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.
Stainless steel vessels see widespread use in superconducting magnets for particle accelerator applications. Their function varies in different magnet designs: they always provide the necessary ...liquid helium containment, but in some cases are also used to provide azimuthal prestress and can also be welded to the magnet end plate to provide additional longitudinal stiffness. A magnet designed with the bladder and key technology does not rely on the structural role of the vessel. They are structurally supported using azimuthally prestressed aluminum shells, and the longitudinal constraint by rods. In this case, the magnet designer would generally like to minimize the interaction between the magnet and the stainless-steel vessel and to minimize the coil stress variation due to the vessel. The stress state in the vessel and in the coil is a function of the circumferential interference, defined as the vessel azimuthal length minus the magnet circumference. The vessel and the magnet azimuthal length machining tolerances are relatively large resulting in significant stress variations in the superconducting coils. In this paper we introduce an interference-control shim, which can significantly limit the stress variation of the coils for a given variation of the interference. The effectiveness of the interference-control shim is evaluated numerically on the MQXF, the low-<inline-formula><tex-math notation="LaTeX">\beta</tex-math></inline-formula> quadrupole for the High Luminosity LHC.
Towards 20 T Hybrid Accelerator Dipole Magnets Ferracin, P.; Ambrosio, G.; Arbelaez, D. ...
IEEE transactions on applied superconductivity,
09/2022, Letnik:
32, Številka:
6
Journal Article
Recenzirano
Odprti dostop
The most effective way to achieve very high collision energies in a circular particle accelerator is to maximize the field strength of the main bending dipoles. In dipole magnets using Nb-Ti ...superconductor the practical field limit is considered to be 8-9 T. When Nb 3 Sn superconductor material is utilized, a field level of 15-16 T can be achieved. To further push the magnetic field beyond the Nb 3 Sn limits, High Temperature Superconductors (HTS) need to be considered in the magnet design. The most promising HTS materials for particle accelerator magnets are Bi2212 and REBCO. However, their outstanding performance comes with a significantly higher cost. Therefore, an economically viable option towards 20 T dipole magnets could consist in an "hybrid" solution, where both HTS and Nb 3 Sn materials are used. We discuss in this paper preliminary conceptual designs of various 20 T hybrid magnet concepts. After the definition of the overall design criteria, the coil dimensions and parameters are investigated with finite element models based on simple sector coils. Preliminary 2D cross-section computation results are then presented and three main layouts compared: cos-theta, block, and common-coil. Both traditional designs and more advanced stress-management options are considered.
A test facility dipole is being developed at LBNL, targeting a 16 T field in a 144 mm wide aperture. The magnet uses a block design, with two double-pancake coils. In order to minimize motion under ...the large Lorentz forces, the coils are preloaded against a thick aluminum shell and iron yoke using bladder and key technology. It is then crucial to verify that the performance of the magnet is not degraded due to strain induced on the Nb 3 Sn conductor during assembly, cool-down and powering. The critical current of extracted strands was measured in a varying background magnetic field and as a function of the applied longitudinal strain. Finite element analysis was used to extract the strain state inside the superconducting strands during magnet assembly and operation. This strain was then compared to the measurements to evaluate potential reversible and irreversible effects on the magnet performances. The results suggest that the magnet can reach 16 T with sufficient margin, with no irreversible degradation in the high field region.
The Nb<inline-formula><tex-math notation="LaTeX">_{3}</tex-math></inline-formula>Sn based test facility dipole magnet (TFD), with a rectangular aperture of 100 × 150 mm, and an operation target bore ...field of 15 T at 1.9 K, is designed to provide a background dipole field for cables and inserts. The design of the magnet is based on four double-layer coils and an aluminum shell-based structure, using key-and-bladder technology, with axial pre-load. The status of the magnet design, and optimization analyses, are here presented. The results of the initial prototyping, initial winding tests, characterization of cable hard-way bend curvature, and layer jump prototype test are also discussed.
High-performance Nb3Sn superconducting wires have become one of the key technologies for the development of next generation accelerator magnets. While their large critical current densities enable ...the design of compact accelerator-quality magnets for their operation above 10 T, the noticeable reduction of the conductor performance due to mechanical strain appears as a new essential characteristic in magnet design. In this work, we extensively investigate the effect of transverse loads, up to 250 MPa, in state-of-the-art Nb3Sn Restacked-Rod-Process round superconducting wires. The tests are performed using a compressive Walters spring device, where the force is applied to the resin-impregnated wire, and the critical current is measured under magnetic fields ranging from 16 to 19 T. As a complement, critical current measurements under axial strain are also performed using a standard Walters spring. Interestingly, the study shows that the wire's electro-mechanical response under transverse stress depends on the initial axial strain condition. Nonetheless, when the main direction load becomes predominant, all tested wires converge to a common behavior. This observation allowed us to combine the results from critical current measurements under the loads exerted in both directions (axial and transverse), shedding some new light on the mechanisms behind critical current degradation.
Within the U.S. Magnet Development Program, LBNL is planning to fabricate and test hybrid magnets combining Nb 3 Sn and Bi-2212 canted-cosine theta (CCT) magnets. The first planned hybrid magnet test ...integrates a 1 m long, 90 mm bore Nb 3 Sn dipole magnet (CCT5), and a 39 cm long, 30.8 mm bore Bi-2212 dipole magnet (BIN5c), which are already fabricated and were tested individually. The second planned hybrid magnet test integrates a 1.5 m long, 120 mm bore Nb 3 Sn dipole magnet (CCT6), still under design, and a 1 m long, 40 mm bore Bi-2212 dipole magnet (BiCCT1), which is under fabrication. This work gives an update of the status of the Bi-2212 CCT magnet program, and focuses on the first hybrid magnet (CCT5/BIN5c). The electromagnetic and mechanical analysis of the CCT5/BIN5c hybrid magnet is presented, as well as the proposed mechanical assembly process. The quench protection strategy associated with the hybrid magnet test is also presented.
This paper shows the influence of the local magnetic forces on the mechanical analysis of a superconducting dipole magnet employing Nb3Sn technology. On high field magnets, the iron yoke has several ...functions: (1) magnetically, it increases the magnetic field in the bore and it has a shielding function; (2) mechanically, iron helps on the coil pre-load, transferring the mechanical forces provided by the external support structure. In many Nb3Sn magnets for particle accelerators, the cable is wound around an iron piece. As a consequence, the magnetic force on the iron can play an important role on the coil stress. The magnetic forces on the iron components can be computed using several different methods and the goal of this paper is to compare three expressions for the Maxwell stress tensor to compute local magnetic forces on iron considering a B-H curve. The influence of these forces on the mechanical behavior of the magnets is then analyzed, taking into account the uncertainties in determining their correct distribution. To achieve this goal, a numerical tool to perform Finite Element Analysis (FEA) was developed using vector potential formulation with first order nodal elements. The solution obtained from the magneto-static problem was used to calculate and compare the magnetic force distribution. The contributions from the Lorentz forces on coils are also taken into account in the mechanical analysis and are compared to the ones computed by using the Maxwell stress tensor on coils.
New high field and large-aperture quadrupole magnets for the low-beta inner triplets (Q1, Q2, Q3) have been built and tested as part of the high-luminosity upgrade of the Large Hadron Collider ...(HL-LHC). These new quadrupole magnets are based on Nb3Sn superconducting technology. The US Accelerator Upgrade Project (US-AUP) is producing the Q1 and Q3 Cryo-Assemblies: a pair of ∼5 m long magnet structures installed in a stainless-steel helium vessel (Cold Mass) and surrounded by cryostat shields, piping, and a vacuum vessel. This paper gives an overview of the design, production, and the results of the horizontal test of the first pre-series Q1/Q3 Cryo-Assembly.
The U.S. High-Luminosity LHC Accelerator Upgrade Project (HL-LHC AUP) has, in the recent years, developed assembly specifications for the 4.5 m long MQXFA magnets, which are 150 mm aperture ...high-field Nb 3 Sn low-β quadrupole magnets that are being built for the CERN Hi-Luminosity LHC (HL-LHC) upgrade. While the specifications were based on lessons learned from the LHC Accelerator Research Program (LARP) effort and the MQXFS and MQXFA prototype magnets, the experience gained from having both MQXFA07 and MQXFA08 magnets not meeting performance specifications during cold testing actually catalyzed a better understanding of the impact of the target assembly specifications and a subsequent refinement of the same. This paper summarizes a body of assembly data from the Pre-Series (MQXFA03-MQXFA07) and Series magnets (MQXFA08-MQXFA11) that have been built to date, and discusses the processes employed to successfully face the challenge of ensuring that the assembly specifications are met for the duration of the project.
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