André Brem

We present a predictive multiscale modeling workflow for Nb ₃ Sn Rutherford cables that bridges strand and cable scales to support the design of accelerator magnets in the 14 T range. This experimentally validated framework enables prediction of critical current reduction under load of Nb ₃ Sn Rutherford cables. The workflow is parameterized in strand and cable architecture, allowing it to be applied across cable designs; as done here on two 21-strand cables of different design (CD1 and SMACC-HF), differing most notably in strand diameter and Cu/non-Cu ratio. The workflow integrates a cabling model capturing strand transposition and compaction-induced deformation, a Python tool for generating FEM-ready 2D cross-sections, homogenization of the Nb ₃ Sn interfilamentary region with inter-strand contact modeling, and a linkage connecting simulated strain to critical current reduction. The workflow was mechanically validated against 10-stack Rutherford cable measurements at 77 K, with the simulated macroscopic stress–strain slope matching the measurement to within 6% in virgin loading and 0.1% in reloading. The predicted critical current reduction was validated against the compression BOX experiment. Across the 10–250 MPa transverse-pressure range covered by the experiments, the predicted cable critical current agrees with the measurement to a mean absolute error of ⩽ 3.2% for CD1, where I c was taken from an independent strand measurement and ⩽ 0.9% for SMACC-HF, where the initial I c is taken from the virgin cable measurement. Both measurement and simulation show a correlation between a lower Cu/non-Cu ratio and retaining a higher fraction of the virgin critical current under transverse pressure.

Pulsating heat pipes (PHPs) are passive devices using thermally induced two-phase flow. They are increasingly studied in cryogenic conditions to improve heat transfer between cryocoolers and high temperature superconducting (HTS) coils. This paper presents the application of two neon PHPs in simulta neous operation transferring the heat load from an insulated HTS coil under alternating current (AC) operation to a cryocooler. The main objective is to demonstrate the feasibility and quantify the potential benefits of using PHPs for this application. The PHPs, the coil, the experimental setup, procedures and campaigns are described. The experimental results of the coil cooldown and of the AC operation, with cycles of current ramping at 500A/s between 0 and 500A, are analyzed and discussed. The maximum magnetic field at the center of the coil is measured at 0.43T and AC losses dissipate an average heat load of nearly 6.5W. The PHP parameters tested include condenser temperatures of 27 and 30K, as well as filling ratios ranging from 37 to 56%. The two neon PHPs effectively transfer the heat while guaranteeing a low thermal resistance (0.18 to 0.24K/W) and without dry-out phenomenon across four experiments. On average, the temperature difference over the thermal buses is reduced by almost 80% compared to an equivalent copper rod, demonstrating the potential of PHPs for this application.

We present a systematic method to tune the turn-to-turn contact resistance $R_{\text{ct}}$ in high-temperature superconducting (HTS) solenoids by applying a polyvinyl butyral (PVB) thermoplastic coating filled with silver-coated copper flakes (SCCF). The filler type and its volume fraction determines the accessible resistance range in the final coil. $R_{\text{ct}}$ strongly affects the charging dynamics and quench resilience of non-insulated (NI) HTS magnets, hence it is important to be able to control this parameter in a reliable and reproducible way. The coating is applied prior to winding in a continuous dip coating process and results in a dry polymeric film on the tape. After winding, the coating layers are fused above $60\,^\circ \text{C}$ and the final $R_{\text{ct}}$ is adjusted through the temperature and the duration of the heat-treatment. This method gives access to a wide range of $R_{\text{ct}}$. Values from about 50 to $10^{6}\,\mu \Omega \cdot \text{cm}^{2}$ can be obtained with the right choice of filler type, concentration and heat-treatment conditions. An 80-turn demonstrator solenoid fabricated using this approach and tested at $\text{77}\,\text{K}$ achieved the targeted $R_{\text{ct}}$ of around $70\,\mu \Omega \cdot \text{cm}^{2}$. A finite-element model, calibrated with the measured field-decay time, was used to extract the average turn-to-turn resistance and validate the tuning method.

This work explores the use of a double-pancake coil for validating electromagnetic numerical models of HTS cables. We present a cost-effective and rapid production method using 3D-printed plastic formers. High-temperature superconducting (HTS) REBCO tapes are a promising solution for high-field magnets, particularly in accelerator applications, due to their ability to maintain superconducting properties in high magnetic field and high critical current density. One of the most effective ways to utilize HTS tapes is through straight soldered stack cables, which offer excellent packing efficiency in racetrack coil geometries-ideal for Block and Common-coil dipole magnet configurations. Measurements were performed in self-field conditions in liquid nitrogen, with AC current applied at frequencies up to 20 Hz. The current was increased in steps of 5 A of peak current, reaching a maximal of 100 A-below the critical current of the tested samples. For each frequency, the AC loss dependence on peak current showed a smooth trend, while AC losses per cycle remained independent of frequency. The validation process involved finite element method (FEM) modeling, using Jc(B) interpolation from experimental data, ensuring high accuracy. Both single REBCO tapes and soldered face-to-face cables were analyzed and compared to models of different modelling depth. The computational model matched the measured results with an accuracy within 10%. These findings provide a foundation for further research on the practical application of HTS straight soldered stack cables in 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.

This paper presents the design of a non-insulated (NI) high-temperature superconductor (HTS) 15 T solenoid with a 72 mm diameter warm bore, intended for use in the Paul Scherrer Institute positron production (P³) experiment. The P³ experiment, scheduled to start in Q3 2026, aims to demonstrate a high-yield positron source that is relevant in the context of FCC-ee. The coils will be solder-impregnated using techniques developed with a small-bore HTS NI coil stack. This magnet produced a magnetic field of 18 T at 12 K and 2 kA, in a cryogen-free, conduction-cooled setup. Similarly, the P³ magnet will be conduction-cooled by two cryocoolers. This larger-bore magnet is designed to operate at 15 K with a nominal operating current of 1.2 kA. To ensure the protection of the NI magnet, quench prevention is the preferred strategy. Several potential failure modes are analyzed, including thermal runaway in the event of failures in the current leads, power supply, or cryocoolers. By enhancing the cold mass’ heat capacity through the addition of a large lead mass, the stored magnetic energy can be safely dissipated in the cold mass through the electrical path formed by the superconductor and the solder. Mechanical analysis indicates that the hoop, radial and axial stresses are kept below allowable limits.

The magnet development (MagDev) laboratory at the Paul Scherrer Institute (PSI) has designed the subscale stress-managed common-coils (subSMCC1) 2-in-1 dipole magnet as part of the CHART (Swiss Accelerator Research and Technology) contribution to the CERN HFM (High-Field Magnet) programme. This R&D vehicle is part of the CHART/MagDev high-field roadmap. The nature of the subscale magnet allows it to be manufactured and tested quickly, so lessons learned from it will be applied to the high-field stress-managed common-coils demonstrator that is currently under development. The magnet has a straight section of 150 mm, two clear bore apertures of 22 mm, and an inter-beam spacing of 120 mm. In the first version, the magnet structure consists mainly of inner and outer pads surrounding the straight sections of the coil, and inner and outer pads at the coil ends, with no iron yoke or shell. Axial pre-load is provided by six rods connected to the end pads. All structural components, including the split-former pieces, are made from the same stainless-steel alloy. The first magnet, with four stress-managed Nb₃Sn common-coils, was manufactured and tested. This contribution presents the detailed manufacturing process as well as test results, training behavior and mechanical measurements.

Stress-management of Nb₃Sn coils offers a promising approach for high-field magnets by reducing conductor stress. The combination of stress-management and the common-coils magnet type simplifies manufacturing processes, such as utilizing the coil former in reaction and impregnation tooling. The common-coils architecture provides an attractive solution for 2-in-1 dipole magnets, with its racetrack geometry facilitating manufacturing and potentially allowing the react-and-wind (R&W) technique for cost-effective large-scale production. A key contribution of this work is overcoming the need for pole coils to achieve field quality by introducing an asymmetric design using only common-coils. The PSI 2-in-1 dipole magnet, featuring eight stress-managed Nb₃Sn layers, employs bladder-and-keys technology for pre-loading at room temperature, eliminating the need for an aluminum shell or pads. The magnet includes two cable grades spliced in the low-field region, along with other features discussed in this paper. This work presents the conceptual design, including 3D magnetic design, mechanical analysis, and protection results.

In the context of high-energy physics, the use of Nb ₃ Sn superconducting magnets as a cost-effective and reliable technology depends on improvements in the following areas: long development and manufacturing cycles, conductor degradation after thermal cycling, long training, as well as a demonstration in accelerator magnets with a beam aperture of the full potential of modern Nb ₃ Sn conductors. In short, performance, robustness, and cost are the three issues to be addressed. The Magnet Development project (MagDev) of the Swiss Accelerator Research and Technology initiative (CHART) at the Paul Scherrer Institute (PSI) aims to contribute to the solutions to each of these issues, re-thinking the manufacturing and design process. In our program, every innovation is to be validated by means of a panoply of fast-turnaround tools: from non-powered and powered samples and coils, tested under background field, to low-field subscale magnets and high field short prototypes. This work presents one element in this panoply of R&D vehicles: a stress-managed Nb ₃ Sn coil called BigBOX, impregnated with paraffin wax, and tested, through a collaboration with the Magnet Development Program of the United States (US-MDP), in the background field of Brookhaven National Laboratory (BNL)’s common coils dipole DCC17.