Coil Fabrication Research Articles

Power Tests of the First Nested Orbit Corrector Prototype for HL-LHC

The first prototype of the short orbit corrector for the upgrade of the LHC has been fabricated at CIEMAT, in collaboration with CERN, in the framework of the HL-LHC project. It consists of two nested dipoles, with an aperture of 150 mm and physical length of 1.5 m. A first power test was performed without the outer dipole coils, which were replaced by a support structure to align the iron yoke with the collared inner dipole. This test was aimed to validate the coil fabrication techniques, which are innovative for a NbTi Rutherford cable. In the power test of the full assembled magnet, individual training of both dipoles was fine to ultimate current. However, the magnet was not able to reach nominal torque in combined operation. Several power tests and re-assemblies were necessary to be able to power both dipoles till nominal current. This paper describes the test results and analysis of the measurements. The magnet is heavily instrumented with voltage taps, collars with strain gauges and bullet gauges at the axial pushers.

Manufacturing Status of CFETR Central Solenoid Model Coil

A central solenoid model coil (CSMC) is to verify the large-scalar superconducting coil manufacture technology for China Fusion Engineering Test Reactor (CFETR) in Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). The CSMC composed of Nb3Sn and NbTi hybrid superconducting magnet can reach to 12 T maximum magnetic field. The main manufacturing processes of the coil are developed and verified by R&Ds and mockup coil fabrication. For now, all the five coils winding and 2/3 of terminals fabricating are finished, and the heat treatment for the Nb3Sn inner coil is completed. The turn releasing and insulation wrapping process and station for the heat treated Nb3Sn coil has been developed. The major remaining processes for the five coils are ground insulation wrapping and vacuum pressure impregnation (VPI). The mockup coil VPI will be performed firstly to validate the process. This article introduces the main manufacturing progress of the CFETR CSMC.

Design and Analysis of NSTX-U Inner Poloidal Field Coils

NSTX-U is an innovative magnetic fusion device constructed at the Princeton Plasma Physics Laboratory (PPPL) and is an upgrade of the original NSTX device that operated successfully for more than ten years. During the initial operation of NSTX-U in 2016, one of the inner poloidal field (PF) coils experienced a turn fault that necessitated a complete shutdown. A precise cause could not be identified but several contributing factors became apparent. It has since been decided to replace all three inner PF upper and lower coil pairs. Prototype and replacement inner PF coils with an improved design and construction have developed that factor in findings from the root cause analysis of the failed coil. Detailed electromagnetic (EM) and transient thermal analyses were performed for both the energized state and the cool down cycle. This article summarizes the effort taken to replace inner PF coils including improvements in design, results of analysis, experience with prototype coils, and progress with the fabrication of replacement coils.

Manufacturing Error Analysis of Field Quality for the HL-LHC CCT Corrector

The High Luminosity LHC (HL-LHC) project, as an upgrade of the Large Hadron Collider (LHC), is to be started in the next decade. A set of canted cosine theta (CCT) twin aperture orbit corrector magnets will be developed in China, with a field quality requirement of 10−3 in the two apertures. Cooperating with CERN, twin aperture correctors are being fabricated as prototypes before the series production. During the fabrication of the CCT coils, certain manufacturing errors including winding error, rotating error and concentricity error appeared which will affect the field quality. This study analyzed the effects on field harmonic components by these manufacturing errors, and the manufacturing process and the first test result of the prototype were introduced briefly. Suggestions from the analysis results are provided for further series production of the 2.2-m orbit correctors.

Development of MgB<sub>2</sub> Rutherford Cable for Large-scale SMES Coils Cooled by Liquid Hydrogen

To realize the practical application of Superconducting Magnetic Energy Storage (SMES) systems cooled by liquid hydrogen, MgB2 is promising as a material for SMES coils in terms of reducing cost in the future and enabling high critical temperature above the temperature of liquid hydrogen. Major technological development issues to realize the application of SMES coils are fabrication of the Rutherford superconducting cable with a kA-class current capacity and fabricating the coils using both Wind and React (W&R) and React and Wind (R&W) methods. Since there is little data regarding the allowable bending strain of the MgB2 strand before heat treatment, an experiment was conducted and it was determined to be approximately 4 % before the fabrication of the cable. The Rutherford cable was designed and developed based on analysis of the space curve theory because the complex bending strain is exposed to the MgB2 filaments during the cable and coil fabrication processes. However, the critical currents measured using cables developed according to the design were degraded approximately 30%. This degradation can be explained as being due to local deformation of the strands caused by compression at the cross-over point between the strands of the conductor. The broken sections of the Nb barrier, which were observed by reviewing SEM images of the strand cross-section, can cause degradation of the critical current. Two double pancake coils, which were primarily wound using the Rutherford cables and manufactured applying both the R&W and W&R methods for the purpose of comparing fabrication processes, were connected in series to excite the coils. As a result, an operating current of 600 A was successfully achieved.

Magnetic Design and Fabrication of Warm-Iron Normal-Conducting Quadrupole Magnets for ReA3 HEBT Line at NSCL

The ReA3 reaccelerator facility at the National Superconducting Cyclotron Laboratory (NSCL), Michigan State University (MSU), East Lansing, MI, USA, provides unique access to high-quality beams of a wide variety of isotopes for scientific research. The high-energy beam transport (HEBT) line from the ReA3 superconducting linear accelerator to the three experimental end stations consists of several iron-dominated normal-conducting quadrupole magnets, dipole magnets, and also combined-function horizontal and vertical corrector magnets. This article focuses on the design and fabrication of a new type of normal-conducting quadrupole magnet with cylindrical split yoke structure and standard quadrupole coils. Electromagnetic simulation results demonstrate that the magnet design fulfills the performance requirements for the ReA3 HEBT beamline. We present the coil fabrication and mechanical assembly details of the magnet. Our analysis results show that the hydraulic requirements for the coils are met.

HTS Dipole Prototype Magnet for NSLS X13A Endstation

This article reports the design, three-dimensional magnetostatic simulation results, coil fabrication, assembly, and low-temperature test results of a C-frame, iron-dominated high-temperature superconductor dipole magnet for ultra high vacuum magnetic-sensitive spectroscopy techniques at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. The magnet is required to provide fields up to ~1 T along two spatial directions. With an added capability to rotate about its vertical axis of symmetry, the magnet system is capable of delivering field along all three principal axes. The combination of this magnet and fast-switched, elliptically polarized soft X-rays produced by the NSLS elliptically polarized wiggler forms the basis for an excellent tool for polarization-sensitive studies of magnetic materials. The prototype magnet consists of a set of pancake coils fabricated from BSCCO tape reinforced with stainless steel. Those pancake coils are clamped in a low-carbon AISI 1006 grade steel pole and yoke assembly. For testing purposes, a cold He gas environment provided cooling the superconducting magnet to subtransition temperatures; for operation in an ultra high vacuum endstation environment, cooling can be provided by an external cryocooler. The low-temperature tests establish the safe-operating parameters needed to achieve the target magnetic field values. The measured magnetic field values are in close agreement with the magnetic simulation results, thereby confirming that the magnet design fulfills the performance requirements. The final section of this article lists some suggestions to improve the magnet performance.

Identification of important error fields in stellarators using the Hessian matrix method

Error fields are predominantly attributed to inevitable coil imperfections. Controlling error fields during coil fabrication and assembly is crucial for stellarators. Excessively tight coil tolerance increases time and cost, and, in part, led to the cancellation of the National Compact Stellarator Experiment and delay of W7-X. In this paper, we improve the recently proposed Hessian matrix method to rapidly identify important coil deviations. Two of the most common figures of merit, magnetic island size and quasi-symmetry, are analytically differentiated over coil parameters. By extracting the eigenvectors of the Hessian matrix, we can directly identify sensitive coil deviations in the order of the eigenvalues. The new method is applied to the upcoming Chinese First Quasi-axisymmetric Stellarator configuration. Important perturbations that enlarge n/m = 4/11 islands and deteriorate quasi-axisymmetry of the magnetic field are successfully determined. The results suggest each modular coil should have separate tolerance and some certain perturbation combinations will produce significant error fields. By relaxing unnecessary coil tolerance, this method will hopefully lead to a substantial reduction in time and cost.

Conceptual design of Japan’s fusion DEMO reactor (JADEMO) and superconducting coil issues

Goals of Japan’s fusion demonstration (DEMO) reactor are to demonstrate (1) steady and stable electric power generation in a power plant scale, (2) self-sufficient production of fuel (tritium), and (3) reasonable availability using a remote maintenance scheme anticipated in a commercial plant. Main design parameters of JA DEMO are a plasma major radius of 8.5 m, fusion output of 1.5-2 GW, magnetic field on the plasma axis of 5.94 T. The superconducting coil system of the reactor consists of a central solenoid (CS), 7 poloidal field (PF) coils and 16 toroidal field (TF) coils. Regarding CS and PF coils, superconducting coil technology on DEMO is basically the same as that on the world largest fusion experimental reactor called ITER. In contrast, TF coils have a technology gap on magnetic energy and the resulting stress between ITER and DEMO due to their size and magnetic field. In particular, the necessity of higher design stress is critical for TF coils, requiring the development of high strength cryogenic steels surpassing the existing ones. The fundamental design strategy to mitigate tolerances in TF coil fabrication is also presented.

Effect of Crossover Turns in Double-Pancake Winding on Magnetic Field Homogeneity of HTS NMR Magnet

Field homogeneity of a superconducting magnet is a critical factor for nuclear magnetic resonance (NMR) spectrometers. For REBCO high-temperature superconducting (HTS) coil fabrication, layer-wound is considered a better option than double-pancake (DP) winding, in terms of field homogeneity. Nonetheless, DP winding is preferred to layer-wound owing to the ease of manufacturing and the difficulty in making long REBCO tapes. However, DP winding has an inevitable crossover turn between the top and bottom layers. For magnets with an HTS insert used in NMR spectrometers, the crossover turn could be critical for the field homogeneity. In this paper, the magnetic field generated by the crossover turn is derived based on the Biot-Savart law, and the effect of crossover turns in stacked DPs on the field homogeneity of an NMR-class HTS magnet is analyzed.

Engineering Design and Fabrication of the Nested Orbit Corrector Prototype for HL-LHC

MCBXF magnets are nested orbit correctors, needed for the upgrade of the large hadron collider (LHC), in the framework of the high luminosity (HL) LHC project. There are two versions with different physical lengths, 1.5 and 2.5 m, which share the same cross section to decrease fabrication costs. These magnets have a large aperture of 150 mm and due to the high radiation dose, a mechanical clamping is necessary to hold the large torque between both dipoles. Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas CIEMAT is developing the short MCBXFB prototype in collaboration with CERN. This paper describes the engineering design of the magnet, which is based on previous magnetic and mechanical calculations. The axial pre-stress on the coil ends is analyzed in detail. Some innovative techniques have been developed for the coil fabrication because of the high number of turns and large aperture.

Mechanical analysis of the Nb3Sn 11 T dipole short models for the High Luminosity Large Hadron Collider

For the Large Hadron Collider luminosity upgrade, two of the NbTi 8.3 T main bending dipoles will be replaced by two shorter Nb3Sn 11.2 T dipoles to create space for the installation of collimators in the dispersion suppression region. With the aim to verify the design features, several 2 m long 11 T models have been constructed and tested at CERN. During the fabrication and assembly of, so far, seven single and two double aperture short model magnets, several challenges were identified and tackled. These include reproducibility in coil fabrication and assembly procedure, as well as control of mechanical stresses in the conductor and surrounding structure. In order to limit coil over-compression and improve reproducibility, the cable insulation was re-optimized. In addition, a review of the collaring procedure of the 11 T magnet was launched with the goal of reducing the risk of conductor degradation due to excessive stress. In this paper, the main fabrication and assembly steps are described, including the description of the actions taken to resolve the identified weakness.

3D-printed dissolvable inserts for efficient and customizable fabrication of NMR transceiver coils.

We describe a simplified method for improving the reproducibility of transceiver coil fabrication for nuclear magnetic resonance (NMR) through single-use templates made from 3D-printed polymer forms. The utility of dissolvable inserts for achieving performance enhanced resonators (DIAPERs) is tested herein by a comparison of RF homogeneity along the rotor axis for variable-pitch solenoids with different inter-turn spacing. Simulated B1 field profiles are compared to experimental homogeneity measurements, demonstrating the potential of this approach for making NMR coils quickly and reproducibly.

Development of design for large scale conductors and coils using MgB2 for superconducting magnetic energy storage device

MgB2 wires are commercially available, and their superconducting characteristics have been continuously developed in the last decade. The relatively high critical temperature of these wires has attracted the attention of researchers, especially in the field of superconducting magnetic energy storage (SMES) coil applications in terms of its relatively high critical temperature, as it enables the use of liquid hydrogen for cooling the coils. The sensitivity of multi-filament MgB2 wires to bending strain makes the design of large-scale conductors and coils for an SMES system technologically difficult, and the careful investigation of the applied strains during manufacturing is required. Two-conventional methods have been introduced for the fabrication of the coils: wind-and-react (W&R) and react-and-wind (R&W). These methods have been demonstrated to be suitable for the production of large-scale MgB2 magnets to maximize the coil performance. The W&R and R&W methods have been successfully applied to the designs of conductors and coils, and small W&R test coil fabrication, as well as stability demonstrations are performed in this study. Our study is the first to demonstrate the feeding of hundreds of amperes of transport current using multifilamentary MgB2 wires at around liquid hydrogen temperature in the practical background magnetic field of 2 T. The minimum quench energy and normal zone propagation velocity are also experimentally investigated for the protection of the actual coils for SMES application.

Fundamental origin of the large impact of strain on superconducting Nb3Sn

Superconductors can carry very high current densities without resistive loss. This makes them highly suitable for the construction of compact high field magnets of several tens of tesla. However, with increasing current density and magnetic field come high Lorentz loads and high strain levels, and it is empirically known that superconducting properties are affected by strain. The superconducting properties of the most commonly used high field material, Nb3Sn, are particularly badly affected by strain. Here we demonstrate that strain causes significant sub-lattice distortion in the A15 lattice structure of Nb3Sn, and show how this leads to the relatively large reduction of its superconducting properties. The changes are found to be primarily due to changes in the electron density of states, with a lesser contribution due to changes in the phonon spectrum. The amount of sub-lattice distortion further depends on crystal orientation. These findings suggest that it is possible to mitigate the reduction of the current carrying capacity by strain through crystal alignment and sub-lattice stabilization. This would enable superconducting magnets to reach a significantly higher magnetic field, and ease their construction by simplifying react-and-wind coil fabrication.

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.

Qualification Program of Lap Joints for ITER Coils

The superconducting coils of the ITER magnet system have hundreds of electrical lap joints interconnecting superconducting cables. The joints operate in a magnetic field of up to 4 T, field derivatives of 0.5 T/s, and currents up to 70 kA. The acceptance tests for the toroidal field (TF), poloidal field (PF), and correction coil (CC) coils will be performed at 77 K, before they are assembled in the pit. Hence there will be no possibility to measure the resistance of the joints in the superconducting state before the whole magnet system is enclosed in the Tokamak cryostat. In addition, no reliable nondestructive method has been found to spot the joints with a failure at room temperature. Therefore, the production of the joints relies on the strict adhesion to established robust manufacturing procedures during the qualification phase. As additional quality monitoring, a periodic test of the joint samples manufactured in parallel with a coil fabrication is foreseen to control the reproducibility of the joint electrical performance. In order to qualify the manufacturing procedures, to establish a series production tools and worker teams, a comprehensive qualification program has been set up for manufacturers of the coils in Russia (Poloidal Coil 1, PF1), China (PF6, feeders, CC), Japan (TF), Europe (TF and PF), and USA (Central Solenoid, CS). This program includes a set of mockups manufactured according to the process to be used for the coils and submitted to different tests. They include mechanical testing of materials, electrical tests of full size joint samples, destructive microscopic examination of the joint mockups, and mechanical testing of the full size joint mockups. All tests are carried out in specialized laboratories qualified for this type of work. This paper describes the main items of the qualification program, the tests performed, and the acceptance criteria. The test results are reported and compared to the criteria.

Magnetic Field Measurement and Analysis of the CSNS/RCS Quadrupole Magnets

The China Spallation Neutron Source/rapid-cycling synchrotron (CSNS/RCS) quadrupole magnets excited by both dc current and 25 Hz ac biased dc current were tested and measured in the past two years. All the magnets had been installed in the tunnel by the end of December, 2015. The challenges of the field measurements included the measurement repeatability of the dc field and the time harmonic measurement of the dc+ac field. The paper will summarize the results of the dc field measurement and the dc+ac field measurement, the key techniques of the rotating coil fabrication and the measurement stability for the CSNS/RCS quadrupole magnets.

Two-Layer 16 T Cos Dipole Design for the FCC

The future circular collider or FCC is a study aimed at exploring the possibility to reach 100 TeV total collision energy that would require 16 T dipoles. Upon the conclusion of the high luminosity upgrade, the US LHC accelerator upgrade project in collaboration with CERN will have an extensive Nb3Sn magnet fabrication experience. This experience includes robust Nb3Sn conductor and insulation scheming, 2-layer cos 2θ coil fabrication, and bladder-and-key structure and assembly. By making improvements and modification to existing technology, the feasibility of a two-layer 16 T dipole is investigated. Preliminary designs indicate that fields up to 16.6 T are feasible with conductor grading while satisfying the HE-LHC and FCC specifications. Key challenges include accommodating high-aspect ratio conductor, narrow wedge design, Nb3Sn conductor grading, and especially quench protection of a 16 T device.

Winding Technology and Experimental Study on 500 kV Superconductive Fault Current Limiter

500 kV saturated iron core superconductive fault current limiter (SFCL) has been built and tested successfully. The superconducting magnet consists of 88 high-temperature superconducting (HTS) double pancake coils with an inner diameter of 1940 mm and an outer diameter of 2040 mm. Double pancake coil plays an important role in 500 kV SFCL, the structure and the winding process have great influence on current carrying capability and reliability of the coil. In this paper, the key technology of winding process of the world's largest HTS coil was introduced, which included bending effect on critical current measurement of HTS tapes, insulation treatment of pancake coil and coil winding process. The V-I curve of the HTS coil was also investigated under the condition of liquid nitrogen in the self-magnetic field. The experimental results show that not only the pancake coil but also the superconducting magnet has good properties and the process is reliable and suitable for manufacturing 500 kV SFCL. The study will not only provide important references for large HTS coils fabrication but also accumulate experience and data for other similar engineering practices.