Emmanuele Ravaioli

The US Magnet Development Program is conducting research on high-field accelerator magnets targeting a bore field of 20 T for the next generation of particle colliders. To reduce the overall cost of the dipole, a viable option consists in a hybrid approach where the inner coils in the high-field region are made of High Temperature Superconductors (HTS), such as Bi-2212 or REBCO coated conductors, and the coils in the lower field region (<16 T) are made of Low Temperature Superconductors (LTS), i.e. Nb3Sn. Quench protection for high-field magnets is demanding, especially in the case of a full-scale 15 m long magnet. To mitigate these challenges, an innovative quench protection method named Energy Shift with Coupling (ESC) has been selected and integrated into the magnet design. This method requires resistive coils that are strongly magnetically coupled to the magnet coils and electrically insulated from them. In this contribution, a 20 T hybrid cosθ dipole configuration is analyzed, investigating the stresses on the conductors during quench transients while limiting the adiabatic hot-spot temperature.

Impedance measurements have long been used to study superconducting magnets in their non-energized state, providing valuable insights into their electromagnetic behaviour. However, studies involving powering have primarily been conducted at low currents, due to the impact of the power converter impedance on the measurement. This paper presents a comprehensive set of impedance measurements on the powered MQXFS7 superconducting magnet, marking the first time such measurements have been performed up to its nominal operating current of I=16.23kA. The magnet's impedance was characterized using two distinct measurement configurations: low-frequency (f=10Hz-1KHz) and high-frequency (f=1kHz-45KHz). Measurements were conducted across all powering phases (ramp-up, steady state, and ramp-down) at two cryogenic temperatures T=1.9K and 4.5K. Impedance variations as a function of current were observed at T=1.9K, with the most notable result being an impedance shift towards the impedance measured at T=4.5K as the nominal operating current was approached. We hypothesize that this may be due to the presence of superconducting (Ti–6Al–4V) magnet poles. Being able to measure the impedance of superconducting magnets in a powered state represent a significant improvement in the understanding of impedance-based diagnostics for superconducting magnets, demonstrating their potential for characterizing magnet behaviour under operating conditions.

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.

The Paul Scherrer Institute (PSI) is developing an integrated system for cooling and protection of high-field superconducting magnets. Increasing the overall current density of superconducting cables reduces the conductor volume required for high fields but also decreases magnet protectability. To mitigate this, a strategy combining auxiliary coils and electrical discharge has been proposed to generate fast transients in the superconducting coils and enhance protection. For effective coupling, the auxiliary conductor must be located close to the magnet coils; however, in multi-layer configurations, this can reduce magnetic efficiency. Another key challenge in high-field magnets is cryogenic efficiency—both in steady-state cooling and in handling transient losses. To avoid the high costs of helium-bath systems in large magnets, PSI is investigating a conduction-cooling approach using forced-flow pipes embedded between coil layers. Yet, this integration can also impact magnetic performance. To address both protection and cooling challenges simultaneously, PSI proposes a dual-function concept using a hollow electrical conductor that serves as both the protection circuit and the cooling channel. The auxiliary coils are wound from this hollow conductor, which offers suitable electrical properties and enhanced thermal performance through forced-flow cooling. This work applies the proposed strategy to the subscale stress-managed asymmetric common-coil magnet under development at PSI. The study includes multi-physics transient analyses to assess system performance and validate the concept as a compact and efficient approach for superconducting accelerator magnets

Energy shift with coupling (ESC) is a novel quench protection method for superconducting magnets, recently developed at European Council for Nuclear Research (CERN). It enables effective protection by actively transitioning most of the coil turns to the normal state and extracting part of the stored magnetic energy, thereby effectively limiting the hot-spot temperature and peak voltage-to-ground. ESC achieves comparable or superior performance to conventional protection systems, such as quench heaters and coupling-loss induced quench, without requiring any electrical connection or close physical contact with the magnet coils, hence substantially improving overall system robustness. The ESC system includes normal-conducting auxiliary coils that are strongly magnetically coupled to the magnet coils. Upon quench detection, a capacitive unit discharges across the ESC auxiliary coils, inducing a fast shift of magnetic energy from the magnet to the ESC coils. As a result, high transient losses are generated in the superconductor, the magnet windings are transitioned very rapidly to the normal state, and the magnet current is quickly discharged. This work presents the first experimental demonstration of ESC, carried out at the CERN magnet test facility on a 50 cm long Nb ₃ Sn racetrack dipole magnet. Over 110 discharges were performed under various operating conditions and ESC unit parameters. The experimental results confirm the essential mechanisms underlying the effectiveness and robustness of ESC, validating its strong potential for use in future high-field accelerator magnets.

The Magnet Development (MagDev) laboratory at the Paul Scherrer Institute (PSI) has designed and successfully manufactured the first subscale stress-managed common coil magnet. The magnet was tested at CERN and reached 98% of its short sample current at 4.5 K. Following the high-field magnet roadmap of the Swiss Accelerator Research and Technology initiative (CHART), a new magnet was assembled after replacing two of the four Nb₃Sn coils and manufacturing and integrating four copper-based coils, which are part of the Energy Shift with Coupling (ESC) quench protection method. The ESC system uses normal-conducting auxiliary coils that are strongly magnetically coupled with the magnet’s Nb₃Sn coils for protection. When a quench is detected, a current is discharged through the auxiliary coils, leading to a rapid transfer of the magnet’s energy from the Nb₃Sn coils to the copper-based coils. This process results in transient losses and quick transition to the normal state of the Nb₃Sn conductor, which facilitates quick energy extraction. This work presents the manufacturing of the copper-based coils, magnet assembly, and test results of the subscale SMCC2.

The FalconD (Future Accelerator post-LHC Cos-$\theta$ Optimized ${\mathrm{Nb}}_{3}$Sn Dipole magnet is part of the High Field Magnet (HFM) CERN programme. In collaboration with the Italian Institute for Nuclear Physics (INFN Genova and LASA Milano), CERN is developing a ${\cos \theta }$ dipole magnet beyond 12 T based on proven ${\mathrm{Nb}}_{3}$Sn technology in the MQXF quadrupole used in the high-luminosity upgrade of the LHC. The magnet, with design aperture diameter of 50 mm, provides a bore field of 12 T for a 73 TeV center-of-mass energy scenario for the Future FCC-hh Circular Collider. The project has specific objectives to investigate cos-$\theta$ short magnet model design based on a two-layers coil with a wide high current cable. Moreover, the study of mechanical dipole structures based on aluminum shell with bladders and key (B&K) developed for the HL-LHC MQXF quadrupole will be presented as a baseline. The structure pre-loading system assembly baseline shall be tested on sub-scale mechanical mock ups at 77 K prior to building two single aperture 1.5 m long ${\mathrm{Nb}}_{3}$Sn magnet models followed by a 2-in-1 dipole magnet assembly. In this paper, we provide an overview of the magnetic and structural design carried out at CERN on an increased aperture diameter variant supported by first preliminary winding trials and give insights of the project timeline. Some preliminary protection scheme assessment is presented as baseline to be tested. Following the cable winding outcomes and the mechanical properties measured, some alternative coil optimized magnetic cross-section is reported.

Thermal transient responses of superconducting magnets can be simulated using the finite element (FE) method. Some accelerator magnets use cables whose electric insulation is significantly thinner than the bare electric conductor. The FE discretisation of such geometries with high-quality meshes leads to many degrees of freedom. This increases the computational time, particularly since non-linear material properties are involved. In this work, we propose to use a thermal thin-shell approximation (TSA) to improve the computational efficiency when solving the heat diffusion equation in two dimensions. We apply the method to compute the thermal transient response of superconducting accelerator magnets used for CERN’s Large Hadron Collider (LHC) and High-Luminosity LHC (HL-LHC). The TSA collapses thin electrical insulation layers into lines while accurately representing the thermal gradient across the insulation’s thickness. It allows considering cryogenic cooling via a temperature-dependent heat transfer coefficient and multi-layered quench heater (QH) regions. The TSA is implemented in the multipole module of the open-source Finite Element Quench Simulator (FiQuS), which can generate the multipole magnet models programmatically from input text files. The modifications implemented in FiQuS when constructing the geometry and mesh for the TSA model are discussed. First, the TSA approach is verified by comparison to classical FE simulations with meshed surface insulation regions for a simple block of four superconducting cables and a detailed model of the single-aperture Nb₃Sn magnet bending high field (MBH) dipole. The results show that the TSA approach reduces the computational time significantly while preserving the accuracy of the solution. Second, the QH delay, i.e. the time between activation of the QH and the point in time where the transport current density exceeds the homogenised critical current density in one of the superconducting half turns, computed with the TSA method is compared to measurements for the MBH magnet. To this end, the thermal transient simulation is coupled to a magnetostatic solution to account for magneto-resistive effects. Third, the TSA’s full capabilities are showcased in non-linear magneto-thermal simulations of several LHC and HL-LHC superconducting magnet models. The full source code, including input files to recreate all simulations, is publicly available.

The Fusillo project at CERN aims to design and build a demonstrator magnet with multi-harmonic corrected fields in a 90°, curved, Canted-Cosine-Theta magnet. Two subscale magnets are built with 30° bending, about 1/30 of the demonstrator conductor length, and increased current to reach coil stresses equivalent to those of the demonstrator. These magnets enable qualification of the technology developments, fabrication methods, winding and assembly procedures, magnetic and quench protection design and measurement setups. The second Fusillo subscale magnet had dedicated instrumentation, including temperature sensors, a heater on the aluminium formers, and a set of sensing coils positioned in the curved bore. The magnet voltage and current were also measured. A specific powering sequence was applied to the magnet, including forced energy extractions. The eddy currents and heat propagation in the formers and the windings are simulated in three dimensions. A cooperative simulation approach was used, which involves tools developed at CERN as part of the STEAM framework: FiQuS, a finite element-based tool, and LEDET, a finite difference-based tool. The measurement results are compared with simulations, and the key aspects of the magnet transient behaviour are discussed. The simulation model inputs are refined using the measurement results, and the methodology for doing this is presented.