Search

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

The Large Hadron Collider luminosity upgrade requires the replacement of standard 8.3 T Nb-Ti main dipole magnets with pairs of shorter 11 T Nb3Sn dipole magnets. After the manufacturing and testing of several 2 m-long 11 T models, the full-length 11 T magnet series production is now ongoing. As a result of a degraded quench performance in several short models, attributed to excessive stress in the coils’ midplanes, an upper hard limit for the coils’ pre-stress was set to avoid any conductor degradation. With the aim of further investigating the influence of a lower pre-stress, an additional double-aperture short model was manufactured. The first aperture was assembled with the standard pre-stress target, in line with the series production. The second aperture was assembled with half of the standard pre-stress. Both apertures were then assembled into a 2-in-1 structure, but connected individually in order to power them independently to allow for comparison studies. Contrary to previously built 11 T magnets, the conductor type is also different as Powder-in-tube (PIT) cable was used, instead of standard Restacked-Rod-Process (RRP) cable. Additionally, the quench heaters for magnet protection were not impregnated as previously done, but placed on the outer radial surface of the coils, in agreement with the final decision for the 11 T series production. This paper describes the manufacturing and testing campaign results of this last 11 T short model.

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.

The critical current of superconducting wires is measured at CERN in four test stations that use high-field superconducting solenoid magnets achieving up to 15 T. The reduction of the uncertainty in the critical-current measurements requires the field mapping of the solenoids in their operating condition. This paper presents the design and application of a solenoidal-field transducer based on a pair of nested induction coils, which are moved along the axis of the magnet. The system allows to extract the longitudinal and transverse field components as a function of the sensor's longitudinal position. The radial dependence of the magnetic flux density can be estimated directly from the nested coils of different diameters validated by applying the theory of pseudo-multipoles in solenoidal magnets. The results and uncertainties of the field mappings are presented.

In a dipole or in a quadrupole accelerator magnet, the displacement of the coil turns induced by the electromagnetic forces can cause quenches limiting the magnet performance. For this reason, an azimuthal preload is applied to avoid azimuthal movements of the coil up to the required operational current. However, several tests showed that accelerator magnets can operate with a partial preload, i.e., that coil unloading during the ramp does not prevent reaching higher currents. This issue is particularly relevant for Nb 3 Sn magnets, where the loads applied to the Nb 3 Sn filaments can reach the degradation limits of critical current. In order to investigate the impact of coil preload on the quench performance, the MQXFS6 short model quadrupole for the High Luminosity Upgrade was tested under an azimuthal pre-load at 80% of the short sample current, reaching 93% of short sample current at 1.9 K. The preload was then released to 60%, still showing ability to operate in the range of 80--85% of short sample current as required by HL-LHC project. With this lower preload, the ability of going above 90% of short sample was lost, and a significant training appeared above 85%. When the preload was restored to the original 80% value, the magnet reached with few quenches 95% of short sample (13.4 T peak field). Magnetic measurements confirm the larger movement of the coil in the case with lower preload, and agree with finite element simulations. In a dipole or in a quadrupole accelerator magnet, the displacement of the coil turns induced by the electromagnetic forces can cause quenches limiting the magnet performance. For this reason, an azimuthal preload is applied to avoid azimuthal movements of the coil up to the required operational current. However, several tests showed that accelerator magnets can operate with a partial preload, i.e. that coil unloading during the ramp does not prevent reaching higher currents. This issue is particularly relevant for Nb$_3$Sn magnets, where the loads applied to the Nb$_3$Sn filaments can reach the degradation limits of critical current. In order to investigate the impact of coil preload on the quench performance, the MQXFS6 short model quadrupole for the High Luminosity Upgrade was tested under an azimuthal preload at 80% of the short sample current, reaching 93% of short sample current at 1.9 K. The preload was then released to 60%, still showing ability to operate in the range of 80-85% of short sample current as required by HL-LHC project. With this lower preload, the ability of going above 90% of short sample was lost, and a significant training appeared above 85%. When the preload was restored to the original 80% value, the magnet reached with few quenches 95% of short sample (13.4 T peak field). Magnetic measurements confirm the larger movement of the coil in the case with lower preload, and agree with finite element simulations. In a dipole or in a quadrupole accelerator magnet, the displacement of the coil turns induced by the electromagnetic forces can cause quenches limiting the magnet performance. For this reason, an azimuthal preload is applied to avoid azimuthal movements of the coil up to the required operational current. However, several tests showed that accelerator magnets can operate with a partial preload, i.e. that coil unloading during the ramp does not prevent reaching higher currents. This issue is particularly relevant for Nb$_3$Sn magnets, where the loads applied to the Nb$_3$Sn filaments can reach the degradation limits of critical current. In order to investigate the impact of coil preload on the quench performance, the MQXFS6 short model quadrupole for the High Luminosity Upgrade was tested under an azimuthal preload at 80% of the short sample current, reaching 93% of short sample current at 1.9 K. The preload was then released to 60%, still showing ability to operate in the range of 80-85% of short sample current as required by HL-LHC project. With this lower preload, the ability of going above 90% of short sample was lost, and a significant training appeared above 85%. When the preload was restored to the original 80% value, the magnet reached with few quenches 95% of short sample (13.4 T peak field). Magnetic measurements confirm the larger movement of the coil in the case with lower preload, and agree with finite element simulations.

Author(s): Mangiarotti, F; Bajas, H; Ambrosio, G; Bajko, M; Bordini, B; Bourcey, N; Duda, M; Desbiolles, V; Feuvrier, J; Fleiter, J; Bermudez, SI; Chiuchiolo, A; Devred, A; Ferracin, P; Fiscarelli, L; Mentink, M; Nobrega, A; Pepitone, K; Ravaioli, E; Schmalzle, J; Todesco, E; Perez, JC; Vallone, G; Willering, G; Yu, M | Abstract: For the high luminosity upgrade of the CERN large hadron collider, lower β∗ quadrupole magnets based on advanced Nb3Sn conductors will be installed on each side of the ATLAS and compact muon solenoid (CMS) experiment insertion zones. As part of the technological developments needed to achieve the required field gradient of 132.6 T/m within a 150-mm aperture, short length model magnets, named MQXFS, are tested both at the CERN SM18 and Fermilab test facilities. The model magnets rely on two types of Nb3Sn conductors (restack rod process (RRP) and powder-in-tube (PIT)) and on an innovative bladders and keys design to provide mechanical support against the Lorentz forces. In 2016 and 2017, the powering tests of the first two models MQXFS3 (RRP) and MQXFS5 (PIT) proved that nominal performance (16.5 kA) could be reached with excellent memory of the quench current after thermal cycle. However both magnets showed a slow training behavior with clear observations of voltage disturbances before the quench. Besides, only MQXFS5 could reach ultimate current (17.9 kA) whereas erratic behavior was observed on MQXFS3 due to conductor local degradation at the head of one of the coils. In 2018, this limiting coil was changed and the applied azimuthal prestress increased. While ultimate current could then be reached, no stable current could be maintained due to identified defect on the outer layer of the new coil. Finally the outcome of the test of the new model MQXFS4, featuring the final RRP conductors that will be used for the series production and variation on the inner layer quench heater designs are here reported in details.

Using magnetization techniques to determine the temperature dependence of critical current in Nb3Sn wires is attractive because of the relative ease compared with using variable-temperature transport measurements. However, there is a known mismatch in the temperature scaling characterizations when using magnetization data compared to transport data. From a practical standpoint, it is the transport properties that matter, as performance prediction, margin calculations, and other aspects of magnet designs rely on the knowledge of the amount of transport current the superconductor can carry in a magnetic field. In this paper, we will identify the underlying issues and propose a procedural solution for determining the temperature dependence of transport critical current in Nb3Sn superconducting wires using magnetization measurements. By using one standard transport measurement at 4.2 K as an ‘anchor’, with this procedural solution it becomes possible to combine the simplicity and economy of quick magnetization measurements at different temperatures with the accurate prediction of transport properties in extrapolated temperature ranges. This study is based on experimental data using internal tin-type wire, but we also address the theoretical implications that would be generally applicable to other wire designs. The strength of our analysis is that our work leads to proposed procedures that improve the accuracy of the temperature scaling even if the assumed pinning curve shape does not fit the data as well at some temperatures, whether those temperatures are close to 4.2 K or to T C.

The inhibition of Nb3Sn grain growth in the presence of ZrO2 nanoparticles appears to be one of the most promising method for pushing the critical current densities of Nb3Sn superconducting wires to levels that meet the requirements set for the Future Circular Collider. We have investigated the effect of ZrO2 nanoparticles formed by the internal oxidation of Zr on the superconducting properties and microstructure of Nb3Sn formed from Nb-1 wt%Zr, Nb-7.5 wt%Ta, Nb-7.5 wt%Ta-1 wt%Zr and Nb-7.5 wt%Ta-2 wt%Zr alloys. A monofilamentary wire configuration was used, with a 0.22 mm outer diameter Nb-alloy tube containing a core of powdered metal oxide (SnO2, CuO or MoO3) as oxygen source and successive deposits of Cu, Sn and Cu on the outer surface. As determined from inductive measurements, the layer critical current densities of the samples based on Nb alloys with internally oxidized Zr were superior to those based on Nb-7.5 wt%Ta. The samples based on Nb-7.5 wt%Ta-1 wt%Zr and Nb-7.5 wt%Ta-2 wt%Zr showed higher critical current densities at high magnetic fields (above 10–15 T), and upper critical fields exceeding 28.5 T at 4.2 K (99% normal state resistivity criterion). A record value of 29.2 T of the upper critical field at 4.2 K was obtained on samples based on Nb-7.5 wt%Ta-2 wt%Zr. Hypotheses are proposed and discussed for explaining this unexpected increase of the upper critical field, by considering the possible effects of non-oxidized Zr on the superconducting properties of Nb3Sn and of the oxidized Zr on the formation and microchemistry of Nb3Sn. Regardless of sample type the Nb3Sn grains observed in our samples have an aspect ratio of 1.5–1.7. When compared in the short axis direction, the mean distance between grain boundary intercepts (lineal intercept method) is ∼40% smaller in the samples with internally oxidized Zr than in the reference samples based on Nb-7.5 wt%Ta. In the long axis direction the reduction is of 20%–30%.

The High Luminosity Large Hadron Collider (HL-LHC) is the new flagship project of CERN. First endorsed in 2013 and approved in 2016, HL-LHC is an upgrade of the accelerator aiming to increase by a factor of ten the statistics of the LHC collisions at the horizon of 2035–2040. HL-LHC relies on cutting edge technologies: among them, large aperture superconducting magnets will replace the present hardware to allow a smaller beam size in two interaction points (IPs). The project involves the construction of about 150 magnets of six different types: the quadrupole triplet, two main dipoles and three orbit correctors. The triplet, manufactured at CERN and in the USA, will consist of 30 magnets based on Nb3Sn technology, with an operational peak field of 11.4 T. These will be the first quadrupole Nb3Sn magnets installed in a particle accelerator. The other five types of magnets, all relying on Nb–Ti technology, present non-trivial challenges in the design and construction; they will be manufactured as part of in-kind contribution under the responsibility of institutes in Japan, China, Spain, and Italy. The project is now in the phase of transition between qualification through short models and prototypes and the beginning of the series construction. In this paper we review the magnet requirements, the reasons for selecting the design, the technological challenges with respect to previous projects, and we summarize the steps that have been taken to validate the baseline.

The upgraded magnet assembly of the EDIPO test facility aims at generating 15 T at 4.2 K and 85% of short sample limit in a large rectangular aperture and over a uniform field length of 1000 mm. During 2020 the design studies have concentrated on two flared-end block-coil design options with four double-pancake coils, two per pole. The coils are planned to be produced using Nb3Sn Rutherford cable with 44 x 1.1 mm diameter strands, which results in significantly larger dimensions compared to the typical cables used in accelerator magnets. Two sections of dummy superconducting cable have been produced, and dedicated tooling was developed to study the wind-ability of such high aspect ratio cable and provide input to define the geometrical parameters of the coil flared ends. This paper describes two magnet designs proposed for EDIPO focusing on the magnetic and the mechanical design.

The high-luminosity upgrade of the Large Hadron Collider (HL-LHC) requires new high field and large-aperture quadrupole magnets for the low-beta inner triplets (MQXF). With a nominal operating gradient of 132.2 T/m in a 150 mm aperture and a conductor peak field of 11.3 T, the new quadrupole magnets are based on Nb3Sn superconducting technology. After a series of short models constructed in close collaboration by LARP (LHC Accelerator Research Program) and CERN, the development program is entering in the series production phase with CERN on one side and the US Accelerator Upgrade Project (US-AUP) on the other side assembling and testing full-length magnets. This paper describes the status of the development activities at CERN, in particular on the cold powering test of the first MQFXB prototype and on the construction of the second full scale prototype. Critical operations such as reaction heat treatment, coil impregnation and magnet assembly are discussed. Finally, the plan towards the series production is described.

High-performance Nb3Sn superconducting wires have become one of the key technologies for the development of next generation accelerator magnets. While their large critical current densities enable the design of compact accelerator-quality magnets for their operation above 10 T, the noticeable reduction of the conductor performance due to mechanical strain appears as a new essential characteristic in magnet design. In this work, we extensively investigate the effect of transverse loads, up to 250 MPa, in state-of-the-art Nb3Sn Restacked-Rod-Process round superconducting wires. The tests are performed using a compressive Walters spring device, where the force is applied to the resin-impregnated wire, and the critical current is measured under magnetic fields ranging from 16 to 19 T. As a complement, critical current measurements under axial strain are also performed using a standard Walters spring. Interestingly, the study shows that the wire’s electro-mechanical response under transverse stress depends on the initial axial strain condition. Nonetheless, when the main direction load becomes predominant, all tested wires converge to a common behavior. This observation allowed us to combine the results from critical current measurements under the loads exerted in both directions (axial and transverse), shedding some new light on the mechanisms behind critical current degradation.

Numerous experiments have shown that the loads applied to Nb3Sn strands and cables can reduce their critical current. Experiments, performed on uniaxially loaded strands, allowed to define clear laws to describe the evolution of the critical surface as a function of the applied current, field, temperature and strain. It is, however, still unclear how these laws can be applied to superconducting magnets. The present paper proposes a methodology to estimate the critical current and temperature margin reduction on superconducting magnets due to stress on the superconducting material. The methodology is tested on the MQXF magnets, a quadrupole developed for the High Luminosity LHC project, and successfully validated by comparing computed strain with data from strain gauge measurements. Results suggested that, because of the stresses arising in winding during assembly, cool-down and powering, the current limit of the magnet is lower than the expected short sample limit, and that the most critical region does not coincide with the peak field location.

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.

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.

Superconducting coils limit current is usually measured on unloaded short samples of strands extracted from the same cable used to wound the coil. However, the critical current density of Nb 3 Sn strands is strongly dependent on the applied strain. This dependence is well reproduced by the so-called exponential scaling laws on strands subject to uniaxial loading. Unfortunately, no standard procedure exists to apply these laws under the complex stress condition experienced by the strands inside Rutherford cables for particle accelerator magnets. As a consequence, an empirical limit from magnet tests was set in the past: 150-200 MPa of maximum equivalent stress within the coil, considered as a block of uniform material. It is not clear how this number relates with the actual stress/strain state in the strands and if it is possible in magnet applications to infer the coil degradation from single strands tests. In this context, this paper presents a model to estimate the critical current degradation of superconducting coils as a function of the applied strain. The model results were compared against the critical current degradation measured on impregnated cable stacks subject to transverse pressure.

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 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.

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.

In the framework of the High-Luminosity Large Hadron Collider, the installation of a new generation of quadrupole magnets is foreseen on each side of ATLAS and CMS experiments. The new magnets are based on Nb3Sn technology and shall be able to reach an ultimate current of 17.9 kA with a peak field of 12.3 T in the coil. In 2016 and 2017, the first two short models, called MQXFS3 and MQXFS5, have been tested at 4.2 and 1.9 K in the two new test benches at the European Organization for Nuclear Research. This paper presents the result of the quench performance of the two models; the first magnet reached nominal but failed to reach ultimate, showing detraining in one coil. MQXFS5 reached ultimate performance without any detraining phenomena, validating the PIT conductor used for the first time in this magnet program.

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.

For the upgrade of the LHC during the long shutdown 2 and for the HL-LHC upgrade during the long shutdown 3, a few 15 m 8.3 T Nb–Ti main dipole magnets are foreseen to be replaced by two 11 T Nb3Sn dipoles, 5.5 m long each. After the production and test of three single aperture models at CERN, the first 2-m-long twin-aperture model has been tested. The model was used to exploit the limits of the model in terms of quench performance and protection. The protection studies included the heater efficiency studies and hotspot temperature studies. In this paper, the performance of the magnet DP101 in terms of training behavior, detection, and protection, and other cold powering test will be presented and discussed.

The quadrupole and dipole magnets for the high-luminosity large hadron collider (HL-LHC) upgrade will be based on Nb 3 Sn Rutherford cables that operate at 1.9 K and experience magnetic fields of up to about 12 T. An important step in the design of these magnets is the development of the high-aspect-ratio Nb 3 Sn cables to achieve the nominal field with sufficient margin. The strong plastic deformation of unreacted Nb 3 Sn strands during the Rutherford cabling process may induce non-negligible I c and Residual Resistivity Ratio degradation. In this paper, the cabling degradation is investigated as a function of the cable geometry for both Powder-in-Tube and Restack Rod Process conductors. Based on this analysis, new baseline geometries for both 11 T and quadrupole magnets of HL-LHC are proposed.

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.

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.

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.

The High Luminosity (HiLumi) Large Hadron Collider (LHC) project has, as the main objective, to increase the LHC peak luminosity by a factor five and the integrated luminosity by a factor ten. This goal will be achieved mainly with a new interaction region layout, which will allow a stronger focusing of the colliding beams. The target will be to reduce the beam size in the interaction points by a factor of two, which requires doubling the aperture of the low-β (or inner triplet) quadrupole magnets. The use of Nb3Sn superconducting material and, as a result, the possibility of operating at magnetic field levels in the windings higher than 11 T will limit the increase in length of these quadrupoles, called MQXF, to acceptable levels. After the initial design phase, where the key parameters were chosen and the magnet's conceptual design finalized, the MQXF project, a joint effort between the U.S. LHC Accelerator Research Program and the Conseil Europeen pour la Recherche Nucleaire (CERN), has now entered the construction and test phase of the short models. Concurrently, the preparation for the development of the full-length prototypes has been initiated. This paper will provide an overview of the project status, describing and reporting on the performance of the superconducting material, the lessons learnt during the fabrication of superconducting coils and support structure, and the fine tuning of the magnet design in view of the start of the prototyping phase.

For the upgrade of the LHC in LS2 and for the High Luminosity upgrade in LS3, a few 15-m Nb–Ti main dipole magnets are foreseen to be replaced by two 5.5-m-long Nb3Sn dipoles each. After a series of dipole models produced and tested by the collaborating partners at FNAL, in 2014 and 2015, the first short dipole model magnets have been produced and tested at CERN in a single aperture configuration. In this paper, the test results and test analyses will be discussed with a focus on training and cold powering performance.

In the framework of the European project EuCARD, the High Field Magnet project, led by a CERN-CEA collaboration, implied the development of a large aperture Nb3Sn dipole magnet called FRESCA2. The magnet uses four double-pancake block-type coils, each about 1.5 m long. In order to characterize strand and cable properties, as well as to qualify the coil fabrication process, CERN started in 2012 the design and fabrication of the Racetrack Model Coil (RMC) magnet, a short model magnet using the same cable as FRESCA2 magnet with only two flat double-pancake coils about 0.8 m long. In 2013, two superconducting coils have been fabricated, making use of two different types of superconductor. In 2014 and 2015, the coils were tested both in a single and in a double-coil configuration in a support structure based on an external aluminum shell pre-loaded with water-pressurized bladders. In this paper, we describe the design of the RMC magnet and its coils, provide the main parameters of the superconductor, and report the results of three powering tests, focusing on quench performance, training, and quench locations.

The high-luminosity upgrade for the LHC (HL-LHC) envisages the replacement of some 15-m-long NbTi dipoles in the dispersion suppressor area by shorter Nb3Sn magnets with a nominal field of 11 T. The new magnets must be compatible with the lattice and other main systems of the LHC. The shorter length of new units will allow the installation of collimators. The successful use of the Nb3Sn technology requires an intense R&D program, and therefore, a CERN–Fermilab joint development program was established. This paper describes the magnetic measurement procedure and presents the analysis of the magnetic measurements on the first 2-m-long single-aperture demonstrators built and tested at CERN. The geometrical field multipoles, the iron saturation effects, and the effects of persistent currents are presented. The experimental data are compared with the magnetic calculations using the CERN field computation program ROXIE and are discussed in view of the requirements for machine operation.

We have compared the mechanical properties and the degradation of the critical current after uniaxial tensile loading at room temperature (RT) and at 77 K of ex situ and in situ MgB2 wires. The strain that the wires can withstand without degradation is at 77 K substantially higher than at RT. In order to explain the mechanical behavior of the wires, the lattice distortions of the different wire constituents and their texture have been measured simultaneously with the composite wire stress and strain in a high-energy synchrotron beamline. The different MgB2 microstructure in both wire types is revealed in filament cross sections prepared by the focused-ion-beam technique and in fracture surfaces.

Porosity is one of several current limiting mechanisms in MgB2 wires. We have compared the microstructural homogeneity and the porosity distribution in different ex situ and in situ MgB2 powder-in-tube wires. The submicrometer structure was determined using focused ion beam nanotomography. The ex situ wires exhibit an isotropic microstructure, which has been quantified in terms of an identical tortuosity in transverse and longitudinal filament direction. The very homogenous microstructure in the new ex situ wire generation is probably one reason for its strongly improved critical current density. The in situ wire has an anisotropic microstructure with a lower tortuosity in the axial direction. The microstructural inhomogeneity of the in situ filaments makes microstructural characterization and the comparison between materials and superconducting properties particularly challenging.

Hysteresis losses of five Nb 3 Sn International Thermonuclear Experimental Reactor reference strands were investigated by means of magnetization loop measurements in a vibrating sample magnetometer in a perpendicularly applied magnetic field. The magnetization loops were recorded while continuously sweeping the applied field between the extreme values ±B m , covering a wide range of maximum applied fields (0.2-10 T). In this paper, we compare the directly determined hysteresis losses based on the area of the smaller measured loops and the losses calculated by the integration of the width ΔM of the B m = 10 T magnetization loop. A suitable fitting function is proposed to describe the ΔM(B) dependence, which leads, for each strand, to an excellent agreement with the experimentally determined hysteresis losses, magnetization, and pinning force. Transport critical current measurements in a perpendicularly applied magnetic field were also performed for all the strands, and on the basis of the comparison with the transport measurements, the critical current densities were determined from the magnetization data.

High field superconducting magnets for particle accelerators often exhibit premature quenches. Once a normal zone is generated within the conductor, the quench may propagate causing temperature and resistive voltage rise along the coil. The resulting thermal gradients can potentially cause new peak stresses that might exceed the tolerable limits, degrading the conductor. The computation of the strain state in the coils during quench then becomes of paramount importance for magnet design, and requires a complete three-dimensional (3-D) analysis of quench phenomena. The objective of this paper is to present the first multiphysics modeling activities towards a new full 3-D methodology for the analysis of magnet mechanics during quench. As a first step, a 3-D thermal-electric finite element model of a Nb 3 Sn superconducting coil is developed and explained here. The model uses direct coupled-field elements to solve the system of thermal and electrical equations. A solving algorithm has also been implemented in order to investigate the physics behind quench transients. The output from this model, built in ANSYS APDL, can be easily coupled in a later stage to a mechanical model in order to estimate the strain state in the coil windings. A very good agreement has been observed between the numerical results and experimental tests performed in individual superconducting cables and real superconducting magnets.

© 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.

The quadrupole magnets for the LHC High Luminosity (Hi-Lumi) upgrade will be based on Nb 3 Sn Rutherford cables that operate at 1.9 K and experience magnetic fields larger than 12 T. Because of the large stored energy, one of the major issues in the design of these magnets is their protection. To study the quench propagation in Nb 3 Sn Rutherford cables for the Hi-Lumi Quadrupole magnets, a campaign of measurements was carried out in the FRESCA test station at CERN. In addition to the critical and stability current measurements at 4.3 K and 1.9 K, we performed a series of tests where the quenches were induced by resistive heaters impregnated in the cable. The longitudinal and transversal speeds of the normal zone propagation were investigated at different magnetic fields and operating currents. In this paper, the numerical and experimental results are reported, compared and discussed.

The luminosity upgrade of the large hadron collider (HL-LHC) requires the development of new type of superconducting cables based on advanced Nb 3 Sn strands. In the framework of the FP7 European project EUCARD, the cables foreseen for the HL-LHC project have been tested recently in a simplified racetrack coil configuration, the so-called Short Model Coil (SMC). In 2013-2014, two SMCs wound with 40-strand (RRP 108/127) cables, with different heat treatment processes, reached during training at 1.9 K a current and peak magnetic field of 15.9 kA, 13.9 T, and 14.3 kA, 12.7 T, respectively. Using the measured signals from the voltage taps, the behavior of the quenches is analyzed in terms of transverse and longitudinal propagation velocity and hot-spot temperature. These measurements are compared with both analytical and numerical calculations from adiabatic models. The coherence of the results from the presented independent methods helps in estimating the relevance of the material properties and the adiabatic assumption for impregnated Nb 3 Sn conductor modeling.

The short model coil (SMC) dipole magnet has been designed and constructed in the framework of the European project EuCARD as a test bed for Nb 3 Sn magnet R&D in terms of coil fabrication technology and testing of Rutherford-type cables. Two pairs of racetrack coils were successfully tested in 2012, obtaining 12.5 T in the winding at 1.9 K, thus demonstrating SMC as a valid magnet design for conductor tests. These successful results prompted a new test campaign of SMC using the cable of the 11-T dipole project. The size of this cable is such that only one double pancake can be accommodated in the mechanical structure. During the latest tests using the 11-T cable, a new record field of 13.5 T was achieved, confirming the excellent capability of SMC to qualify a conductor, providing precise information of the stability and general cable performance, and to test insulation techniques, instrumentation systems, and coil fabrication processes. This paper describes the challenges of this new assembly, indicating novelties in the fabrication technology and reports on the measurements and the performance of SMC using CERN's 11-T-dipole-type conductor.

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.

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.

International audience; The Nb3Sn block-coil dipole magnet FRESCA2 was developed within the framework of a collaboration between CEA Saclay and CERN, follow up of the European project EuCARD. The magnet is aimed at upgrading the cable test facility at CERN to a bore field of 13 T at 10.6 kA in an aperture of 100 mm as deliverable. The design features four 1.6-m-long double-layer coils wound with a 40 strand, 21-mm-wide Rutherford cable. The first assembly was limited at 12.2 T with a known production defect of the cable close to a splice in one of the coils. Following replacement of this coil, the second assembly reached the target for this run of 13.3 T. In this paper, the performance of the assemblies in terms of training behavior, quench detection, protection, magnetic measurements, and other cold powering tests will be discussed.

The so-called CERN-LHC DS upgrade relies on the use of 11-T dipole magnets. For these magnets, 40 strands Nb3Sn type Rutherford cables based on 0.7-mm wires are being developed. Recently, four samples of the cables were characterized in the CERN FRESCA cable test station. The critical current and the premature quench current due to magneto-thermal instability were measured at 1.9 and 4.3 K in a background magnetic field between 0 and 9.6 T (the peak magnetic field on the conductor, including the self-field of the cable, ranges from ~ 2 T to ~ 12 T). Two cable samples were based on Powder-In-Tube (PIT) wire and two on Restacked-Rod-Process (RRP) wire. The PIT samples were identical and without a core in the cable while one of the RRP samples features a 25- μm-thick stainless steel core. All cables samples tested have a width and a thickness of about 14.7 and 1.25 mm, respectively. Cables and sample holders were manufactured at CERN. In this paper, we report and discuss the cable test results and compare them to the performance of witness strands, heat treated and measured on ITER-VAMAS type sample holders.

The mechanisms causing the critical-current degradation of ITER-type Nb3Sn Cable-in-Conduit-Conductors have been studied in the past decade. One of the origins of the degradation of the performance is the Lorentz load during magnet operations. The strands within the cable subjected to the transverse loads locally experience larger strains than the ones caused by tensile loads. Results obtained with experiments on full size cables are limited due to their cost and difficulty so, to predict the degradations of a Nb3Sn CICC under transverse loads, empirical laws should be established. Strain scaling laws are available but have never been used to determine the behavior of strands under transverse load. We propose a new method, by which the critical-current behavior of a subsized cable under transverse loads is derived from the critical-current behavior under uni-axial strains. This method is based on the strain state of the Nb3Sn filaments obtained by finite element analysis (FEA). The FEA strain results are used to find the critical-current using tensile scaling laws available. The critical-current behavior obtained by this method is compared with the available experimental data of triplets. The results of this new methodology, its benefits, and limitations are discussed.