Marika D'ADdazio

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.

The US Magnet Development Program is leading an effort to design and manufacture a 20 T accelerator dipole magnet. Various designs are under development featuring cos-theta, block-coil, or common-coil geometries. To achieve the target field while maintaining cost-effectiveness the magnet cross-section includes high-temperature superconductor (HTS) inner coils and Nb$_{3}$Sn outer coils, which are all powered in series. The quench protection of this class of high-field accelerator magnets is extremely challenging due to the high energy density, high current density, slow quench propagation, in particular in the HTS coils, and highly inhomogeneous thermal properties of HTS and Nb$_{3}$Sn coils. The magnet quench discharge is simulated with the STEAM-LEDET program as coupled electro-magnetic and thermal transients. The peak temperature and voltage to ground in the magnet coils during the transient are presented. The performance of a CLIQ-based quench protection system applied to 1 m long model magnets in terms of peak temperature and voltage to ground is investigated. Furthermore, the scalability of the proposed solution to full-scale, 15 m long magnets is discussed.

An investigation of instance segmentation of cracks in Nb₃Sn 4-stack 40-strand Rutherford cables using machine learning is presented. Three samples were uniaxially and biaxially loaded before metallographic inspections were performed. The Mask R-CNN model was used in the Detectron2 framework with pre-trained weights but fine-tuned to detect and segment cracks. The model detected cracks with bounding box and mask average precisions (AP) of 42.8 and 27.9, respectively, and was used for instance segmentation of all cracks in the three samples. More cracks were found in the sample pre-loaded along the z-axis (i.e., along the cable length). Pre-loading along the x-axis (i.e., on the cables’ edges) reduced the number of cracks and changed the crack orientation distribution, away from being highly aligned with the y-axis (i.e., normal to the cables’ broad faces), i.e., the direction with the highest applied load. Fine-tuning of the Segment Anything Model (SAM) was also studied but performed poorly without human-provided prompts. However, the zero-shot capability of SAM showed high promises to accelerate the image annotation process for applications beyond this study.

To reach high collision energy for future high-energy particle colliders, like the Future Circular Collider (FCC) or the Muon Collider, it is required to achieve high field strength of the bending dipoles. Currently, the practical limit for Nb$_{\text{3}}$ Sn technology is around 16 T and, in order to further increase the magnetic field, the superconducting magnet community is considering High Temperature Superconductors (HTS), in particular Bi-2212 and REBCO conductors. However, their relevant higher cost has led the community to consider a hybrid approach where HTS materials are used in the high field region of the coils with so-called “insert coils”, and Low Temperature Superconductors are involved in the lower field part ($< $ 16 T) with so-called “outsert coils”. This paper describes the conceptual mechanical design of a 20 T hybrid cos$\theta$ dipole configuration. The high stress levels that the structure is facing due to the high magnetic field are discussed. Moreover, it presents the results of the optimization analysis of the shell-based support structure based on the key-and-bladder technology that provides the azimuthal pre-stress during room temperature assembly and cooldown to cryogenic temperatures. The aim of this work is to present a feasible design that satisfies the stress requirements.

Superconducting magnet coils are subject to large thermo-mechanical loads applied during magnet assembly, cooldown and operation. These loads can cause the reduction of their critical current due to mechanical strains or local filament failures. Measurements on longitudinally stretched strands and Rutherford cables under transverse pressure have allowed exploration of material limits in two directions. However, no systematic study of the effect of multi-axial loading conditions has been done. Finite Element (FE) models show that, indeed, the actual limits of the material are strongly dependent on the nature of the applied load and that the strength under multi-axial loading can be significantly higher with respect to uniaxial loading conditions. In this paper, we try, for the first time, to measure the effect of multi-direction loading conditions on Nb ₃ Sn Rutherford cables. The experiments are performed on impregnated cable stacks under transverse, lateral, and longitudinal constraints. The integrity of the cables is verified by destructive metallography inspection, evaluating the damage as a function of the applied loading condition.