy. Electromagnetic-thermal transient simulations were performed with the STEAM-LEDET. Results show that QH cannot protect this magnet, while CLIQ is effective only within a narrow energy range, limited by its switching current capability. ESC provides robust protection across the full current range with lower component stress and intrinsic redundancy from its per-pole implementation. ESC, therefore, emerges as the most promising candidate for FCC-hh MQ protection.

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