Nb3Sn Accelerator Magnets Research Articles

Training-free demonstration of a 5.4 T Nb3Sn Canted–Cosine–Theta accelerator dipole impregnated with paraffin wax

Lawrence Berkeley National Laboratory is pursuing stress-managed Nb3Sn Canted–Cosine–Theta (CCT) magnet technology for high field accelerator magnets. Although promising results have been reported, improvements in the training performance are desired. This work describes the fabrication and testing campaigns of two subscale Nb3Sn CCT magnets; a baseline impregnated with National High Magnetic Field Laboratory Mix-61 and a magnet impregnated with paraffin wax. The paraffin magnet reached the short sample limit of the conductor, to within the measurement uncertainty, with no training quenches inside the magnet. In contrast, the baseline magnet reached 80% of the short sample limit after approximately 20 quenches and exhibits some loss of memory after thermal cycles. Inter-layer flexible quench antennas combined with voltage tap data show that all quenches appear identical and originate from a region corresponding to the location of peak field in the cable. Although this success should be replicated at higher fields, these first test results demonstrate the potential for training free Nb3Sn accelerator magnets operating near the short-sample limit.

Efficient multiscale investigation of mechanical behavior in Nb3Sn superconducting accelerator magnet based on self-consistent clustering analysis

The High Luminosity Large Hadron Collider (HL-LHC) is one of the large-scale projects currently being developed at the European Organization for Nuclear Research (CERN). Many Nb3Sn superconducting magnets are employed in this project, which exhibit complicated multiscale characteristics. The mechanical behaviors of magnets, which are directly related to the safety and stability of the overall structure, are affected by the electromagnetic–mechanical-thermal loading in actual service conditions. However, detailed modeling of magnets requires significant computing resources. A reduced-order method named self-consistent clustering analysis (SCA) has been proposed recently to reduce the cost. We have combined the SCA and finite element method (FEM) as an efficient multiscale framework, which is utilized to analyze the mechanical behaviors of accelerator magnets. Firstly, the computation results by presented framework are compared with direct numerical simulation (DNS) and existed experimental data to validate the accuracy and efficiency. Then, the FEM-SCA2 framework is utilized to study the mechanical behaviors of MQXFS accelerator magnet model under assembly, cool-down and excitation steps. The influences of material parameters, friction coefficient and stiffness degradation are also discussed. This paper provides an efficient multiscale analysis framework for investigating mechanical behavior in Nb3Sn accelerator magnet, which could be employed to study other magnets.

A methodology for the analysis of the three-dimensional mechanical behavior of a Nb3Sn superconducting accelerator magnet during a quench

The fast thermal and electromagnetic transients that occur in a superconducting magnet in case of a quench have the potential of generating large mechanical stresses both in the superconducting coils and in the magnet structure. While the investigation of such quench loads should generally be conducted to ensure a safe operation of the system, its importance is greatly enlarged in the case of high-field magnets based on strain sensitive superconductors. For these, a rigorous analysis of the magnet mechanics during a quench becomes critical. The scope of this work is hence to bring, for the first time, a detailed understanding of the three-dimensional mechanical behavior of a Nb3Sn accelerator magnet during a quench discharge. The study relies on the use of finite element models, where various multi-domain simulations are employed together to solve the coupled physics of the problem. Our analysis elaborates on the case study of the new MQXF quadrupole magnet, currently being developed for the high-luminosity upgrade of the LHC. Notably, we could find a very good agreement between the results of the simulation and experimental data from full-scale magnet tests. The validated model confirms the appearance of new peak stresses in the superconducting coils. An increase in the most relevant transverse coil stresses of 20–40 MPa with respect to the values after magnet cool-down has been found for the examined case.

Mechanical Tests, Analysis, and Validation of the Support Structure of the eRMC and RMM Magnets of the FCC R&D at CERN

The enhanced racetrack model coil and racetrack model magnet constitute the current R&D Nb3Sn magnets under development at CERN aiming to achieve dipole fields of 16-18 T, the design baseline of the Future Circular Collider (FCC). This article reports the mechanical behavior of their common support structure, which underwent three different levels of room-temperature preload (with the bladders and key method) and two cooldown cycles to 80 K. The structure was mounted using aluminum dummy blocks in lieu of the actual Nb3Sn coils. Strain gauges were placed in the external aluminum shell, tie-rods, and dummy coils. The measurements agree well with the results of the finite-element models (FEM). This work validates the FEM and a support structure for testing Nb3Sn accelerator magnets in the 16-T regime.

Conceptual Design of a HTS Dipole Insert Based on Bi2212 Rutherford Cable

The U.S. Magnet Development Program (US-MDP) is aimed at developing high-field accelerator magnets with magnetic fields beyond the limits of Nb3Sn technology. Recent progress with composite wires and Rutherford cables based on the first generation high-temperature superconductor Bi2Sr2CaCu2O8−x (Bi2212) allows considering them for this purpose. However, Bi2212 wires and cables are sensitive to transverse stresses and strains, which are large in high-field accelerator magnets. This requires magnet designs with stress management concepts to control azimuthal and radial strains in the coil windings and prevent the degradation of the current carrying capability of Bi2212 conductor or even its permanent damage. This paper describes a novel stress management approach, which was developed at Fermilab for high-field large-aperture Nb3Sn accelerator magnets, and is now being applied to high-field dipole inserts based on Bi2212 Rutherford cables. The insert conceptual design and main parameters, including the superconducting wire and cable, as well as the coil stress management structure, key technological steps and approaches, test configurations and their target parameters, are presented and discussed.

Heat Diffusion in High-Cp Nb3Sn Composite Superconducting Wires

A major focus of Nb3Sn accelerator magnets is on significantly reducing or eliminating their training. Demonstration of an approach to increase the Cp of Nb3Sn magnets using new materials and technologies is very important both for particle accelerators and light sources. It would improve thermal stability and lead to much shorter magnet training, with substantial savings in machines’ commissioning costs. Both Hypertech and Bruker-OST have attempted to introduce high-Cp elements in their wire design. This paper includes a description of these advanced wires, the finite element model of their heat diffusion properties as compared with the standard wires, and whenever available, a comparison between the minimum quench energy (MQE) calculated by the model and actual MQE measurements on wires.

Design of a High Toughness Epoxy for Superconducting Magnets and Its Key Properties

Author(s): Yin, S; Swanson, J; Shen, T | Abstract: Nb3Sn accelerator magnets are poised to enable the luminosity upgrade of the Large Hadron Collider (LHC) at CERN, improving its potential for exploring physics beyond the standard model of particle physics. The prototype Nb3Sn magnets consistently need 10-25 quenches to achieve their best performance. A hypothesis is that the long training of these magnets may at least be partially induced by epoxy cracking and bonding failures. In 2018, we showed that several existing epoxies have a higher toughness and less tendency to crack at low temperatures than CTD-101K, the epoxy resin with which almost all Nb3Sn accelerator magnets have been impregnated. Here we explore a new high toughness formulation for reducing quench training of Nb3Sn accelerator magnets, through combining two amine curing agents to achieve a good glass transition temperature $(T_{g})$, adding a viscosity reducer to achieve low viscosity, and experimenting a coupling agent for improved bonding strengths. We report results of comprehensive materials tests, including thermal shock, tensile, compressive, shear tests, viscosity and $T_{g}$ tests.

Modeling of Quench Protection Concepts for Canted-Cosine-Theta Type High-Field Magnets

An innovative high-field superconducting magnet of Canted-Cosine-Theta (CCT) type has been proposed for Future Circular Collider 16 T dipole magnet design. The unique mechanical structure intercepts the accumulated forces lowering the stress on the windings, constituting intrinsic stress management in high-field Nb 3 Sn accelerator magnets. However, this structure also constitutes a barrier for heat to quickly propagate in case of a quench. To succeed in the CCT-type magnet design and construction, quench protection is a challenging task that requires a detailed investigation of the electrothermal behavior of the magnet. In this paper, the protectability of a two-layer short model CD1 (Canted Dipole) built at PSI is studied using multiphysics simulations. Two protection methods are considered: energy extraction and coupling-loss induced quench. The 2D User-Defined Elements (UDEs) developed at Lawrence Berkeley National Laboratory in ANSYS Parametric Design Language, which support the multi-dependence material properties and include the effect of inter-filament coupling currents, are adapted and used in the coupled electrothermal, electrodynamic and electrical circuits calculations. A first-of-a-kind CCT-type magnet protection study using UDEs is presented. The generic model method will be validated through CD1 cold tests. Furthermore, these studies will prepare the ground for four-layer CCT protection concepts for FCC.

Effect of the fabrication route on the phase and volume changes during the reaction heat treatment of Nb3Sn superconducting wires

Accelerator magnets that can reach magnetic fields well beyond the Nb-Ti performance limits are presently being built and developed, using Nb3Sn superconductors. This technology requires reaction heat treatment (RHT) of the magnet coils, during which Nb3Sn is formed from its ductile precursor materials (a “wind and react” approach). The Nb3Sn microstructure and microchemistry are strongly influenced by the conductor fabrication route, and by the phase changes during RHT. By combining in situ differential scanning calorimetry, high energy synchrotron x-ray diffraction, and micro-tomography experiments, we have acquired a unique data set that describes in great detail the phase and microstructure changes that take place during the processing of restacked rod process (RRP), powder-in-tube (PIT), and internal tin (IT) Nb3Sn wires. At temperatures below 450 °C the phase evolutions in the three wire types are similar, with respectively solid state interdiffusion of Cu and Sn, Cu6Sn5 formation, and Cu6Sn5 peritectic transformation. Distinct differences in phase evolutions in the wires are found when temperatures exceed 450 °C. The volume changes of the conductor during RHT are a difficulty in the production of Nb3Sn accelerator magnets. We compare the wire diameter changes measured in situ by dilatometry with the phase and void volume evolution of the three types of Nb3Sn wire. Unlike the Nb3Sn wire length changes, the wire diameter evolution is characteristic for each Nb3Sn wire type. The strongest volume increase, of about 5%, is observed in the RRP wire, where the main diameter increase occurs above 600 °C upon Nb3Sn formation.

A cable-scale experiment to explore new materials for optimizing superconductor accelerator magnets

A previously commissioned system for exploring critical current pressure sensitivity in superconducting cables, the Transverse Pressure Insert, was modified to examine signals resulting from strand motion and insulation/impregnation cracking in a fully-excited Nb3Sn single strand embedded in an insulation wrapped and impregnated Cu dummy Rutherford cable stack. Measurements were performed in LHe with cyclic transverse pressures up to 68.5 MPa and transverse applied fields up to 13 T. Pressure was applied and removed to simulate stresses seen during magnet operation. Voltage and acoustic signal traces from a CTD-101 K impregnated cable stack were compared to those of a sample impregnated with NHMFL Mix–61. The experiments in this document were in the pursuit of developing a smaller scale, lower cost, and shorter turn around time experiment for exploring materials and processes to decrease training in next-generation Nb3Sn accelerator magnets.

Epoxy Resins for Vacuum Impregnating Superconducting Magnets: A Review and Tests of Key Properties

Epoxy impregnated superconducting magnets often have a long quench training and an uncertain performance. This is particularly an issue for Nb3Sn accelerator magnets planned for future high-energy proton colliders, which need thousands of such magnets. This paper reviews epoxies used in major Nb3Sn superconducting magnet systems, sorts them into three groups, and examines differences of their resistances to cracking when subjected to repeated thermal shocks and other key properties. The first group, including CTD-101K using which nearly all Nb3Sn accelerator magnets have been potted, is found to be brittle and prone to cracking. A second epoxy group is shown to have a much higher toughness. Their mechanical properties, potting characteristics, and glass transition temperatures were measured, compared, and discussed in terms of the potential uses of the second epoxy group for impregnating Nb3Sn accelerator magnets with a minimized training.

Analysis of Nb3Sn Accelerator Magnet Training

Nb 3 Sn accelerator magnet technology has made significant progress during the past decades. For the first time, it is planned to be used in a real accelerator. A relatively small number of Nb 3 Sn quadrupoles and dipoles will be installed in the Large Hadron Collider (LHC) to increase machine luminosity. Although it will prove the possibility of using Nb 3 Sn magnets in real machines, many questions of scaling this technology up remain. One of them is related to slow training of Nb 3 Sn magnets compared to the traditional Nb-Ti accelerator magnets. Since the goal is to operate thousands of Nb 3 Sn magnets in a future post-LHC accelerator, the slow training will affect both the practical design margin and the nominal operation field. Consequently, the cost of the project to reach the design field level is also increased. To improve our understanding of slow magnet training the existing Fermilab data from Nb 3 Sn magnet tests were reanalyzed. A summary of coil training features and correlations with fabrication parameters observed is presented in this paper.

Tripled critical current in racetrack coils made of Bi-2212 Rutherford cables with overpressure processing and leakage control

We fabricated three racetrack coils (RC1, RC2, and RC3) from Bi-2212 Rutherford cables (17-strand, thickness × width = 1.44 mm × 7.8 mm, strand diameter = 0.8 mm) and applied overpressure processing heat treatment (OPHT). The quench currents of RC1 and RC2 reached 5268 A and 5781 A, respectively, despite them still, surprisingly, exhibiting some Bi-2212 leakage to the surface. After removing most of the leakages using a simple-to-implement insulation scheme, the quench current of RC3 improved to 6485 A, which is about three times the average quench current of a dozen racetrack coils that had been fabricated and reacted using the conventional 1 bar heat treatment. The results confirm the effectiveness of the OPHT technology and the new leakage control scheme for coils made from Bi-2212 Rutherford cables. Coils exhibited an increased quench current with increasing the current ramp rate from 5 to 200 A s−1; they were quite stable against point and transient disturbances, and were capable of adsorbing persistent Joule heating at ∼80 mW for >15 s before quenching. These behaviors are different from Nb–Ti and Nb3Sn accelerator magnets. Overall, our results provide a critical evaluation and verification of Bi-2212 wire and magnet technologies (wire, insulation, heat treatment, coil fabrication, and coil operation), reveal crucial new stability features of Bi-2212 magnets, and demonstrate technological options for it to become a practical high-field magnet technology.

“Block-Type” Coils Fabrication Procedure for the Nb3Sn Dipole Magnet FRESCA2

The Nb3Sn FRESCA2 dipole magnet is dedicated to upgrade the CERN cable test facility FRESCA. It is also a technological demonstrator of large-aperture Nb3Sn accelerator magnet. It has an aperture of 100 mm and a target bore magnetic field of 13 T. It is composed of four 1.5-m-long double-pancake “block-type” coils, manufactured following the wind, react and impregnation technique. It is developed by CEA and CERN in the framework of a collaboration agreement, in the continuity of the EuCARD program. Through the fabrication of two full-scale copper prototypes, the different steps of the coil fabrication process (winding, heat treatment, splicing, instrumentation, impregnation, and transport) and the corresponding tooling have been adjusted. The final winding and reaction procedure integrates the possibility to open longitudinal gaps in the winding table and in the central post, in order to accommodate partially the longitudinal contraction of the cable during reaction. This new feature has been experienced through full-scale tests using superconducting cable with a reduced number of turns. This paper reports on the fabrication procedure of these coils. The production phase of the superconducting magnets has started in June 2015.

15 Years of R&D on high field accelerator magnets at FNAL

The High Field Magnet (HFM) Program at Fermi National Accelerator Laboratory (FNAL) has been developing Nb3Sn superconducting magnets, materials and technologies for present and future particle accelerators since the late 1990s. This paper summarizes the main results of the Nb3Sn accelerator magnet and superconductor R&D at FNAL and outlines the Program next steps.

Progress with High-Field Superconducting Magnets for High-Energy Colliders

One of the possible next steps for high-energy physics research relies on a high-energy hadron or muon collider. The energy of a circular collider is limited by the strength of bending dipoles, and its maximum luminosity is determined by the strength of final focus quadrupoles. For this reason, the high-energy physics and accelerator communities have shown much interest in higher-field and higher-gradient superconducting accelerator magnets. The maximum field of NbTi magnets used in all present high-energy machines, including the LHC, is limited to ∼10 T at 1.9 K. Fields above 10 T became possible with the use of Nb3Sn superconductors. Nb3Sn accelerator magnets can provide operating fields up to ∼15 T and can significantly increase the coil temperature margin. Accelerator magnets with operating fields above 15 T require high-temperature superconductors. This review discusses the status and main results of Nb3Sn accelerator magnet research and development and work toward 20-T magnets.

Protecting a Full-Scale <inline-formula> <tex-math notation="TeX">$\hbox{Nb}_{3}\hbox{Sn}$</tex-math></inline-formula> Magnet With CLIQ, the New Coupling-Loss-Induced Quench System

A new protection system for superconducting magnets called coupling-loss induced quench system (CLIQ) has been recently developed at CERN. Recent tests on Nb-Ti coils have shown that CLIQ is a valid, efficient, and promising method for the protection of high-magnetic-field superconducting magnets. However, the protection of new-generation Nb3Sn accelerator magnets is even more challenging due to the much higher stored energy per unit volume and to the significantly larger enthalpy needed to initiate and propagate a normal zone in such coils. Now, the CLIQ system is tested for the first time on a Nb3Sn magnet in the CERN magnet test facility in order to investigate its performance in practice, thereby validating the method for this type of superconducting magnets as well. Furthermore, we successfully reproduced the electrothermal transients during a CLIQ discharge. Finally, the implementation of various CLIQ-based protection schemes for the full-scale Nb3Sn quadrupole magnet for the LHC high luminosity upgrade is discussed. The impact of key system parameters on CLIQ performance and the advantages and drawbacks of using multiple CLIQ units on a single magnet are discussed.

Advanced Accelerator Magnets for Upgrading the LHC

The Large Hadron Collider is working at about half its design value, limited by the defective splices of the magnet interconnections. While the full energy will be attained after the splice consolidation in 2014, CERN is preparing a plan for a Luminosity upgrade (High Luminosity LHC) around 2020 and has launched a pre-study for exploring an Energy upgrade (High Energy LHC) around 2030. Both upgrades strongly rely on advanced accelerator magnet technology, requiring dipoles and quadrupoles of accelerator quality and operating fields in the 11-13 T range for the luminosity upgrade and 16-20 T range for the energy upgrade. The paper will review the last ten year of Nb3Sn accelerator magnet R&D and compare it to the needs of the upgrades and will critically assess the results of the Nb3Sn and HTS technology and the planned R&D programs also based on the inputs of first year of LHC operation.

Quench Margin Measurement in ${\rm Nb}_{3}{\rm Sn}$ Quadrupole Magnet

One of the possible practical applications of the Nb3Sn accelerator magnets is the LHC luminosity upgrade that involves replacing the present NbTi focusing quadrupoles in two high-luminosity interaction regions (IR). The IR magnets are exposed to strong radiation from the interaction point that requires a detailed investigation of the magnet operating margins under the expected radiation-induced heat depositions. This paper presents the results of simulation and measurement of quench limits and temperature margins for a Nb3Sn model magnet using a special midplane strip heater.

Design Study of 13–16 T<tex>$rm Nb_3rm Sn$</tex>Accelerator Dipole Based on Shell-Type Coils

This paper presents results of design studies of high-field dipole magnets based on Nb3Sn shell-type coils performed to explore the possibilities and limitations of Nb3Sn accelerator magnets. To achieve the maximum bore field of 13-16 T, 3- and 4-layer coils were used. The coil design concepts, conductor parameters, results of magnetic and mechanical analysis are presented and discussed. Different conductor grading techniques are described and compared. An innovative design of a 3-layer coil without interlayer splices is described