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For the High Luminosity Upgrade project (HL-LHC) of the CERN Large Hadron Collider (LHC), lower $\beta$ * quadrupole magnets based on advanced Nb $_\text{3}$ Sn conductors will be installed on each side of the ATLAS and CMS interaction points. To quantify the endurance and technological limits of these magnets, beyond their maximum operational conditions, two short length model magnets have been extensively tested at the CERN SM18 test facility. Both magnets were subjected to eight thermal cycles. One of them was trained beyond its ultimate current (17.89 kA, corresponding to 143 T/m field gradient and 12.2 T peak field), reaching a maximum of 19.57 kA at 1.9 K (corresponding to 155 T/m, 13.4 T peak field and 95.4% of the short sample limit) in a 150 mm diameter bore. This magnet currently has the record highest field gradient of this quadrupole magnet class. The second short model had zero re-training quenches up to nominal (16.47 kA) and ultimate current at 1.9 K during the thermal cycles; more than 1000 current cycles to nominal current; and provoked quenches to simulate the most severe failure scenarios of the protection system. After all these tests, both magnets continue to perform beyond requirements for operating current and temperature. In this paper, the tests performed on the two magnets are discussed.

A large aperture Nb 3 Sn dipole is proposed to replace the magnet assembly of EDIPO, which was irreversibly damaged in 2016. The goal is to generate a background field of 15 T at 4.2 K in a clear aperture of approximately 100×150 mm 2 and over a uniform length of 1000 mm in order to test superconducting cables for both fusion and high-energy physics applications. The magnet features a block-type coil design wound with wide Rutherford cable (two alternative coil cross sections are considered) and supported by a mechanical structure based on keys-and-bladders technology. In the end regions, the coils tilt up (flare) through a hard-way bend of the cables to provide room for the test well, following a layout already adopted in the LBNL HD2 and CERN-CEA FRESCA2 magnets. The two considered coil design alternatives aim at minimizing the mechanical stress in the coil windings. One coil pack design makes the use of two double pancake coils per pole, whereas the other alternative features three double pancakes per pole. Both design options are presented focusing on the results of numerical computations carried out with finite-element models to investigate peak stresses in the coils during room-temperature pre-loading, cool down, and powering.

As part of the Large Hadron Collider (LHC) accelerator upgrades foreseen by the high luminosity-LHC project, the CERN 11 T program is aimed at replacing standard LHC Nb-Ti main dipole magnets, operating with a bore field of 8.3 T, with pairs of shorter Nb 3 Sn dipole magnets with a bore field of 11 T and the same total integrated field, thus providing space for additional collimators in the dispersion suppressor region. At the time of the submission of this paper, six single-aperture and two double-aperture short models have been fabricated and tested. As a result of a degraded quench performance observed in some of the short models, attributed to excessive stress on the Nb3Sn coil mid-planes, a thorough investigation of the room temperature loading procedure, and in particular of the collaring process, has been launched. A 150-mm-long collared coil mockup, instrumented with strain gauges and pressure sensitive films, has been used to study the peak stresses experienced by the brittle and strain sensitive Nb 3 Sn cables in the different phases of the collaring and as a function of coils' size and collaring force. In this paper, the results of the test campaign are described.

The planned upgrade of the Large Hadron Collider collimation system requires the installation of 11 T Nb 3 Sn dipole magnets in the dispersion suppressor areas. Due to the large stored energy density and the low copper stabilizer section, the quench protection of these magnets is particularly challenging. The baseline protection scheme after installation in the main dipole circuit is based on quench heaters and a bypass diode. The maximum allowable temperature during quench has a primary importance. In one of the latest short model magnets, full protection studies were performed up to a quench integral and hot spot temperatures well beyond the design value in order to understand the limits. Measurements are compared to electro-transient and thermo-mechanical models to evaluate quench propagation, temperature rise in the conductor, and thermal stress due to temperature gradients in the coil and surrounding structure.

For reaching very high magnetic fields in fully superconducting magnets, beyond 16 T for particle accelerators dipoles and beyond 23 T for solenoids, the use of high-temperature superconductors (HTS) is unavoidable. Due to the high minimum quench energy in HTS these coils are much more difficult to protect against quenches using conventional methods, such as quench heaters or coupling loss induced quench (CLIQ). Although it is possible to use a dump resistor on a short HTS magnet, extracting the energy externally, this does not provide a solution for longer magnets or magnets operated in a string, because the extraction voltage becomes unacceptably high. Here, a method named E 3 SPreSSO (External Energy Extraction Symbiotic Protection System for Series Operation) is proposed that allows for fast energy extraction in HTS magnets. The E 3 SPreSSO comprises of units with a near-zero self-inductance superconducting circuit, connected in series with the main magnet. When the protection is triggered, these devices are turned resistive, using quench heaters, overcurrent or CLIQ, causing them to absorb the energy of the system. The units can be located outside the main magnet and do not generate magnetic field. Therefore, it is possible to use relatively cost-efficient and robust Nb-Ti or possibly MgB 2 (at higher temperatures). This paper introduces the concept and provides an analytical method weighing the different options for designing the E 3 SPreSSO units themselves.

The EuCARD-2 project, co-funded by the European Commission, supports the design, production, and standalone cold testing of a high-temperature superconducting demonstrator insert-magnet. The so-called Feather-M2 dipole magnet was built at CERN, using Roebel cable consisting of REBCO coated conductor tapes. In this paper, we will discuss the preparation and results of the magnetic measurements performed on the Feather-M2 model magnet, operated in forced-flow helium gas. For this test, the magnet was instrumented with cryogenic Hall-Sensors, which are cross-calibrated “ in situ ” using induction-coil sensors. We present the measurement procedure, the instrumentation used, and the measurement results. These include the transfer function and dynamic field effects. The most exciting scientific result is the low level of persistent current effects observed in this model magnet.

A large number of training quenches at various currents, temperatures, and ramp rates, have been performed on six 11 T dipole model magnets. Quenches in the midplane of these magnets were of special interest, since the quench current in the last three models measured in 2016 was limited to between 84% and 92% of the magnets short sample limit. Measurements of quench propagation velocity, based on both voltage taps and quench antennas, yield a high propagation velocity of 50 to 80 m/s. Due to the high magnetic field gradient over the width of the midplane turn such a high propagation speed cannot be explained by propagation in longitudinal direction of the strand following the twist pitch. In these cases, current and heat sharing at the thin cable edge (where the field, stress, and cable compaction are high) are likely to provoke strand-to-strand quench propagation at higher velocities than along the strands. This investigation is focused on analyzing the quench propagation along the strands and strand-to-strand of various measured cases.

The aim of this paper is to estimate the change of the superconducting properties T c, J c, and B c2 in the quadrupoles of the future accelerators HiLumi-LHC and FCC, submitted to irradiation by multiple high-energy sources. The estimations are performed based on the number of displacements per atom ( dpa ), calculated by means of the FLUKA code. It is found that the dpa can be used as a single parameter for describing the radiation induced change of T c, the degree of atomic order parameter S , and the lattice parameter a in the case of multiple energy sources. The mechanism leading to the enhancement of $J_{\text{c}}$ after proton and neutron irradiation is analyzed based on a possible correlation with the concentration of Frenkel defects produced by the irradiation. Experimental data on J c/ J co versus dpa at 4.2 K allow estimating J c in the quadrupoles of HiLumi-LHC and FCC/Run1, where the values of dpa for lifetime are 2.5 × 10−4 and 5 × 10−4, respectively. The dpa in FCC/Run2 is much higher, 3 × 10 −3, thus reaching well beyond the maximum of J c versus dpa. An accurate estimation of the variation of T c, J c, and B c2 is performed based on earlier irradiation data with 14-MeV neutrons.

The high-luminosity Large Hadron Collider (HL-LHC) project aims at allowing to increase the collisions in the LHC by a factor of ten in the decade 2025-2035. One essential element is the superconducting magnet around the interaction region points, where the large aperture magnets will be installed to allow to further reduce the beam size in the interaction point. The core of this upgrade is the Nb3Sn triplet, made up of 150-mm aperture quadrupoles in the range of 7-8 m. The project is being shared between the European Organization for Nuclear Research and the US Accelerator Upgrade Program, based on the same design, and on the two strand technologies. The project is ending the short model phase, and entering the prototype construction. We will report on the main results of the short model program, including the quench performance and field quality. A second important element is the 11 T dipole that replaces a standard dipole making space for additional collimators. The magnet is also ending the model development and entering the prototype phase. A critical point in the design of this magnet is the large current density, allowing increase of the field from 8 to 11 T with the same coil cross section as in the LHC dipoles. This is also the first two-in-one Nb3Sn magnet developed so far. We will report the main results on the test and the critical aspects.

The Roebel cable utilized in High Field accelerator magnets is subject to high transversal electromagnetic forces. The conductor response to exerted pressure depends from the geometry and materials of the cable. A transverse loading test was performed for an impregnated cable in cryogenic conditions. The test revealed Roebel cable being able to withstand elevated average pressure level common to dipole magnets, when the pressure load is exerted by a stiff press tool. However, the mechanism for irreversible current degradation during the transverse loading during powering remains so far unknown. This paper focuses on finding likely failure mechanisms when a magnet is powered. The cable is wound with a glass-fiber sleeve and impregnated with epoxy. Epoxy has much lower stiffness than the coated conductor. When the cable is subjected to transverse loading, abrupt changes in cable thickness and material properties may lead to irreversible degradation of the conductor. As the tape crosses the epoxy-filled central gap region of the cable, the discontinuous change of the support stiffness generates bending strains and shear stress in the conductor. The cable is mechanically modeled. By modeling, the measured axial strain limit of the conductor is connected to transverse pressure limit of the cable.

Rutherford cables for particle accelerator magnets can be subjected to a time-varying magnetic field during the typical accelerator operating cycle, which in turn induces coupling currents flowing in the loops formed at the contacts between different strands. A proper analysis of the losses generated by these currents must be carried out for the design of the cryogenic system. A continuum model with nonuniform contact conductances between the strands was recently developed, starting from a previous continuum model based on uniform conductances between strands along the cable length. The model allows one to attain the required level of detail in the description of the interstrand currents. This model is applied here to analyze the effect of the insertion of a resistive core between the two layers of the cable, and to assess quantitatively the impact of the main core parameters, such as its width, positioning, and electrical resistance.

The large hadron collider (LHC) at CERN has been operating and generating physics experimental data since September 2008, and following its first long shut down, it has entered a second, 4-year-long physics run. It is to date the largest superconducting installation ever built, counting over 9000 magnets along its 27-km long circumference. A significant operational experience has been accumulated, including the occurrence and consequences of electrical faults at the level of the superconducting magnets, as well as their protection and instrumentation circuits. The purpose of this paper is to provide a first overview of the most common electrical faults and their frequency of occurrence in the first years of operation, and to perform a statistical analysis that can provide reference values for future productions of similar dimensions and nature.

The upgrade of the LHC collimation system requires the installation of 11 T Nb3Sn dipole magnets. Due to the large stored energy density and the low copper stabilizer section, the quench protection of these magnets is particularly challenging. A total of ten coils assembled in five single aperture and one double aperture short model magnets have been tested at CERN with different heater to coil insulation lay outs. This paper reports on the test results of the model program, which are used to validate numerical models and to optimize the quench protection performance. A parametric analysis on the impact of different conductor and operation parameters on the peak temperature is presented. Coil voltage to ground and turn-to-turn voltages are also evaluated under nominal conditions and failure case scenarios.

The luminosity upgrade of the Large Hadron Collider (LHC) at CERN requires the installation of additional collimators in the dispersion suppressor regions of the accelerator. The upgrade foresees the installation of one additional collimator on either side of interaction point 7 (IP7) at the location of the existing main dipoles (MBs) that will be replaced by shorter and more powerful dipoles, and of one additional collimator on either side of IP2 at the location of existing empty cryostats. This paper describes the design and the construction status of the full-length prototype of the 11-T dipole magnet, which is needed for IP7. This magnet features a two-in-one structure, like the LHC MB, impregnated coils made of Nb3Sn conductor, an inner bore of 60 mm, and a magnetic length of about 5.3 m. Two 11-T magnets are needed to replace a 15-m long MB. A by-pass cryostat placed in between the two magnets allows creating a room temperature space for the additional collimators. The magnet is designed to provide the same integrated field as the MB at nominal field. However, due to the difference in transfer function at lower field, a correction by means of a trim current has been considered. A full-length prototype is currently under construction at CERN with the goal of developing the manufacturing and inspection procedures prior to launch the series production. For this, new tooling has been developed and optimized during the fabrication of fully representative practice coils. This paper describes the design of the magnet, the main manufacturing steps, and corresponding quality indicators, which will be used to monitor the series production. Finally, the production and installation schedule will be presented.

Advances in the performance of low- and high-temperature superconductors enable the development of advanced magnets for a range of applications, including tokamaks for fusion energy, dipoles, and quadrupoles for hadron colliders, and solenoids for nuclear magnetic resonance studies. The capability of testing prototype cables and insert coils at high field is critical to these developments. We present here the conceptual design of a test facility dipole with features suitable to support the advanced magnet development efforts of both Fusion and HEP communities. In particular, a background field of 15 T is provided over a minimum homogeneous length of 1000 mm, and the magnet clear bore of approximately 144 × 94 mm can accommodate large fusion conductors as well as prototype coils for high field dipoles, along with flexible cryogenic and mechanical provisions for sample characterization. Two technical solutions are considered. The first uses a forced flow cable-in-conduit conductor and follows the design developed by EFDA for the EDIPO magnet. The second uses a bath cooled Rutherford cable and follows a block-coil design similar to the CERN FRESCA2 dipole, and the LBNL HD and LD1 dipoles. Following a comparison between the two approaches, we present two designs for the Rutherford cable approach including performance objectives, key parameters, and preliminary magnetic, mechanical and quench protection analysis.

For the upgrade of the LHC, two 15-m NbTi main dipole magnets are foreseen to be replaced by two 11 T 5.7-m-long Nb 3Sn dipoles each. A series of model magnets has been produced to verify the design choices that are important for the prototype and series production, focusing on mechanical aspects and protection studies. A fourth and a fifth 2-m single aperture models were produced, assembled, and tested. In this paper, the cold powering tests of the single aperture models SP104 and SP105 will be presented and the results will be compared with the previous models. Special attention will be given to the upper limit in magnet current due to quenches in the midplane turns.

Scaling analysis of several thousand Nb 3 Sn critical-current (I c ) measurements is used to derive the extrapolative scaling expression (ESE), a fitting equation that can quickly and accurately extrapolate (or interpolate) limited datasets to obtain full three-dimensional dependences of I c on magnetic field (B), temperature (T), and mechanical strain (e). Unlike nonextrapolative fitting equations, the ESE relation is determined completely by fundamental raw scaling data from master pinning-force curves. The results show that extrapolation errors with ESE approach typical I c measurement errors. The scaling expression is simple and robust, providing straightforward extrapolation capability for conductor characterization and magnet design. The ESE relation offers the prospect for extrapolations in several new areas, including: a reduction in the measurement space required for full I c (B, T, e) characterization to about one fifth the size; ability to combine data from separate temperature and strain apparatuses; extrapolation of transport I c data from above 4 K to lower temperatures, where heating effects and instabilities are problematic for transport measurements; and extrapolation of full I c (B, T, e) datasets from as little as a single I c (B) curve when several core parameters have been measured in similar conductors (particularly applicable to qualifying production wires). Accuracies are evaluated for concatenations of these different extrapolation capabilities. Examples are given for practical Nb 3 Sn conductors, including those for high luminosity-LHC magnets, ITER, and cryocooled NMR magnets.

In 2015, the 1232 Nb-Ti dipole magnets in the Large Hadron Collider (LHC) have been commissioned to 7.8 T operational field, with 172 quenches. More than 80% of these quenches occurred in the magnets of one of the three cold mass assemblers (3000 series), confirming what was already observed in 2008. In this paper, the recent analysis carried out on the quench performance of the Large Hadron Collider dipole magnets is reported, including the individual reception tests and the 2008 and 2015 commissioning campaigns, to better understand the above-mentioned anomaly and give an outlook for future operation and possible increase of the operational field. The lower part of the quench probability spectrum is compatible with Gaussian distributions; therefore, the training curve can be fit through error functions. An essential ingredient in this analysis is the estimate of the error to be associated with the training data due to sampling of rare events, allowing to test different hypothesis. Using this approach, an estimate of the number of quenches required to reach 8.3 T (corresponding to the 7 TeV nominal energy) is given, and we propose to have two LHC sectors trained toward this target before the next warm up of the LHC.

The high-luminosity large hadron collider (LHC) project at CERN entered into the production phase in October 2015 after the completion of the design study phase. In the meantime, the development of the 11 T dipole needed for the upgrade of the collimation system of the machine made significant progress with very good performance of the first two-in-one magnet model of 2-m length made at CERN. The 11 T dipole, which is more powerful than the current main dipoles of LHC, can be made shorter with an equivalent integrated field. This will allow creating space for the installation of additional collimators in specific locations of the dispersion suppressor regions. Following tests carried out during heavy ions runs of LHC in the end of 2015, and a more recent review of the project budget, the installation plan for the 11 T dipole was revised. Consequently, one 11 T dipole full assembly containing two 11 T dipoles of 5.5-m length will be installed on either side of interaction point 7. These two units shall be installed during the long shutdown 2 in years 2019-2020. After a brief reminder on the design features of the magnet, this paper describes the current status of the development activities, in particular the short model programme and the construction of the first full scale prototype at CERN. Critical operations such as the reaction treatment and the coil impregnation are discussed, the quench performance tests results of the two-in-one model are reviewed and finally, the plan toward the production for the long shut down 2 is described.

Two different processes are known to create “artificial” pinning centers in superconductors: 1) high energy irradiation; and 2) nanoinclusions. Own results on the critical current density J c of Nb3Sn wires after irradiation by protons are compared to the effects obtained by neutrons (on the same wires) and by nanoinclusions from the literature. Characteristic differences and similarities are discussed. After high energy irradiation, the observed enhancement of J c is attributed to the formation of defect clusters. The contributions due to grain boundary pinning on one hand and of point pinning on the other can be separated, using a two-component model. This is in contrast to wires with nanoinclusions, where point pinning effects occur simultaneously to those due to enhanced grain boundary pinning, as a consequence of a reduction of the A15 grain size. In both cases, the enhancement Δ J c in Nb3Sn wires decreases gradually with increasing applied field and vanishes at Bc2. In binary Nb3Sn wires with oxide nanoinclusions, an increase of J c by 100 A/mm2 with respect to the present RRP wires is expected at fields between 17 and 18 T.

As a part of the large hadron collider luminosity upgrade (HiLumi-LHC) program, CERN is planning to replace some of the 8.33-T 15-m-long Nb-Ti LHC main dipoles with shorter 11 T Nb3Sn magnets providing longitudinal space for additional collimators. Whereas the present design of the 11 T dipole enables the use of RRP conductor with critical current degradation after cabling at the level of 5%, new cross sections of the cable have been studied in order to further decrease the degradation of both critical current and resistivity of the copper matrix. This change is particularly beneficial for the PIT conductor. The coil layout is reoptimized to accommodate the new cable geometry, using the ROXIE code. A set of additional design changes are implemented, such as reduction of the outer yoke diameter. In this paper, we review the main parameters of the present design, describe the changes implemented in the new design, and discuss their impact on both the electromagnetic and structural properties.

In this paper we summarize the evolution and contributions of superconducting magnets to particle accelerators as chronicled over the last 50 years of Particle Accelerator Conferences (PAC, NA-PAC and IPAC). We begin with an historical overview based primarily on PAC Proceedings augmented with references to key milestones in the development of superconducting magnets for particle accelerators. We then provide some illustrative examples of applications that have occurred over the past 50 years, focusing on those that have either been realized in practice or provided technical development for other projects, with discussion of possible future applications.

The electrical resistance of Large Hadron Collider main busbar cable lap splices produced by soft soldering has been measured with two independent methods as a function of intercable contact area and for splices made of cables with various defects. For defect-free lap splices, the resistance increases from 0.3 to 10 nΩ (at 4.3 K in self-field) when reducing the cable overlap length from 120 to 3 mm, as expected assuming that the resistance is inversely proportional to the intercable contact area. The resistance of bridge splices that connect side-by-side cables can be predicted from the lap splice resistances and the overlap areas involved.

In this paper we discuss results of a one-dimensional numerical model to simulate electrical and thermal transients in the superconducting cables for the LHC machine, with specific reference to the analysis of quench due to the heat released by beam losses. Two models have been developed, one based on the analysis of the behavior of a single strand, whereas the other accounts for all the strands in the multi-strand cable. As a first step the stability margin is computed considering the single strand subjected to a variable magnetic field and heat deposit along its length. The impact of the field non-uniformity is assessed by com- parison with computations performed assuming a uniform field in the cable cross section. The results of this model are compared to those obtained with the multi-strand model. The numerical results are discussed in terms of current and heat redistribution between strands, and stability margin of the cable.

Recent studies highlight a renewed interest in a 100-TeV hadron collider as the next HEP discovery machine. The arc dipoles are the major technology challenge and cost driver for this application. Among the possible magnetic layouts, block coils offer several potential advantages in terms of high conductor packing, separation between the high-field and high-stress locations, use of flat cables, and fewer parts of simple geometry. In order to experimentally assess these properties, several short models were designed, fabricated, and tested at LBNL during the last decade (HD series). In this paper, we discuss the design features and performance characteristics of Nb 3 Sn block-coil dipoles in the design range of interest, in light of the experimental experience provided by the HD dipoles, and outline areas where further development is required to fully evaluate the potential of this approach for a 100-TeV collider.

The HQ magnet is a 120 mm aperture, 1-meter-long Nb$_{3}$Sn quadrupole developed by the LARP collaboration in the framework of the High-Luminosity LHC project. A first series of coils was assembled and tested in 5 assemblies of the HQ01 series. The HQ01e model achieved a maximum gradient of 170 T/m at 4.5 K at LBNL in 2010-2011 and reached 184 T/m at 1.9 K at CERN in 2012. A new series of coils incorporating major design changes was fabricated for the HQ02 series. The first model, HQ02a, was tested at Fermilab where it reached 98% of the short sample limit at 4.5 K with a gradient of 182 T/m in 2013. However, the full training of the coils at 1.9 K could not be performed due to a current limit of 15 kA. Following this test, the azimuthal coil pre-load was increased by about 30 MPa and an additional current lead was installed at the electrical center of the magnet for quench protection studies. The test name of this magnet changed to HQ02b. In 2014, HQ02b was then shipped to CERN as the first opportunity for full training at 1.9 K. In this paper, we present a comprehensive summary of the HQ02 test results including: magnet training at 1.9 K with increased pre-load, quench origin and propagation, and ramp rate dependence. A series of powering tests was also performed to assess changes in magnet performance with a gradual increase of the MIITs. We also present the results of quench protection studies using different setting for detection, heater coverage, energy extraction and the Coupling-Loss Induced Quench (CLIQ) system.

Next generation of dipole magnets with a field higher than 16 T are considered for future particle colliders. To do so, combined-technology magnets, made of Nb-Ti, Nb3Sn and high-temperature superconductor (HTS) materials, have to be developed to reduce the cost of such a magnet. Therefore, in the framework of the European Coordination for Accelerator Research and Development (EuCARD-2) project, many HTS dipole magnet designs have to be investigated to find the most effective design for the HTS insert in a graded magnet. This paper discusses the cos θ option. A 5-T stand-alone configuration of the HTS accelerator magnet (the first goal of EuCARD-2) appears to be achievable, whereas mechanical stress distribution shows that its use as an insert in a graded magnet is very challenging. This paves the way for alternative designs as the so-called slot or motor-like design, which is briefly introduced here.

A new cryogenic dc current transformer (Cryo-DCCT) has recently been designed and assembled at CERN. The device, whose design is based on that of a high-accuracy 600 A market solution suitable for room temperature applications, is optimized for measuring currents of up to 80 kA and for operation at 4.2 K. The CryoDCCT has been conceived with the objective of preserving the metrological performance of the original commercial device in the new extended range of operation. For reducing the effect of interfering magnetic fields arising from test conditions, it incorporates ferromagnetic and MgB 2 superconducting shields. In this paper, the design of the CryoDCCT and the results of the commissioning of the device at CERN are reported. The effectiveness of the current transducer is analysed and discussed. This new device will be used for measuring the secondary current of a 80 kA superconducting transformer feeding a sample of NbSn 3 cable at the Facility for Research on Superconducting Cables (FRESCA) at CERN.

A recent proton irradiation study on Ta and Ti alloyed industrial multifilamentary Nb 3 Sn wires has now been extended to fluences up to 1.38 × 10 21 p/m 2 , the Bragg peak region being located outside of the wire. In the present work, magnetization measurements were performed up to 14 K and 10 T, to follow and determine with more precision the development of the upper critical field B c2 with fluence. It was found that the critical temperature, T c , decreases linearly with increasing fluences, about 3% up to the highest fluence 1.38 × 10 21 p/m 2 . The transition width does not change after irradiation, thus reflecting a homogeneous damage in analogy to neutron irradiation. Both the critical current density, J c , and the upper critical field, B c2 , were found to increase in the considered fluence range. It was obtained that the larger enhancement of J c , (about 45% for Ta alloyed wires and 100% for Ti alloyed wires at 10 T) is not correlated to that of B c2 (about 5% for Ti alloyed and 10% for Ta alloyed up to the highest fluence). The enhancement of J c in Ta alloyed wires is very similar for both PIT and RRP processing, thus assigning a major importance to the nature of the additive. The present results after irradiation were analysed applying the two-mechanism model on the volume pinning force, taking into account both grain boundary pinning and point pinning. A comparison between the present results and those achieved after neutron irradiation on the same Nb 3 Sn wires shows that protons cause considerably higher damage than neutrons: the same effect on J c and T c is already observed at fluences being one order of magnitude smaller.

Roebel cables made from REBCO coated conductors meet the requirements of accelerator magnets for high-current carrying capability, compactness and transposition. Previous work evidenced that the transverse stress distribution and the electrical protection of the first generation Roebel cables measured at CERN had to be improved. New Roebel cables incorporating copper stabilizer have been designed, produced at KIT and extensively characterized at CERN at 4.2 K and in external fields of up to 9.6 T. In this paper, we report on the optimisation of the cable design and on the results of the electrical and mechanical measurements performed at CERN on a new generation of cables.

Some of the bronze-route Nb3Sn wires produced for the ITER toroidal field magnets have values of the Cu residual resistivity ratio (RRR) lower than the specification (RRR > 100) when reacted with the longest ITER heat treatment cycle (cycle A: high-temperature plateau 200 h at 650 ◦C). As the low RRR value was suspected to be due to the Cr plating, CERN has carried out extensive investigations on two different Cr-plated Nb3Sn wires in order to assess how the RRR is influenced by the Cr plating. Each type of wire was reacted with three heat treatments differing for the duration of the high-temperature plateau (from 100 to 200 h). The presence of Cr in the Cu stabilizer was investigated using a transmission electron microscope together with energy-dispersive X-ray spectrometry. In the most critical wire reacted for 200 h, CrS nanoparticles and Cr in solid solution (0.02 at.%) with Cu were found up to 13 μm from the Cr–Cu interface. At larger distance, we could not quantify the Cr concentrations because of the detector sensitivity limit (about 0.016 at.% Cr in Cu). We provide a model that can explain the measured RRR degradation purely in terms of the Cr contamination that we observed.

Within the framework of the European Fusion Programme (EFDA) a design activity has been started in late 2004 by the EFDA Close Support Unit at Garching (FRG) to design a 12.5 T superconducting dipole magnet. The final goal of this activity is to define a "minimum cost" and "minimum risk" dipole to be built within the next 3 years by European industries. The newly built magnet shall be hosted in one of the existing cryogenic laboratories in the EU to test, in particular, the full size conductor samples that shall be produced during the ITER magnets procurement. As a result of this study-carried out in cooperation with several European laboratories-different concepts have been analyzed and optimized. All designs are based on the use of high current density Nb3Sn strands (j(c) similar to 2000 A/mm(2) at 12 T/4.2 K). It is assumed that these strands can be cabled either as Cable In Conduit Conductors (CICC) or as Rutherford-type Cables (RC) and then wound as planar racetrack, non-planar (saddle) or cos theta coils. The aim of this paper is to provide an overview of this study activity with particular emphasis to the optimization steps followed in the design of each magnet concept. Four different types are described: a planar CICC/RC-based coil, a saddle CICC-based coil and a cos theta RC-based coil. Each design is described together with the salient optimization steps that include electromagnetic, structure mechanic and thermo-hydraulic analyses as well as different manufacturing constraints.

We present here the results of the analysis of the stability experiment SeCRETS, performed on two Nb/sub 3/Sn cable-in-conduit conductors with the same amount of total copper stabilizer, but different degree or segregation. The model used for the analysis, including superconducting strands, conductor jacket and helium, is solved with the code Gandalf/sup /spl trade//. We obtain a qualitative agreement of simulation results and experimental values. The simulation results confirm that in the operation regime explored in the experiment the segregated copper is not effective for stability. The details of the current sharing and the approximation taken for the transient heat transfer are shown to be critical for the interpretation.

Known expressions, for the critical temperature, critical field and pinning force in NbTi are combined into a self-consistent fit formula that provides the critical current density as a function of temperature and field. The main advantage of such a fit is the extended validity range. Data available in literature and measurements on LHC strands are used to demonstrate the accuracy of the fit. The J/sub c/ data-sets used to cover a range of field from 0 T to 9 T and temperature from 1.9 K to 9 K. The standard deviation of the fits presented is of the order of 5% or better. This accuracy is generally sufficient for design purposes, extrapolation and scaling of measured results. Better accuracy, e.g. for short sample limit prediction, can be achieved restricting the domain of validity.

High performance Nb/sub 3/Sn strands for the high field ITER (International Thermonuclear Experimental Reactor) magnets have been manufactured using diffusion barriers among the multifilamentary cores and the stabilizing copper. In the conductor development programme, billets using a single Ta or a double NbV barrier have been assembled. The barrier is shown to have an influence on magnetization characteristics. The measurements on samples from the two types of strands are shown and analyzed to correlate them with the magnetic behaviour of Nb at low field. The results show that at low field hysteretic losses are greatly increased by the pure Nb of the barrier. At high field, losses of samples with Ta barrier are larger, due to their higher J/sub c/. >

Designers of conductors for ITER TF coils selected Nb/sub 3/Sn as the strand material. As the influence of stress and strain on the superconducting property in Nb/sub 3/Al conductors is smaller than in Nb/sub 3/Sn conductors, Nb/sub 3/Al has become a potentially attractive candidate material. However, its industrial production has not yet been developed at the level of Nb/sub 3/Sn. Of interest is the evaluation of the use of cable-in-conduit-conductors for ITER TF coils. The assessment is carried out considering alternative jacket material (stainless steel), manufacturing approaches (winding of reacted conductors) and performance of the final conductor in the operating conditions of ITER TF coils. >

The coupling current losses of a full size, fusion dedicated Nb/sub 3/Sn cable-in-conduit superconductor are measured as a function of the transverse load in the SULTAN III test facility. Applying a broad range of transport current (up to 56 kA) and background field (up to 12.1 T), the change of the transverse resistance is observed through the shift of the O-crossing in the pick-up coil voltage. Despite the difficulty to extract useful information from the signal, the role of the strand Cr plating as resistive barrier is experimentally assessed under operating conditions. The friction factor of the conductor is also evaluated from pressure drop measurements. >

A significant amount (over 70 runs) of experimental data on stability margin and quench behaviour in a two-dimensional, forced-flow cooled NbTi winding pack has been collected. The measurements performed over a wide range of operating conditions (operating current, field and temperature) have been used as the experimental data-base for the comparison with the results of the simulations done using quench analysis code SARUMAN. Here we present the results of the experimental and validation activities, which show that SARUMAN ran successfully predict stability margin and quench propagation in a 3-D winding. >

We present the structure and features of a quench code for the analysis of normal operation and safety-relevant transients. The code gives a self-consistent simulation of 3-D quench development in the winding pack of a force-flow cooled coil, including the evolution of the operating current and magnetic field and the change of the helium state at the coil inlet and outlet manifolds due to the transient response of the cryogenic plant. >

An optimisation procedure for the design of CICC's is applied to a parametric scan on the effect of the superconductor critical properties (Jc, Tc) operating conditions (B, strain) and other design parameters (RRR, strand diameter, limit on the strand Cu:non-Cu ratio). The parameter with the strongest influence on the optimal design performance is Tc, while surprisingly large variations in Jc can be accommodated at minor loss in performance. >

A method for the calculation of the hysteresis loss in a superconducting slab for large, bipolar changes of the applied magnetic field is presented. The method is purely analytical and uses the Kim model for the critical current density J/sub c/. Arbitrary change in the external magnetic field can be handled, before or after full penetration. Magnetization, dissipated power and energy are computed as a function of time, independent of the presence of closed loops in the magnetic field change. Scaling relations are given to extend the results to the hysteresis loss in a superconducting cylinder in a changing transverse field. An example of an application shows excellent agreement between measured and computed dissipated energy. >

Experiments of quench propagation and stability margin have been performed on a NbTi hollow conductor test module allowing heat conduction only between adjacent layers, while the turns in a layer were thermally insulated. Some of the experimental data on heat propagation and stability margin are reported, and the results of two selected runs are compared with the prediction of a 2-D version of the SARUMAN code. It is shown that the simulation can reproduce the general features of the quench propagation experiments, in one case reaching a quantitative agreement. >

The authors present a procedure for the analysis of the operating margin of a pulsed magnet, based on the sequential calculation of magnetic field, AC losses, temperature, temperature margin, and energy margin. The procedure is suited to the analysis of supercritical helium, force-flow cooled magnets. An example of application to the ITER (International Thermonuclear Experimental Reactor) central solenoid and the diverter coils is given, showing the capabilities achieved by an integrated analysis code and the level of detail in the information gained. >

The major part of the program involves the industrial fabrication of long lengths of full-size conductors and the manufacturing of test coils that will be thoroughly tested in a single test facility (TOSKA, at KfK Karlsruhe, Germany). A total of 3.5 km of 40-kA conductor is required to form circular model coils with an inner bore of 2 m and a peak field of over 13.5 T. Short (5-m) lengths of full-size 40-kA conductors have been manufactured. The present results on the industrial fabrication of 12-T, 40-kA cable-in-conduit conductors for the Next European Torus (NET) indicate that the machine design criteria can be satisfied and that the conductor is suitable for large-scale industrial fabrication.

The transient stability behavior of the different conductors proposed for the Next European Torus toroidal and poloidal field magnets has been studied. A conceptually simple, length-independent computer model has been refined to analyze the time-dependent response of the conductors to sudden energy inputs over a wide range of pulse duration, pulse shape, and transient-heat-transfer coefficients. Some conductors exhibit a sudden transition from a high-stability (well-cooled) to a low-stability (ill-cooled) regime under certain operating conditions. A simple expression is given for prediction of this transition point. Implications for the NET coil designs are discussed. >