Accelerator Magnets Research Articles

Image Analysis Capabilities and Methodologies of Nb3Sn Rutherford Cables

The unique Rutherford cabling facility at the Berkeley Center for Magnet Technology, LBNL, has been leading the production of a range of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn cables for a variety of magnet projects domestically and internationally. Cable fabrication is a critical step in accelerator magnet production: to ensure tight dimensional control of the multi-turn coils, fine tolerances must be kept on the cable width, thickness, keystone angle [1] with minimal degradation to the conductor. In addition to these dimensional measurements, we implemented an in-line image acquisition system that can monitor for critical defects such as cross-overs, as well as track the extent of strand deformation at the cable edges. At LBNL, we have collected a large dataset to establish the association between the cable edge facet dimension and the cross-sectional subelement damages. We thus can use image analysis on the facet as an integral and critical quick turn-around-time quality control monitor. Although such measurements are not direct and require a baseline for a given conductor and cable geometry combination, they are non-destructive. They can be measured across the entire length of the cable, which contrasts with critical current or residual resistance ratio measurements, which are only made at the point and tail of the cable and require a lengthy heat-treatment. Moreover, the high frequency of image acquisition coupled with Fast Fourier Transform analysis can give us insights into the key components of the cabling machine as they tend to produce periodic variations within the cable. Such analysis can help find potential faults and inform maintenance planning. This work describes our setup of in-line imaging of Rutherford cables during manufacture, the subsequent image analysis, and data processing. Several case studies illustrate typical usage of the system and lessons learned.

Design Optimization, Cabling and Stability of Large-Diameter High Jc Nb3Sn Wires

In the framework of the High Field Magnets (HFM) program, CERN is developing and qualifying Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn Rutherford cables to support magnet development towards the requirements of a future energy-frontier collider, using both state-of-the-art commercial wires and experimental wires under development with industrial partners. The trend towards higher current density and larger diameter wires imposes challenges for magneto-thermal stability. In this study, rolling trials and Rutherford cabling have been performed at CERN for two designs of a 1 mm diameter distributed tin Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wire produced by KAT, and for 1 mm and 1.1 mm diameter RRP Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wires procured from Bruker OST, and the self-field stability and cabling degradation have been analyzed. The 1 mm RRP wire shows significant degradation in <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c</i> and stability on cabling. Although the latter is not expected to impact the performance of research magnets, the potential of heat treatment optimization to improve stability has also been quantified. The distributed tin wire shows substantially poorer stability, but promising indications of low cabling degradation. The influence of wire design characteristics on cabling behavior and stability have been assessed, and the implications for future wire optimization towards high field accelerator magnet applications have been discussed.

FFT-Based Approach for the Mechanical Analysis of Superconducting Rutherford-Type Cables

Low-temperature superconductors are widely used in high field accelerator magnets, mostly within Rutherford type cables. We have proposed the “CoCaSCOPE” approach in order to build a numerical model of impregnated low-temperature Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconducting Rutherford-type cables dedicated to mechanical computation. Here the strand structure is described as a bi-metallic material: a copper core, a superconducting bundle region and an outer copper ring. In this paper, we explore the use of the distributed solver AMITEX_FFTP based on Fast Fourier Transform (FFT). The main interest of this method is to reduce the simulation time. This approach is investigated for structures with linear material behavior in this paper. This work compares the results from the FFT approach (using AMITEX) and from the 3D Finite Elements approach (using Cast3M). In this way, we can demonstrate the higher performances of FFT-based solving method compared to the finite elements method for multi-scale structure such as Rutherford-type cables in terms of computation time. With this approach, we can reduce the computation time or, alternatively, increase the size of our models in order to analyze the behavior of a macroscale model under mechanical solicitation.

Training Curves of Nb3Sn Rutherford Cables With a Wide Range of Impregnation Materials Measured in the BOX Facility

Training of accelerator magnets is a costly and time consuming process. The number of training quenches must therefore be reduced to a minimum. We investigate training of impregnated Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn Rutherford cable in a small-scale experiment named BOX (BOnding Experiment). The test involves a Rutherford cable impregnated in a meandering channel simulating the environment of a canted-cosine-theta (CCT) coil. The sample is powered using a transformer and the Lorentz force is generated by an externally applied magnetic field. The low material and helium consumption enable the test of a larger number of samples. In this article, we present training of samples impregnated with alumina-filled epoxy resins, a modified resin with paraffin-like mechanical properties, and a new tough resin in development at ETH Zürich. These new data are compared with previous results published earlier. Compared to samples with unfilled epoxy resin, those with alumina-filled epoxy show favorable training properties with higher initial quench currents and fewer training quenches before reaching 80% of the critical current.

Straight and Curved Canted Cosine Theta Superconducting Dipoles for Ion Therapy: Comparison Between Various Design Options and Technologies for Ramping Operation

Canted Cosine Theta layout for accelerator magnets is a very attractive since such magnets can be manufactured and assembled without big tooling, and with a relatively modest number of parts and tools. In the frame of European Horizon2020 funds, two collaborations, HITRI <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">plus</i> and I.FAST, are developing a CCT design, of 80 mm free bore, 4 T central dipole field, and 0.4 T/s ramp-rate. This magnet is expected to be the bending element of a gantry, to control the beam delivery in therapy with ions (hadrontherapy). The paper illustrates first a comparison between CCT and more classical cosine theta layout, followed by the comparison between Nb-Ti, Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn, MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and HTS tapes coils. Relevant requirement for the magnets of this study is to be operated at low current, to limit the heat generation, in sight of a liquid-free cooling system. The results of the comparison is then applied to the design of two magnet demonstrators. Both adopt a low-losses Nb-Ti rope, consistently with the need for keeping the heat generation as low as possible. The first is a straight combined function dipole-quadrupole, while the second is a curved CCT dipole. The paper concludes with the first manufacturing tests for the CCT formers, for which aluminium -bronze, stainless steel and charged PEEK polymer are being explored as basic material.

Conceptual Design of a 20 T Hybrid Cos-Theta Dipole Superconducting Magnet for Future High-Energy Particle Accelerators

High energy physics research will need more and more powerful circular accelerators in the next decades. It is therefore desirable to have dipole magnets able to produce the largest possible magnetic field, in order to keep the machine diameter within a reasonable size. A 20 T dipole is considered a desired achievement since it would allow the construction of an 80 km machine, able to circulate 100 TeV proton beams. In order to reach 20 T, a hybrid Low-Temperature Superconductor (LTS) - High-Temperature Superconductor (HTS) magnet is needed, since LTS technology is presently limited to ∼16 T for accelerator magnet applications. In this paper, we present the design of a 6 layers 20 T hybrid dipole magnet using Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn (LTS) and Bi2212 (HTS). We show that it is possible to achieve this magnetic field with accelerator field quality, with sufficient margin on a realistic conductor, keeping the stresses within safe limit, avoiding conductor degradation.

Conceptual Design of 20 T Hybrid Accelerator Dipole Magnets

Hybrid magnets are currently under consideration as an economically viable option towards 20 T dipole magnets for next generation of particle accelerators. In these magnets, High Temperature Superconducting (HTS) materials are used in the high field part of the coil with so-called insert coils, and Low Temperature Superconductors (LTS) like Nb3Sn and Nb-Ti superconductors are used in the lower field region with so-called outsert coils. The attractiveness of the hybrid option lays on the fact that, on the one hand, the 20 T field level is beyond the Nb3Sn practical limits of 15-16 T for accelerator magnets and can be achieved only via HTS materials; on the other hand, the high cost of HTS superconductors compared to LTS superconductors makes it advantageous exploring a hybrid approach, where the HTS portion of the coil is minimized. We present in this paper an overview of different design options aimed at generating 20 T field in a 50 mm clear aperture. The coil layouts investigated include the Cos-theta design (CT), with its variations to reduce the conductor peak stress, namely the Canted Cos-theta design (CCT) and the Stress Management Cos-theta design (SMCT), and, in addition, the Block-type design (BL) including a form of stress management and the Common-Coil design (CC). Results from a magnetic and mechanical analysis are discussed, with particular focus on the comparison between the different options regarding quantity of superconducting material, field quality, conductor peak stress, and quench protection.

A Metallurgical Inspection Method to Assess the Damage in Performance-Limiting Nb3Sn Accelerator Magnet Coils

The design and production of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn-based dipole and quadrupole magnets is critical for the realization of the High-Luminosity Large Hadron Collider (HL-LHC) at the European Organization for Nuclear Research (CERN). Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconducting coils are aimed at enhancing the bending and focusing strengths of accelerator magnets for HL-LHC and beyond. Due to the brittle nature of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn, the coil fabrication steps are very challenging and require very careful QA/QC. Flaws in the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn filaments may lead to quenches, and eventually, performance limitation below nominal during magnet testing. A novel inspection method, including advanced non-destructive and destructive techniques, was developed to explore the root-causes of quenches occurring in performance-limiting coils. The most relevant results obtained for MQXF coils through this innovative inspection method are presented. This approach allows for precise assessment of the physical events associated with the quenches experienced by magnet coils, mainly occurring in the form of damaged strands with transversely broken sub-elements. Coil-slice preparation, micro-optical observations of transverse and longitudinal cross-sections, and a deep etching technique of copper will be illustrated in the present work, with a focus on the results achieved for a CERN coil from a non-conforming quadrupole magnet prototype, and two coils fabricated in the US, in the framework of the Accelerator Upgrade Project (AUP) collaboration, from two different non-conforming quadrupole magnets, respectively. The results obtained through the proposed inspection method will be illustrated.

External Coil Coupled Loss Induced Quench (E-CLIQ) System for the Protection of LTS Magnets

The next generation of Nb <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{3}$</tex-math></inline-formula> Sn accelerator magnets are pushed to higher magnetic fields, higher stored energy and higher energy density. This brings additional challenges when it comes to quench protection. At CERN, a fast quench protection system is under development that introduces normal zones directly in the coil's conductor by inductive heating. This system is called External coil Coupled Loss Induced Quench (E-CLIQ). The E-CLIQ device comprises many small copper coils that are strategically placed near the main coils. Once a quench is detected, a capacitor is discharged in to the E-CLIQ coils, resulting in the characteristic damped oscillating current of an RLC circuit. This results in a large, but local applied dB/dt on the conductor of the main magnet system, which locally heats it by induced interfilament and interstrand coupling loss. Such a system has the advantage of being fast by introducing normal zones in potentially less than 10 ms, it doesn't rely on heat flow through the insulation layers of the coil, as is the case with traditional quench heaters, and it is not electrically connected to the main circuit. In this paper, the E-CLIQ system is introduced, several first demonstrator coils are presented and the results of a first proof-of-concept test are shown.

Design of a 4 T Curved Demonstrator Magnet for a Superconducting Ion Gantry

The Superconducting Ion Gantry (SIG) project is the contribution from INFN (the Italian National Institute for Nuclear Physics) to the international SIGRUM project with the aim of exploring new technological solutions for the critical elements of a 430 MeV/u carbon ion gantry. The project includes the design and construction of a cos <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\theta$</tex-math></inline-formula> 4 T superconducting dipole demonstrator magnet whose main scope is to prove the feasibility of winding and assembling an accelerator magnet type with a relatively small radius of curvature (1.65 m). In addition to the complexity due to the curvature, the target field ramp rate is 0.4 T/s and the cooling system must not adopt liquid helium. This paper discusses the design activities carried out in the last year on the electromagnetic and thermal domains and reports on the present concepts and infrastructure for the first winding trials.

An Open-Source Finite Element Quench Simulation Tool for Superconducting Magnets

An open-source Finite Element Quench Simulator (FiQuS) is being developed as part of the STEAM framework following CERN's open science policy [1]. The tool is based solely on open-source software and uses Python to generate geometries and meshes with Gmsh, and compute solutions with GetDP. FiQuS scripts have a modular structure in order to accommodate a broad range of geometries and simulation requirements with the main focus on superconducting accelerator magnets. The tool, at its advanced stage, will be capable of 1D, 2D, and 3D geometry generation of superconducting elements such as bus bars, multi-pole, solenoid, and canted-cos-theta (CCT) magnets. It already has the capability for parametrized mesh control and subsequent model generation of 2D multi-pole and 3D CCT magnets. It will be possible to perform either electromagnetic (EM), thermal (TH), or coupled EM-TH simulations for static or transient analysis. The focus is on aspects related to the powering and quench transients, enabling parametric analyses and co-simulations to support comprehensive quench protection studies. In this contribution, we lay the foundation of FiQuS by presenting its structure and three specific capabilities that represent the basis upon which the future modules will be built. These capabilities showcase the integration of FiQuS with software developed at CERN, at other national laboratories, and within the STEAM framework. The first capability is the integration with the STEAM framework, which enables cooperative simulations with the other tools of the framework, the automatization of multi-step studies, and the Single Source Of Truth (SSOT) practice through the centralization of model and material data. The second capability is the integration with the magnet design software ROXIE via a dedicated input files parser, which seamlessly integrates magnet design details around a single input file. Examples of magnetostatic solutions obtained in FiQuS are shown for the 2D cross-section of LHC magnets based on ROXIE input files. The third capability is the integration with the parametric analysis software Dakota developed by Sandia National Laboratories. An example is shown of multi-objective optimization of the model mesh to minimize the error of the computed magnetic field.

Investigation on nonuniform current density and shape deformation affecting the magnetic field performance of a saddle-shaped no-insulation HTS cosine–theta dipole magnet

High magnetic fields are desirable for discovering new particles in particle accelerators. Dipole magnets using superconductors have played a key role in creating the required field intensity and uniformity. In contrast, high temperature superconductor (HTS) dipole magnets have recently been spotlit because of their ability to generate higher magnetic fields compared to their low temperature superconductor counterpart. Similar needs have emerged in other fields using magnets, and no-insulation (NI) technology is considered a feasible option to reach high magnetic fields by overcoming the disadvantages of HTS magnets. However, research has rarely been carried out on the utilization of NI HTS magnet technology for dipole magnets in high-field accelerators. Here we show the design, fabrication, and test results of an NI HTS dipole magnet with numerical analysis results. This paper aims to investigate the effect of nonuniform current density and undesirable shape deformation on the magnetic field distribution of a saddle-shaped NI HTS dipole magnet. The magnet is designed and constructed considering the ‘constant perimeter winding’ technique and tested in liquid nitrogen. The field mapping process is also performed along a designated mapping trajectory to obtain the magnetic field distribution. A T-A formulation-based simulation model, named the ‘sequential simulation model,’ is suggested to reproduce the measurements and employed considering the current distribution and shape deformation. As a result of quantitative analysis of the transverse direction measurements, the magnetic field error decreased by 0.02 percent point (pp) when the nonuniform current density is considered. It decreased by 0.13 pp when the shape deformation is considered. Moreover, the critical current calculated through an additional numerical analysis shows an error of up to 10%. In conclusion, the saddle-shaped NI HTS dipole magnet can produce a sufficient magnetic field level for particle accelerator research, even though the field distribution shows a uniformity of 0.37% within this study.

Single Stretch Wire and vibrating wire measurement system for characterization of multipole accelerator magnets

A stretch wire magnetometer is a versatile instrument and can be treated as the primary standard of magnetic qualification of accelerator magnets. Bhabha Atomic Research Centre has developed a Stretch wire magnetic measurement system to cater to the requirements of magnetic measurements on accelerator magnets. This bench also includes a vibrating wire system. The scheme used in this instrument for measurements of magnetic flux ensures drift compensations after each scan. This feature along with post-signal processing gives an accuracy of homogeneity measurements comparable to that of a rotating coil magnetometer. Unlike conventional stretch wire systems, wire moves in the r-z plane (along r-axis) at different azimuthal angles. This provides the opportunity for drift compensation at each scan. This has immensely enhanced the measurement accuracy of non-homogeneity of integral fields to less than 3 units. The same is cross-verified with a rotating coil system. The comparison between rotating coil and stretch wire is presented. The Paper gives the step-wise procedure for the measurement of key performance indicators of an accelerator magnet. This paper brings out salient features of the system along with design and calibration results.

The effect of ground motion on the LHC and HL-LHC beam orbit

The High Luminosity Large Hadron Collider (HL-LHC) will require unprecedented orbit stability at the low-beta experiments, ATLAS and CMS. Because the effect of seismic noise might become a relevant source of luminosity loss, several studies have been conducted to characterise the actual ground motion in the LHC tunnel. In preparation for the official groundbreaking of the civil engineering work for the high luminosity upgrade, that started on 15 June 2018 in parallel to LHC beam operation, seismic sensors were installed to permanently monitor the ground stability close to these experiments. Compactor work and shaft excavation were expected to induce vibrations in the accelerator magnets that would cause orbit disturbance, beam loss and potentially premature beam dumps. This paper summarises the observations made on the LHC beams and uses this data to benchmark estimates of the impact of low frequency vibrations on the closed orbit at the main interaction points and collimators. The impact of ground motion on HL-LHC operation is then estimated using the expected operational scenario and the estimated mechanical stability of the new HL-LHC triplet magnets that is based on recent measurements of mechanical magnet transfer functions.

Significant reduction in the low-field magnetization of Nb3Sn superconducting strands using the internal oxidation APC approach

Nb3Sn superconductors are promising for building accelerator magnets for future energy-frontier circular colliders. A critical factor for this application is the low-field persistent-current magnetization because it leads to several critical issues: e.g. low-field instability (including flux jumps), hysteresis loss, and field errors in magnet bores. Suppression of low-field magnetization requires reduction of low-field critical current density (J c) or effective subelement size (d eff). However, reduction of d eff of state-of-the-art Nb3Sn conductors—the restacked-rod-process (RRP®) type—below 40–50 μm without a pronounced decrease in high-field J c is difficult. On the other hand, the internal oxidation method which forms artificial pinning centers (APC) in Nb3Sn offers an alternative approach to reducing the low-field magnetization. Compared with a conventional Nb3Sn conductor whose flux pinning force versus field (F p–B) curve peaks at ∼20% of its irreversibility field (B irr), the F p–B curve peaks of APC conductors shift to higher fields due to the point pinning effect, leading to flattening of the J c–B curves. The goal of this paper is to quantitatively study how much the APC approach can reduce the low-field magnetization. We measured the J c–B curves of an RRP® conductor and two APC conductors (reacted at 700 °C) from zero field to B irr using a high-field vibrating sample magnetometer. The results showed that the APC conductors have higher non-Cu J c at high fields (e.g. 32%–41% higher at 16 T) and simultaneously lower non-Cu J c at low fields (e.g. 28%–34% lower at 1 T) compared with the RRP®. This effect is due to a competition between their Nb3Sn layer fraction ratios and layer F p ratios. Suppose they reach the same 16 T non-Cu J c, then the 1 T non-Cu J c and magnetization of the APC conductors are only half or even less compared with the RRP® conductor.

Investigating the magnetic field outside small accelerator magnet analogs via experiment, simulation, and theory

Particle accelerators use powerful and complex magnetic fields to turn, shape, and eventually collide beams of near-light-speed particles, yet the fundamental magnetic principles behind the accelerator magnets can be understood by undergraduate students. In this paper, we use small-scale accelerator magnet analogs in a multi-faceted, low-cost exploration of the magnetic field exterior to accelerator magnets. These fields are best understood using the multipole expansion of the field. If we assume that the magnetic field is created by ideal magnetic dipoles, we can derive a theoretical model that shows that each accelerator magnet configuration is dominated by a single multipole moment and obeys B∝1/rl+2, where l is the multipole order (with l=1,2,3, and 4 for the dipole, quadrupole, octopole, and hexadecapole moments, respectively). Using commercially available NdFeB magnets and the magnetic field sensor inside a smartphone, we experimentally verify the power-law dependence of the accelerator magnet configurations. Finally, we use the open-source Python library Magpylib to simulate the magnetic field of the permanent magnet configurations, showing good agreement among theory, experiment, and simulation.

Degradation of I c due to residual stress in high-performance Nb3Sn wires submitted to compressive transverse force

Future particle colliders in search for new physics at the energy frontier require the development of accelerator magnets capable of producing fields well beyond those attainable with Nb-Ti. As the next generation of high-field accelerator magnets is presently planned to be based on Nb3Sn, it becomes crucial to establish precisely the mechanical limits at which this brittle and strain sensitive superconductor can operate safely. This paper reports on the stress dependence and the permanent reduction of the critical current under transverse compressive loads up to 240 MPa in state-of-the-art restacked-rod-process (RRP®) and powder-in-tube Nb3Sn wires. Single-wire experiments were performed at 4.2 K in magnetic fields ranging between 16 T and 19 T on resin-impregnated samples to imitate the operating conditions of a wire in the Rutherford cable of an accelerator magnet. Depending on the wire technology, we measured irreversible stress limit values—defined as the transverse stress value, leading to a permanent reduction in the critical current of 5%, assessed by convention at 19 T—ranging between 110 MPa and 175 MPa. This permanent reduction of the critical current after mechanical unload can occur for two reasons, which can be concomitant: the plastic deformation of the Cu matrix that produces residual stresses on the Nb3Sn lattice and the formation of cracks. We developed a method to identify the dominant degradation mechanism in our experiments that allowed us to predict the fraction of critical current lost due to residual stresses. Interestingly, we found that in the RRP® wires the measured reduction of Ic after unload from stresses as high as 240 MPa can be fully ascribed to residual stresses. An independent confirmation of this conclusion coming from a study combining x-ray tomography and deep learning Convolutional Neural Networks is also reported.

Development of superconducting model dipole magnets beyond 12 T with a combined common-coil configuration

The superconducting magnet group at IHEP (the Institute of High Energy Physics, Chinese Academy of Sciences, China) has been engaging in the R&D of high field accelerator magnets since 2014 for the pre-study of the next-generation high energy particle accelerators like CEPC-SPPC (Circular Electron Positron Collider-Super Proton Proton Collider), FCC (Future Circular Collider), etc. A superconducting model dipole magnet named LPF1 was first fabricated with NbTi and Nb3Sn in a combined common-coil configuration in 2017 and tested in 2018, a main field of 10.23 T was reached within two 10 mm apertures. The magnet was reassembled with higher pre-stress in 2019 to investigate the influence of the pre-stress loading level on the performance of common-coil dipole magnets. The main field of this upgraded magnet named LPF1-S was increased to 10.71 T within two 12 mm apertures. By improving the key processing technologies in Rutherford cable fabrication, coil winding, Nb3Sn & NbTi splices soldering and coil impregnation, the recently fabricated model dipole magnet named LPF1-U attained a 12.47 T main field in two 14 mm apertures in 2021. To fully take advantage of Nb3Sn and NbTi superconductors in different field regions, LPF1-U was still designed and developed with a combined coil configuration with Nb3Sn for the inner two coils and NbTi for the outer four coils. Additionally, graded coil topology and different coil LSS (length of straight section) settings were adopted in the optimization of LPF1-U layouts. Bladder and key technology was used for the support structure, and fiber optic sensors based on FBG (fiber Bragg grating) were utilized to monitor the stress distribution and variation in the structures and coils through the whole process from magnet assembly to cool-down and excitation. During the performance test, a relatively large number of trainings were carried out and LPF1-U became more stable after the thermal cycle. A peak field of 12.7 T in the coils (12.47 T main field in apertures) corresponds to an operating current of 6865 A and a load line ratio of 90% was finally attained. It is worth mentioning that LPF1-U is the first hybrid—Nb3Sn, NbTi—common coil dipole magnet that has achieved a main field of more than 12 T in two apertures. The design, fabrication and performance analysis of the LPF series dipole magnets are presented in this paper.

Optimizing the use of pressurized bladders for the assembly of HL-LHC MQXFB magnets

The use of pressurized bladders for stress control of superconducting magnets was firstly proposed at Lawrence Berkeley National Laboratory in the early 2000s. Since then, the so-called ‘bladders and keys’ procedure has become one of the reference techniques for the assembly of high-field accelerator magnets and demonstrators. Exploiting the advantages of this method is today of critical importance for Nb3Sn-based accelerator magnets, whose production requires the preservation of tight stress targets in the superconducting coils to limit the effects of the strain sensitivity and brittleness of the conductor. The present manuscript reports on the results of an experimental campaign focused on the optimization of the ‘bladders and keys’ assembly process in the MQXFB quadrupoles. These 7.2 m long magnets shall be among the first Nb3Sn cryomagnets to be installed in a particle accelerator as a part of the High Luminosity upgrade of the LHC. One of the main practical implications of the bladders technique, especially important when applied to long magnets like MQXFB, is that to insert the loading keys, the opening of a certain clearance in the support structure is required. The procedure used so far for MQXF magnets involved an overstress in the coils during bladder inflation. The work presented here shows that such an overshoot can be eliminated thanks to additional bladders properly positioned in the structure. This optimized method was validated in a short model magnet and in a full-length mechanical model, becoming the new baseline for the series production at CERN Furthermore, the results are supported by numerical predictions using finite element models.

Uni-layer magnets: a new concept for LTS and HTS based superconducting magnets

A novel geometrical configuration to form a magnetic field perpendicular to an aperture, created by an asymmetric current distribution, within a single layer, and using a continuous ideal current line, named the uni-layer (UL) magnet, is here presented. The idea is compared to existing concepts in superconducting magnets, namely, the sector magnet, stress managed and canted . The UL magnet allows for a design with a continuous unit length (no layer jump), and an increased minimum bending radius of the conductor in relation to traditional and canted designs. The specific characteristics of the UL design are especially advantageous for strain-sensitive and prone to winding degradation high-temperature superconductors, in very high field accelerator magnet applications, in which, high efficiency in the use of conductor, and a small aperture are required. The advantages with regard to the design and fabrication of UL magnets in relation to other concepts are also discussed.