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ReBCO racetrack coils may be used in high-dynamic superconducting linear motor systems, typically replacing either permanent- or electromagnets in the DC stator. Even so, in order to achieve a significant increase in force density, the superconductor needs to carry a high transport current while simultaneously experiencing the time-varying magnetic field from the copper mover coils. To aid with the design of such devices, a 2D numerical model has been developed that predicts the AC loss under motor-relevant conditions, i.e. under the combined influence of a stationary transport current and an alternating external magnetic field. The main aim of the experiments described in this paper is to validate this model with dedicated AC loss measurements. To this end, we constructed a set-up that simultaneously measures magnetization-, transport current- and overall AC loss. Two identical insulated sub-scale ReBCO racetrack coils were tested at 4.2 K while carrying a stationary transport current of up to 700 A in a sinusoidal, alternating magnetic field up to 1.5 T, applied perpendicular to the broad face of the windings. Just like with metallic superconductors, the transport current significantly increases the AC loss level and lowers the penetration field. The inductive, electric and calorimetric data were found to be consistent with each other, validating the experimental calibration methods involved. Furthermore, the numerical model accurately predicted all AC loss components in the coils without any fitting to the data and can thus reliably be used in the design of superconducting machines.

In this article, electromagnet layouts are presented, which generate a magnetic field with a magnitude gradient that does not vary significantly in a horizontal plane but decreases monotonically with the vertical height above the magnet. Such a one-direction magnetic field gradient is a specific requirement for magnetic density separation (MDS), a novel recycling technology that combines a vertical magnetic field gradient with a ferrofluid to separate a mixture of non-magnetic materials based on their mass density. We are assembling the first superconducting magnet to be used for this application. In contrast to other separation technologies that use ferrofluid, multiple products can be separated in a single process step. First, the idealized current distribution is introduced that produces such a magnetic field with a magnitude that decays only in one direction. This ideal field can be approximated with practical coil configurations, which are evaluated with a Fourier analysis to derive an optimal cross-sectional layout based on flat racetrack coils. The analysis concludes with a discussion of the effect of winding pack thickness on the value of the magnetic field above the magnet system and the peak field inside the winding pack. The conclusions of this study are applicable not just for MDS but for any application that requires a magnetic field gradient that changes only in one direction.

In the framework of the High-Luminosity Large Hadron Collider (HL-LHC) project, 11 T dipole and MQXF quadrupole magnets employing Nb3Sn technology have been tested in short and long test configurations. Nb3Sn magnets are more sensitive than Nb-Ti magnets to a potential degradation of their conductors during production, testing, and cycling operation. At CERN, new diagnostic tools and measurement procedures have been developed to investigate, in detail, the performance of Nb3Sn accelerator type magnets. This is accomplished by V-I measurements extracted from voltage taps on conductor sections as well as entire coils. A leading hypothesis for the cause of decaying voltages on current plateaus of the V-I measurements is the presence of an inhomogeneous defect in the Rutherford cable. Current redistribution for bypassing such defects takes place through a current diffusion process, which leads to a decaying voltage over the affected cable sections. Using the simulation software THEA, the general behavior of this phenomenon has been studied. Good qualitative agreement is found between simulation and magnet test results.

This article discusses the optimum layout of coils of a superconducting magnet system for magnetic density separation (MDS). MDS is a novel separation technology that combines a vertical magnetic field gradient with a ferrofluid to separate mixtures of non-magnetic particles based on their mass density. The MDS process can separate more than two types of particles in a single process step, thereby distinguishing it from other separation techniques using a ferrofluid. The authors are currently constructing a superconducting MDS demonstrator. Ideally, the gradient of the magnetic field magnitude should change only with the distance above the magnet but remain constant in a horizontal plane. In principle, such an ideal field profile can be generated with an infinite harmonic sheet current. In practice, edge effects appear due to the necessity of using a finite number of coils. These cause a horizontal component in the field gradient and also change the vertical component. We compare the vertical magnetic field gradient of various coil layouts to see which configuration performs best. To facilitate ease of production, the analysis is restricted to flat racetrack coils. The main result is that the specific shape of a racetrack coil has a larger influence on the vertical gradient than the number of coils. The feed particles need to be pushed through the separation chamber from the insertion to the collection point. One option to realize this is to use an MDS setup in which the magnet is inclined with respect to the horizontal plane. This tilting results in a horizontal magnetic force component, which drives feed particles through the fluid bed. We show that a three-coil layout provides the largest usable fluid bed depth for a wide range of tilt angles.

In the scope of the Future Circular electron positron Collider study (FCC-ee), the IDEA detector is developed. It comprises a superconducting solenoid with free bore of 4 m, 6 m long and a central magnetic field of 2 T. The positioning of the magnet between the inner tracker and the electronic calorimeter heavily constrains the magnet design, as it is required to have the lowest possible radiation length, so minimum thickness and lowest density material. With respect to the classical solution of a solenoid enclosing the calorimeters, a cost reduction of about 50% is expected due to size reduction. An optimization of the different components of the magnet system has been carried out, resulting in the development of a new composite high-strength conductor that can be used to build a 30 mm thin solenoid. The quench analysis of the solenoid will be presented as it is of critical importance given the high energy density in the magnet of 21 kJ/kg. A cryostat made of concentric aluminium shells would account for about 50% of the radiation length of the magnet and most of this material is used in the outer vacuum shell of the cryostat to prevent buckling. In order to further reduce the radiation length, two fundamentally different approaches are being analysed. The first method focuses on reducing drastically the outer shell thickness. This leads to use honeycomb composites, reinforcing bars and corrugated shells for the outer shell of the cryostat. The second approach consists of supporting very thin cryostat shells directly on the solenoid cold mass using proper support. This can be achieved by replacing the thick walls and MLI insulation by a material that can sustain 1 atm while having low radiation length and low thermal conductivity. Cryogel Z has shown promising properties and its suitability for this project is being analysed. This novel approach has never been used so far for superconducting magnets.

High temperature superconducting (HTS) technologies are expected to be a key enabler for lightweight and cost-effective direct-drive (DD) trains for large wind turbines. This paper reports the designing and basic experimental validation of the world's first full-scale DD HTS generator demonstrated on a commercial wind turbine. The HTS generator has its rotor with an HTS field winding working below 30 K, which is achieved by using off-the-shelf Gifford-McMahon cryocoolers. The stator of the generator is essentially conventional, except that the armature winding has four segments to limit fault torques in case of sudden short circuits due to converter failures. Compared to an existing DD permanent magnet generator on the turbine, the air gap shearing stress of the HTS generator is doubled, and the weight is reduced by 24%. The overall design requirements from the turbine integration perspective, as well as the topological considerations, are first described in this paper. The electromagnetic and cryogenic designs are then presented, followed by performance testing of HTS coils. The basic experimental validation shows that the cryogenic design is satisfactory and the measured no-load voltage matches the finite element calculation very well.

In the frame of the High-Luminosity Large Hadron Collider construction and Future Circular Collider development program, the magnetic field in the accelerator dipole magnets is being enhanced to 11 T, and 15 T to 16 T level, respectively. Advanced Nb3Sn superconductors with a non-copper critical current density exceeding 2500 A mm−2 at 4.2 K and 12 T, are being developed using the Restacked-Rod-Process (RRP) and Powder-In-Tube (PIT) wire technologies. However, since Nb3Sn is extremely brittle, it is a significant challenge to construct the high-field dipole magnets with such very strain-susceptible superconductor. The high-level of stress acting on the wide face of the Rutherford cables in the coils of 120 MPa to 200 MPa generated by the Lorenz’ force, causes initially a reversible reduction and eventually at some stress level followed by permanent degradation of the critical current when strain goes to high. This study sets out to examine the critical current and upper critical field performance of state-of-the-art RRP and PIT Nb3Sn Rutherford cables in terms of transverse pressure. The variation of the critical current and upper critical field due to the thermal- and mechanical load-cycling was investigated as well. For reference, the critical current of witness wires characterized on standard ITER type barrels were also measured. The results indicate that the RRP type of Nb3Sn Rutherford cables, when fully impregnated with epoxy resin, are able to withstand a transverse stress of 170 MPa to 250 MPa without noticeable irreversible critical current reduction. For the transverse pressure limit for present PIT type of Nb3Sn Rutherford cables somewhat lower values are found at the level of 50 MPa to 120 MPa. Therefore, given the present cables, the high-field dipole magnet construction can be realized using the RRP Nb3Sn Rutherford cables, while for PIT type cables more cable development is requested to enhance the onset of irreversible degradation. The reversible critical current reduction in RRP type of cables of 10% at 150 MPa to 250 MPa needs to be taken into account when predicting magnet performance. Finally, extreme care needs to be taken into account for Nb3Sn coil fabrication, since the experimental results show significant critical current reduction due to stress concentrations already at 0.2° misalignment angles between the pressure applying surface and the surface of the impregnated cable.

High temperature superconducting (HTS) generators could enable a lightweight and cost-effective direct drive (DD) wind turbines with large power ratings. The EU-funded EcoSwing project successfully demonstrated the world's first full-scale MW-class HTS generator on a commercial DD wind turbine. This paper focuses on the commissioning of the EcoSwing HTS generator on the wind turbine. The commissioning campaigns, including the rotor cool-down, excitation of the HTS field winding, and the power production of the generator, are presented in the paper. In the testing period, the generator was grid-connected for more than 650 hours and accumulatively produced more than 600 MWh to the grid. The target output power of the 3 MW class was reached. Throughout the real-life testing on the wind turbine, the generator performed well from the electromagnetic, thermal, and mechanical perspectives. Moreover, the generator even sustained three sudden short circuits in the converter system. The work reported has shown that HTS generators are technologically feasible for wind turbine applications, and the technology readiness level of HTS wind turbine generators has been improved to 6 $\sim$ 7 for the first time.

The measured critical current reduction in Nb3Sn Rutherford cables under magnet-relevant transverse pressure levels is analyzed in terms of the strain state of the filaments inside their strands. Several straightforward mechanical 2D FE models of the cables’ cross-section are used to translate the stress that is applied to the surface of the impregnated cables into a strain distribution on its strands. The resulting critical current reduction of the cable is then estimated from the average deviatoric strain in the strands’ filamentary zone, using the well-established strain scaling relations obtained for isolated strands. This allows to identify the main factors that influence the pressure response of impregnated Nb3Sn accelerator cables. The analysis is presented for state-of-the-art cable samples that were measured at the University of Twente and shows how especially stiff and incompressible resins reduces the deviatoric strain in the filamentary zone of the cable strands, but also how relatively small alignment errors can lead to stress concentrations that reduce the critical current density significantly.

The EU-funded EcoSwing project addresses the world's first full-scale direct-drive (DD) high temperature superconducting (HTS) wind turbine generator. Before the generator was installed on a commercial wind turbine, the generator had been tested on the ground at the Dynamic Nacelle Testing Laboratory (DyNaLab) in Fraunhofer Institute for Wind Energy Systems (IWES), serving as an experimental validation of the generator design. This article is centered on the ground testing of the EcoSwing HTS generator, which includes corresponding tests for validating the generator's cryogenic design and electromagnetic performance. Despite one HTS coil with a defect limiting the operating current significantly, partial power production up to 1 MW at 14.5 rpm was reached. The critical auxiliary systems, i.e., vacuum system and rotary helium coupling, behaved as expected or even better than anticipated. The work reported in the article successfully validates the generator design, and also provides firsthand information prior to installing and testing the HTS generator on the wind turbine.

The focus of this paper is on the mechanical design of a NbTi-based demonstrator magnet for magnetic density separation (MDS) that is being constructed at the University of Twente. MDS is a new recycling technology that allows the separation of non-magnetic particles based on their mass density, using a vertical magnetic field gradient and a ferrofluid. The unique mechanical design challenge for this type of magnet is the desired minimization of the distance between a sim1 m2 planar array of cryogenic racetrack coils and the ambient-temperature ferrofluid bath. The optimization of the magnet geometry results in a distance between the coils and ferrofluid of 50 mm. This is made possible by opting for conduction-cooling, for the inclusion of room-temperature rods that pass through the cold mass to support the cryostat, and for the geometry of the cassette that reacts to the Lorentz force.

For 30 years, training and unpredictable degradation in accelerator type high-field Nb3Sn magnets have seriously hampered Nb3Sn application. Training and deterioration have to be solved or at least better controlled. The global picture shows that most of the R&D and short model magnets start to train at some 40%-70% of the critical current and then creep up to almost the critical current within some 10-50 training steps. A typical class of failures leading to quenches is largely characterized by cracking and debonding at the interfaces between cable and glass-resin insulation, as well as between insulation and coil former. The study of training by means of testing demonstrator coils is rather expensive and time consuming. However, advances in magnet design and fabrication can also be assessed and benchmarked using BOX, the bonding experiment presented here, that produces maximum uniaxial Lorentz forces at some 7.5 T in a controlled experiment performed in 11 T solenoid facility at the University of Twente. BOX samples use only one meter of Nb3Sn cable inserted in a three-wave meandering slot in a flat metallic sample holder, reproducing magnet-relevant interactions between cable, insulation, impregnated materials and coil former. The meander shape exposes seven straight cable sections to a transverse magnetic field, thereby generating a representative level of shear stress at the interfaces. In this way, characteristic training curves of magnets can be mimicked and solutions studied. We aim to demonstrate with various samples failure mechanisms of high-field Nb3Sn magnets without the need to manufacture complete magnets. BOX may thus be expected to allow for quick and affordable testing of novel insulations, impregnation materials, coatings and interfaces for Nb3Sn magnets achieved by investigating various resins, fillers and more.

The main aim of the EU H2020 project EcoSwing was to demonstrate a technical readiness level of 6–7 for high-temperature superconducting (HTS) technology operating in a wind generator. To reach this goal, a full-scale synchronous HTS generator was successfully designed, built and field-tested in a 3.6 MW turbine. The generator has a rotor with 40 superconducting coils of 1.4 m long. The required >20 km of coated conductor was produced within the project’s time schedule. All coils were tested prior to assembly, with >90% of them behaving as expected. The technical readiness level of HTS coils was thus increased to level 7. Simultaneously, the maturing of cryogenic cooling technology over the last decade was illustrated by the several Gifford-McMahon cold-heads that were installed on-board the rotor and connected with the stationary compressors through a rotating coupling. The cryogenic system outperformed design expectations, enabling stable coil temperatures far below the design temperature of 30 K after only 14 d of cool-down. After ground-based testing at the IWES facility in Bremerhaven, Germany, the generator was installed on an existing turbine in Thyborøn, Denmark. Here, the generator reached the target power range and produced power for over 650 h of grid operation.

In the frame of the EuCARD2 collaboration, aimed at developing the technology for 20 T class accelerator magnets, several demonstrator dipole magnets are being built using high critical current density and fully transposed ReBCO tape-based Roebel-type cables. In accelerator magnets the dynamic magnetic field quality is one of the key parameters, which is affected by the effective inter-strand resistances in the cables. For this reason, measurements of the inter-strand resistances on ReBCO Roebel cables were carried out at 4.2 and 77 K. Acquiring these data is also essential for input of cable simulation models. The cable samples are impregnated with epoxy resin to reduce the effect of transverse stress degradation due to Lorentz forces acting on the strands in the Roebel cables. The measured inter-strand resistance is used to estimate the AC coupling loss in different magnetic field orientations. Moreover, the contributions of diverse interface contact resistances to overall inter-strand resistance of Roebel cables were determined using a novel theoretical model. For validation, the AC loss of cables were examined in various orientations of applied field at 4.2 K. With three analytical models the hysteresis loss was calculated and compared to the measured data. The average inter-strand resistance of the cable samples impregnated with the unfilled epoxy CTD-101K range from 3 to 16 μΩ at 77 K and 1.5 to 9 μΩ at 4.2 K. Between the tapes the copper to copper interface resistance dominates the inter-strand resistance of impregnated Roebel cables. The calculated and measured AC loss for the CTD-101K impregnated Roebel cable lead to equivalent conclusions that the coupling loss is lower than the hysteresis loss within the range of the experiment. These observations substantially differ from earlier results extracted from a similar cable but impregnated with the alumina-filled epoxy resin CTD-101G, which showed considerable coupling loss when exposed to magnetic field parallel the wide face of the cable.

ReBCO Roebel cables are being developed as one of the promising routes towards much higher-field magnets for future particle accelerators. The Lorentz forces generated in such magnets lead to sizeable transverse pressure on the superconducting cables, in the present 15-20 T magnet designs of the order of 100-150 MPa. Contrary to single ReBCO coated conductors, unprotected Roebel cable due to their uneven surface, start to degrade already with 40 MPa as shown in earlier work. Previous publications also showed that proper impregnation with epoxy resin can dramatically improve the transverse pressure tolerance of a relatively simple model cable with 10 ReBCO strands, assembled from SuperPower tape. However, the epoxy resins used until now are not suitable for large-scale magnet vacuum impregnation. For this reason, an alternative impregnation method using CTD-101K with S2 glass fibers has been implemented. The present paper expands further on the effect of impregnation on the transverse pressure susceptibility of Roebel cables. Three cables, with an architecture that is directly relevant for the so-called EuCARD-2 accelerator demonstrator magnet, were investigated with a variable transverse mechanical load at 4.2 K in a 10.5 T perpendicular magnetic field. The new cables feature a relatively long transposition length of 226 mm and comprise 15 strands of either SuperPower or Bruker coated conductor. For reference, one of the new SuperPower tape based cables was impregnated with the same resin as the one used in the earlier publications. The other cables were impregnated using the new method. The critical current at 4.2 K of the Bruker tape Roebel cable is 2.5 to 3 times higher than that of the SuperPower tape ones with similar architecture. Remarkably no critical current degradation was observed for transverse pressure up to 440 MPa in the SuperPower tape cables and up to 370 MPa in the Bruker tape cable. These values by far satisfy the design requirements of presently envisaged 20 T class accelerator demonstrator magnets.

International audience; A future circular collider (FCC) with a center-of-mass energy of 100 TeV and a circumference of around 100 km, or an energy upgrade of the LHC (HE-LHC) to 27 TeV require bending magnets providing 16 T in a 50-mm aperture. Several development programs for these magnets, based on Nb$_3$Sn technology, are being pursued in Europe and in the U.S. In these programs, cos-theta, block-type, common-coil, and canted-cos-theta magnets are explored; first model magnets are under manufacture; limits on conductor stress levels are studied; and a conductor with enhanced characteristics is developed. This paper summarizes and discusses the status, plans, and preliminary results of these programs.

A proper understanding of the degradation of the transport properties of cable-in-conduit conductors (CICCs) due to a changing strain distribution or to crack formation in the filaments is essential to determine the operational limits of the conductors and to optimize their design. Based on the electrical and strain properties of the superconducting strand, the performance of short samples of the CICC can be analyzed, such as the ones tested in the SULTAN facility, or the full-size CICC used in real magnets. Mathematical fitting expressions are proposed to implement strand properties into the cable model. In combination with the strain maps generated by the mechanical model MULTIFIL, these expressions are introduced in the electromagnetic code JackPot to predict the current sharing temperature of the CICC of the international thermonuclear experimental reactor (ITER) Central Solenoid. A comparison is made with SULTAN short samples tests as well.

The European Organization for Nuclear Research (CERN) is developing high-current ReBCO-CORC strand-based cables for use in future large-scale detector magnets. A six-around-one, forced flow gas-cooled ReBCO-CORC cable-in-conduit conductor (CICC) is envisioned for application in magnets operating in the 20-40 K temperature range. A CICC, rated for 45 kA at 4.2 K and 10 T, is designed and in production. The CICC comprises a cable of six CORC strands helically wound around a tube. The cable has an expected current density of 105 A/mm2 at 10 T/4.2 K, which corresponds to an overall current density of 53 A/mm2. A cable current density of 110 A/mm2 can be reached when increasing the temperature to 20 K and operating in a magnetic field of 5 T.

CERN is developing high-current ReBCO conductor on round core (CORC) cables for application in future detector and accelerator magnets. A characterization test on a ReBCO-CORC cable sample and its joints is performed in the 10-T FRESCA cable test facility at CERN. The sample is taken from the first 12-m-long CORC production. Key is the characterization of the field- and temperature-dependent critical currents of the CORC cable at 1.9 K and 4.2 K. Secondary objectives include evaluating the response of the CORC cable to quenches and the performance of cylindrical low resistive cable terminals especially designed and manufactured for use on CORC cables. The 7.6-mm CORC cable features 8 kA at 4.2 K and 10 T, and the joint terminals show a $25\pm 5-{n}\Omega$ resistance for 20-cm length.

The objective of the EuCARD2 WP10 (Future Magnets) research activity is to demonstrate high-temperature superconducting magnet technology for accelerator applications by building a short demonstrator dipole with an aperture of 40 mm, operating field of 5 T, and understood field quality. One of the magnet requirements is of small inductance, for use in long magnet strings; hence, the superconducting cable must have large current-carrying capacity, in the range of 10 kA at the operating conditions of 4.2 K and 5 T. An initial down-selection of the cable material and geometry resulted in the choice of REBCO tapes assembled in a Roebel cable as the baseline layout. In this paper, we described the requirements derived from magnet design, the selection process that led to the choice of material and geometry, the reference design of the cable, and its options. Activities have started to address fundamental issues, such as tape performance and tape processing through the cable construction, and key performance parameters such as cable critical current under stress or magnetization. Here, we report the main highlights from this work.

Many high-temperature superconductor (HTS) applications require superconducting cables with high currents while operating in an alternating magnetic field. HTS cables should be composed of numerous superconducting tapes to achieve the required current capacity. Alternating current and magnetic fields cause AC losses in such cables and can provoke conductor instability. AC losses and contact resistances were measured of several cable designs based on commercially available REBCO tapes at the University of Twente. The AC loss was measured under identical conditions for eight REBCO conductors manufactured according to three types of cabling methods—CORC® (Conductor on Round Core), Roebel, and stacked tape, including a full-size REBCO CICC (cable in conduit conductor). The measurements were done at T = 4.2 K without transport current in a sinusoidal AC magnetic field of 0.4 T amplitude and frequencies from 5 to 55 mHz. The AC loss was measured simultaneously by calibrated gas flow calorimeter utilizing the helium boil-off method and by the magnetization method using pick-up coils. Also, the AC loss of two CORC® conductors and a Roebel cable was measured at 77 K. Each conductor was measured with and without background field of 1 T. The measured AC coupling loss in the CORC® and Roebel conductors is negligible at 4.2 K for the applied conditions while at 77 K coupling loss was observed for all conductors. The absence of coupling loss at 4.2 K can be explained by shielding of the conductor interior; this is confirmed with measurement and calculation of the penetration field of CORC® and Roebel cables. The inter-tape contact resistance was measured for CORC® and stacked tape samples at 4.2 and 77 K. It was demonstrated that a short heat treatment of CORC® conductor with solder-coated tapes activates tape-to-tape soldering and decreases the contact resistance. The reduction of contact resistance by two orders in magnitude to tens of nΩm is comparable with the interstrand contact resistance in ITER Nb3Sn type conductors.

© 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. A next step of energy increase of hadron colliders beyond the LHC requires high-field superconducting magnets capable of providing a dipolar field in the range of 16 T in a 50-mm aperture with accelerator quality. These characteristics could meet the requirements for an upgrade of the LHC to twice the present beam energy or for a 100-TeV center of mass energy future circular collider. This paper summarizes the activities and plans for the development of these magnets, in particular within the 16 T Magnet Technology Program, the WP5 of the EuroCirCol, and the U.S. Magnet Development Program.

International audience; The EuCARD2 collaboration aims at the development of a 10 kA-class superconducting, high current density cable suitable for accelerator magnets, to be tested in small coils and magnets capable to deliver 3-5 T when energized in stand-alone mode, and 15-18 T when inserted in a 12-13 T background magnet. REBCO tape, assembled in a Roebel cable, was selected as conductor. The developed REBCO tape has reached a record engineering critical current density, at 4.2 K and 18 T of ${\text{956 A/mm}}^{2}$. Roebel cable carried up to 13 kA at 20 K when tested in a small coil (FeatherM0.4). Then a first dipole magnet, wound with two low-grade Roebel cables of 25 m each, was assembled and tested. The dipole reached the short sample critical current of 6 kA generating more than 3 T central field at about 5.7 K, with indications of good current transfer among cable strands and of relatively soft transition. The construction of a costheta dipole is also discussed. Eucard2 is reaching its objective and is continuing with the H2020-ARIES program aiming at doubling the Je at 20 T to obtain 6 T as standalone and 18 T as insert in a high field facility.

International audience; This paper describes the standalone magnet cold testing of the high temperature superconducting (HTS) magnet Feather-M2.1-2. This magnet was constructed within the European funded FP7-EUCARD2 collaboration to test a Roebel type HTS cable, and is one of the first high temperature superconducting dipole magnets in the world. The magnet was operated in forced flow helium gas with temperatures ranging between 5 and 85 K. During the tests a magnetic dipole field of 3.1 T was reached inside the aperture at a current of 6.5 kA and a temperature of 5.7 K. These values are in agreement with the self-field critical current of the used SuperOx cable assembled with Sunam tapes (low-performance batch), thereby confirming that no degradation occurred during winding, impregnation, assembly and cool-down of the magnet. The magnet was quenched many tens of times by ramping over the critical current and no degradation nor training was evident. During the tests the voltage over the coil was monitored in the microvolt range. An inductive cancellation wire was used to remove the inductive component, thereby significantly reducing noise levels. Close to the quench current, drift was detected both in temperature and voltage over the coil. This drifting happens in a time scale of minutes and is a clear indication that the magnet has reached its limit. All quenches happened approximately at the same average electric field and thus none of the quenches occurred unexpectedly.

Two new ReBCO-CORC® based cable-in-conduit conductors (CICC) are developed by CERN in collaboration with ACT-Boulder. Both conductors feature a critical current of about 80 kA at 4.5 K and 12 T. One conductor is designed for operation in large detector magnets, while the other is aimed for application in fusion type magnets. The conductors use a six-around-one cable geometry with six flexible ReBCO CORC® strands twisted around a central tube. The fusion CICC is designed to be cooled by the internal forced flow of either helium gas or supercritical helium to cope with high heat loads in superconducting magnets in large fusion experimental reactors. In addition, the cable is enclosed by a stainless steel jacket to accommodate with the high level of Lorentz forces present in such magnets. Detector type magnets require stable, high-current conductors. Therefore, the detector CORC® CICC comprises an OFHC copper jacket with external conduction cooling, which is advantageous due to its simplicity. A 2.8 m long sample of each conductor is manufactured and prepared for testing in the Sultan facility at PSI Villigen. In the paper, the conductor design and assembly steps for both CORC® CICCs are highlighted.

CERN has designed and manufactured the first 1.7-m-long fully operational ReBCO CORC cable-in-conduit conductor. This CORC cable-in-conduit conductor is envisioned for use in large, high current and high energy magnets. The conductor has been successfully tested at 4.5 K/9 T and in 77 K/self-field. At 4.5 K, the CICC is fully superconducting up to the current limit of the test facility. The critical current of the conductor at 77 K is 12.3 to 13.0 kA with an n-value of 12 to 15, depending on ramp rate. The current distribution is observed to be homogeneous at both test temperatures. A resistance of 1.9 to 5.8 n \Omega is found for the joint terminals at 4.5 K in various magnetic fields and ramp rates, and 6.8 to 9.9 n\Omega at 77 K in self-field. The values found for the critical current and the resistance of the terminals at both 4.2 and 77 K are conform to the expectations based on a simulation model. The conductor underwent multiple thermal cycles no trace of degradation due to cabling, conduit assembly, joints and the high magnetic field tests was observed.

The paper describes a temperature-dependent extension of the classical laminate theory (CLT) that may be used to predict the mechanical behaviour of Fibre Metal Laminates (FML) at cryogenic conditions, including crack initiation. FML are considered as a possible alternative class of structural materials for the transport and storage of liquified gasses such as LNG. Combining different constituents in a laminate opens up the possibility to enhance its functionality, e.g. offering lower specific weight and increased damage tolerance. To explore this possibility, a test programme is underway at the University of Twente to study transverse crack initiation in different material combinations under combined thermal and mechanical loading. Specifically, the samples are tested in a three-point bending experiment at temperatures ranging from 77 to 293 K. These tests will serve as a validation of the model presented in this paper which, by incorporating temperature-dependent mechanical properties and differential thermal expansion, will allow to select optimal material combinations and laminate layouts. By combining the temperature-dependent mechanical properties and the differential thermal contraction explicitly, the model allows for a more accurate estimate of the resulting thermal stresses which can then be compared to the strength of the constituent materials.

The effect of disorder on the normal state resistivity and the superconducting properties of Nb3Sn Sn is explored in a combination of ab initio calculations and microscopic theory. The crystal symmetry is calculated to be preferentially tetragonal at a normal state resistivity below 27.0 ±1.4 μcm, and preferentially cubic above this value, which is shown to be consistent with the experimentally observed transition point. The phonon density of states, the Eliashberg spectrum a2 (w)F (w), the electronphonon coupling constant, the characteristic frequency, the critical temperature Tc, and the upper critical magnetic field at 0 K Hc2 (0) are calculated over a large normal state resistivity range and shown to be consistent with experimental observations. The high degree of consistency between the calculation results and experimental observations is a strong indication that the calculation approach utilized here, a combination of ab initio calculations and microscopic theory, is a useful tool for understanding the superconducting and normal state properties of Nb3Sn.

The paper describes the development and the experimental validation of a cryogenic magnetic shielding system for transition edge sensor based space detector arrays. The system consists of an outer mu-metal shield and an inner superconducting niobium shield. First, a basic comparison is made between thin-walled mu-metal and superconducting shields, giving an off-axis expression for the field inside a cup-shaped superconductor as a function of the transverse external field. Starting from these preliminary analytical considerations, the design of an adequate and realistic shielding configuration for future space flight applications (either X-IFU [D. Barret et al., e-print arXiv:1308.6784 [astro-ph.IM] (2013)] or SAFARI [B. Jackson et al., IEEE Trans. Terahertz Sci. Technol. 2, 12 (2012)]) is described in more detail. The numerical design and verification tools (static and dynamic finite element method (FEM) models) are discussed together with their required input, i.e., the magnetic-field dependent permeability data. Next, the actual manufacturing of the shields is described, including a method to create a superconducting joint between the two superconducting shield elements that avoid flux penetration through the seam. The final part of the paper presents the experimental verification of the model predictions and the validation of the shield's performance. The shields were cooled through the superconducting transition temperature of niobium in zero applied magnetic field (

International audience; Abstract:A key challenge for a future circular collider (FCC) with centre-of-mass energy of 100 TeV and a circumference in the range of 100 km is the development of high-field superconducting accelerator magnets, capable of providing a 16 T dipolar field of accelerator quality in a 50 mm aperture. This paper summarizes the strategy and actions being undertaken in the framework of the FCC 16 T Magnet Technology Program and the Work Package 5 of the EuroCirCol.

Current-voltage characteristics of high-critical-current Bi2Sr2CaCu2O10/Ag ribbons were measured using both transport and magnetization techniques. The slope of these curves changes with magnetic field and temperature in a way very similar to the observed jc(H, T) behavior. This correspondence between the critical current and the slope of the I-V characteristics can be explained within the thermally activated flux creep framework. The dependence of jc on the angle between field and ribbon is compared to the existing intrinsic anisotropy models. © 1995 Plenum Publishing Corporation.

With the Large Hadron Collider (LHC) up and running, studies have started for its successor. Under study is the Future Circular Collider (FCC), which has a circumference of about 100 km, aiming at a proton-proton collision energy of 100 TeV. Consequently, the main bending dipole magnets have to operate at a magnetic field of 16 T. As a first step towards its realization, this paper presents the results of a parametric study of the cross-sectional layout for dipole magnets with a field in the range of 13-17 T using Nb-Ti and Nb3Sn superconductors. The principal layouts included are the classical Cosine-Theta, the Canted Cosine-Theta, and the Block type. Conductor cost can be reduced significantly when a graded hybrid solution is chosen. Optimizing such complex magnet layouts requires an iterative algorithm, which arranges the positions of the various blocks of coil windings in the coil cross section, thereby finding the thickness of the coil layers. The iterative algorithm is coupled to an adiabatic quench model, which finds an optimal copper-to-superconductor fraction for each of the layers. Outside the iterative cycle, a pattern search algorithm is applied to find a cost optimal distribution of the magnetic field generated by each coil layer.

EuCARD-2 aims to research ReBCO superconducting magnets for future accelerator applications. The properties of ReBCO conductors are very different from low-temperature superconductors. To investigate dynamic field quality, stability, and normal zone propagation, an electrical network model for coated conductor cables was developed. To validate the model, two identical samples were prepared at CERN, after which measurements were taken at the University of Twente and Southampton University. The model predicts that for a Roebel cable, in a changing magnetic field applied in the perpendicular direction, hysteresis loss is much larger than coupling loss. In the case of a changing magnetic field applied parallel to the cable, coupling loss is dominant. In the first case, the experiment is in good agreement with the model. In the second case, the data can only be compared qualitatively because the calibration for the inductive measurement is not available.

The European Organization for Nuclear Research (CERN) is developing a six-around-one conductor on round core (CORC)-strand-based cable-in-conduit conductor (CICC) for use in detector and other large magnet systems. The CICC comprises six ReBCO-CORC strands helically wound around a central tube or rod and inserted in a square aluminum jacket. A major design challenge is finding a simple yet low-resistive method of injecting current homogeneously into the CORC strands of the CICC. In the production of joints for single-CORC cables, we are currently pursuing a method in which the different ReBCO layers at both ends of the CORC cable are trimmed into a staircase-like geometry. A similar trimming method is developed for joint terminals for the ReBCO-CORC-based CICC. A demonstration joint terminal is made to test the various steps of the trimming and manufacturing process before fabricating a joint terminal with real CORC strands. This paper presents an overview of CIC-joint terminal design, simulation results, and the different steps in the manufacturing process.

Roebel cable with second-generation YBCO strands is one of the promising high-temperature superconducting solutions of fully transposed high current conductors for high field accelerator magnets. Following the considerable research effort on the manufacturing of Roebel cables in recent years, sample conductors are now available in useful lengths with reproducible performances to allow for detailed characterizations beyond the standard critical current measurements. The ac loss and strands coupling are of significant interest for the field quality of the accelerator magnets. We report a set of systematic ac loss measurements on two different Roebel cable samples prepared for the EuCARD-2 collaboration. The measurements were performed over a wide range of temperature values between 5 and 90 K, and the results were analyzed in the context of strands architecture and coupling. The results show that the transposed bundles are partially decoupled, and the strands in transposition sections behave as an isolated single tape if the strands are insulated.

EuCARD-2 is a project partly supported by FP7 European Commission aiming at exploring accelerator magnet technology for 20-T dipole operating field. The EuCARD-2 collaboration is liaising with similar programs for high-field magnets in the U.S. and Japan. EuCARD-2 focuses, through the work package 10 “future magnets,” on the development of a 10-kA-class superconducting high-current-density cable suitable for accelerator magnets, for a 5-T stand-alone dipole of 40-mm bore and about 1-m length. After stand-alone testing, the magnet will be inserted in a large bore background dipole, 10–18 T. This paper reports on the design and development of models, which are called Feather0, wound with REBCO Roebel cable. Based on aligned block design to take advantage of the anisotropy of the REBCO tapes, Feather0 is a precursor of Feather2, which should reach the project goals in 2016. Feather0 is planned to be tested both in stand alone and as an insert mounted in the CERN Fresca facility providing 10-T background field. The progress of other designs pursued in the collaboration, one based on classical $\cos\vartheta$ layout with Roebel cable and the other based on coil block with stacked tape cable, will be also reported.

The effect of tantalum and titanium additions on the composition, the superconducting properties, and their sensitivity to strain of bulk Nb3Sn is investigated. Using heat capacity analysis and Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDX), it is found that the binary Nb3Sn bulk and Nb3Sn bulk with added titanium and tantalum consist of stoichiometric Nb3Sn and niobium(-oxide). Furthermore, it is found that the niobium-to-tin ratio decreases in the presence of tantalum and increases in the presence of titanium, which suggests that tantalum is replacing niobium and titanium is replacing tin in the A15 crystal structure. Using a 10% resistivity criterion, it is observed that the critical magnetic field of unstrained binary bulk is 26.7 T, while the presence of tantalum and titanium raises the critical magnetic field to 29.3 and 30.1 T, respectively. The curves of normalized critical magnetic field as function of strain of all three samples nearly overlap, a strong indication that the variation in strain sensitivity observed in wires is not caused by the titanium and tantalum additions. Understanding the effect of additions on the composition, superconducting properties, and strain sensitivity of Nb3Sn is important for optimizing Nb3Sn conductor technology.

The cable-in-conduit cabling schemes chosen for conductors inherently have distributed axial, bending, and contact loads acting on the multifilamentary strands under the operating conditions of the magnet. Due to the brittle nature of filaments, such mechanical loads can lead to fractures in the filaments and cause a permanent degradation in the conductor performance, depending on the chosen cabling pattern, aspect ratio and void fraction. Therefore, the knowledge of filament fracture mechanisms and the corresponding degradation in strand's transport property is an important factor in optimizing the design of Nb3Sn strands and cabled conductors. Previously, we have developed a metallographic technique to resolve microscopic cracks in a multifilamentary strand and the crack density growth with applied axial tensile strain was investigated. Separately, the evolution of strand's voltage-current characteristics at zero applied strain after various tensile strains loads was measured. Here, we present a numerical model of a strand by a three-dimensional electrical network and simulation results by introducing a crack in the system.

The current transfer length in composite MgB2 wires with different metallic sheaths and filament numbers was analysed. The electric potential gradient along the wire in the vicinity of the current contact was monitored as a function of transport current in various external magnetic fields up to 14 T. In all samples the characteristic current transfer length is of the order of a millimetre, varying with detailed sheath composition in the range 0.35–2 mm. For the wires with a clear reaction layer at the filament/matrix interface, the electrical resistivity of the material constituting such a barrier was estimated.

Within CERN the development of REBCO-CORC (Conductor On Round Core) type cables is pursued in view of possible application in future detector and accelerator magnets. An important issue is the design and qualification of terminations for connecting CORC cables mutually or to bus-bars. A termination design is envisaged that combines a simple manufacturing process with a lowest possible joint terminal resistance in the few nΩ range at 4.2 K, first for a single CORC cable and subsequently for CORC based Cable-in-Conduit Conductors. The investigation concerns the effect of tapering the CORC cable within the joint to form a staircase like geometry, which allows current to pass more directly from the copper joint casing to the inner REBCO layers of the CORC cable. Simulations have shown a substantial decrease in joint resistance at operating current in the case both CORC cable and joint casing are tapered. The CORC cable and new joint were tested at CERN. In this paper, some details of the new joint design, fabrication process, and model are presented and the results are summarized.

Multifilamentary NbTi wires for ac applications are manufactured by embedding filament bundles into a metal matrix. In this stage of the manufacturing process, it is possible to affect the layout of the cross section and to choose whether to use few large or many small bundles in order to achieve a certain amount of filaments. All in all, up to 100 000 filaments are attainable for wire having the diameter of 1 mm. In this paper, ac loss measurements in external magnetic field on differently stacked NbTi samples are described. The measurements were performed in a LHe-cooled cryostat. The amplitude of the external field was varied between 250 mT and 3 T at frequencies of 0.02 and 0.12 Hz. We discuss possibilities to simulate the losses with finite element method. In particular, we concentrate on the filament bundle approximation and the possibilities to exploit it in the research and development process of new NbTi wires. In this approach, the filament bundles are considered as a homogenous mixture of matrix and superconducting filaments. According to the results, the bundle approximation greatly overestimates the losses. Furthermore, it should not be used for comparing, e.g., two wire structures where one has bundles of different size than the other. However, when considering how to situate the bundles on the cross section to achieve minimal ac loss, the bundle approximation can be a useful tool

REBCO Roebel cables are considered for application in high-temperature superconducting inserts for accelerator magnets because of their fully transposed geometry, high-engineering current density, and adequate bending tolerance. In these magnets the cables experience Lorentz forces leading to transverse stresses up to 100–150 MPa. Previous reports have shown bare Roebel cables to degrade under such high stresses so that additional reinforcement is required. In this work, two identical Roebel cables are vacuum impregnated with a mixture of epoxy and fused silica in order to improve their tolerance to transverse stress. After impregnation, the critical current of the cables is measured under transverse mechanical loading at T = 4.2 K, ⊥ = B 10.5 T. A reference cable without impregnation is tested as well. Pressures up to 350 MPa are applied to a short (30 mm) section of each cable. No degradation was observed for pressures up to 250 MPa and 170 MPa in the two impregnated cables. The critical current of the non-impregnated cable, in contrast, started to decrease at stresses as low as 40 MPa.

International audience; EuCARD-2 is a project supported by FP7-European Commission that includes, inter alia, a work-package (WP10) called ldquoFuture Magnets.rdquo This project is part of the long term development that CERN is launching to explore magnet technology at 16 T to 20 T dipole operating field, within the scope of a study on Future Circular Colliders. The EuCARD2 collaboration is closely liaising with similar programs for high field accelerator magnets in the USA and Japan. The main focus of EuCARD2 WP10 is the development of a 10 kA-class superconducting, high current density cable suitable for accelerator magnets, The cable will be used to wind a stand-alone magnet 500 mm long and with an aperture of 40 mm. This magnet should yield 5 T, when stand-alone, and will enable to reach a 15 to 18 T dipole field by placing it in a large bore background dipole of 12-15 T. REBCO based Roebel cables is the baseline. Various magnet configurations with HTS tapes are under investigation and also use of Bi-2212 round wire based cables is considered. The paper presents the structure of the collaboration and describes the main choices made in the first year of the program, which has a breadth of five to six years of which four are covered by the FP7 frame.

A design study is presented for the coil layout of the EuCARD-2 Fiver Tesla HTS Research (FeaTHeR) magnet. The angular dependence of the critical current in the used ReBCO Roebel cable is taken into account. This leads to a new coil layout named aligned block. This layout makes optimal use of the anisotropy of the ReBCO coated conductor, by aligning all tapes with the magnetic field lines. Both two dimensional cross sections and three dimensional coil layouts are presented. In the layouts the magnetic field angle is highest at the edges of the cable causing a large variation of the critical current over its width. Different approaches to the calculation of the critical current, with and without current sharing in and between the tapes, are presented. The values are compared to the values found using a non-linear network model of the cable, in which the electrical properties of the elements are calculated as a function of magnetic field and magnetic field angle. The model also includes electrical contact between the strands using additional network elements.

Formation of bismuth-based high-Tc superconducting ceramics 2212 and 2223 with partial replacement of Bi by p-elements of group III (Ga, In, Tl), IV (Ge, Sn, Pb), or V (Sb) was studied. XRPD, DTA/TG, SEM/EDX, SQUID, and VSM were used for phase identification, structural analysis, grain morphology studies, chemical analysis and magnetic measurements.

This paper reports on the cryogenic fatigue life of Al 2024 / Stycast 2850 FT composite sandwiches loaded under cyclic strain, as well as on the strength of their constituent materials at 77 K. These Fibre Metal Laminate (FML) specimen serve as a model for an alternative class of cryogenic structural materials that might be used e.g. in downstream LNG applications. FMLs, such as the GLARE ™, are already used in the aeronautic industry, where they provide better damage tolerance, corrosion resistance and lower specific weight. Their cryogenic performance, however, is yet to be understood. Preliminary results show that the metal/filled- epoxy combination presented here withstands repeated cool-down to 77 K. Moreover, its cryogenic fatigue life is at least 20 times longer than at room temperature. These observations are consistent with the measured stress-strain behaviour of the metal and the epoxy, as well as with the shear strength of the bond between them. The Youngs modulus, yield strength and tensile strength of the Stycast 2850 FT roughly double when cooled down to 77 K. In addition to this, the bond strength with the GLARE-type coated Al increases significantly. These preliminary experiments indicate that cryogenic FML are technically feasible. © 2015 Published by Elsevier B.V.

The reversible axial strain dependence of the critical current of MgB2 conductors is shown to vary with the temperature and magnetic field. The measured critical temperature and irreversibility field are also found to change reversibly with the axial strain. Combining these effects, we show empirically how the strain dependence of the whole critical surface can be scaled with just three parameters: the strain dependences of its three corner points.

Single-core MgB2 composite wires prepared by in situ and ex situ PIT processes were subjected to axial tension and pressure. Their transport critical currents Ic were measured as a function of axial strain e with a U-shaped spring set-up at liquid helium temperature and external magnetic fields ranging from 3.5 to 6 T. For all wires, Ic changes linearly and reversibly with applied strain, up to a sample-dependent reversible strain limit eirr. The value of eirr depends mainly on the degree of pre-compression, on the mechanical properties of the sheath and also on the structure of the MgB2 core. In contrast to ex situ wires, the aspect ratio of in situ wires does not affect the value of eirr. On the other hand, in situ wires are typically more sensitive to axial strain, i.e. they display a stronger reversible Ic variation and a steeper irreversible degradation, which is attributed mainly to the higher porosity of in situ cores.

A-priori knowledge of the normal zone development in MgB/sub 2/ conductors is essential for quench protection of applications. Therefore the normal zone propagation in a monofilament MgB/sub 2//Fe conductor under near-adiabatic conditions at 4.2 K has been measured and simulated. The results show normal zone propagation velocities up to several meters per second. In addition, by including the voltage-current relation into the computational model, the influence of the n-value on the normal zone propagation is determined. The simulations show that lower n-values suppress the normal zone propagation velocity due to lower heat generation in the MgB/sub 2/ filaments.

Addition of tungsten metallic particles has led to an improvement of the current carrying capacity of ex situ MgB2 wires, due to an improved internal stability. The best performance was observed for 5–10 wt% of W addition and therefore this content was also used for multi-core wires made by the rectangular wire-in-tube technique (RWIT). Transport current measurements show how the variable wire shape (rectangular, circular or tape) as well as additional mechanical deformations (twisting, bending and axial stresses) influence the critical current density of four-filament MgB2–W/Fe wires in external magnetic fields ranging up to 11 T. Critical current degradation by twisting, bending at room temperature and axial stresses at 4.2 K are discussed. It was found that the addition of W particles has no effect on the value of the irreversible strain limit.