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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.

MQXF is the Nb 3 Sn Low-β quadrupole magnet that the HL-LHC project is planning to install in the LHC interaction regions in 2026 to increase the LHC integrated luminosity. The magnet will be fabricated in two different lengths: 4.2 m for MQXFA, built in the US by the Accelerator Upgrade Project (AUP), and 7.15 m for MQXFB, fabricated by CERN. In order to qualify the magnet design and characterize its performance with different conductors, cable geometries and pre-load configurations, five short model magnets, called MQXFS, were fabricated, assembled and tested. We compare the mechanical behavior of short model magnets using experimental data and new numerical models that take into account the measured coil sizes as a function of position. -MQXF is the Nb3Sn Low-β quadrupole magnet that the HL-LHC project is planning to install in the LHC interaction regions in 2026 to increase the LHC integrated luminosity. The magnet will be fabricated in two different lengths: 4.2 m for MQXFA, built in the US by the Accelerator Upgrade Project (AUP), and 7.15 m for MQXFB, fabricated by CERN. In order to qualify the magnet design and characterize its performance with different conductors, cable geometries and pre-load configurations, five short model magnets, called MQXFS, were fabricated, assembled and tested. We compare the mechanical behavior of short model magnets using experimental data and new numerical models that take into account the measured coil sizes as a function of position.

The recombination dipoles D2 (MBRD) for the luminosity upgrade of the Large Hadron Collider (LHC) are double aperture magnets to be placed on each side of ATLAS and CMS experiments, generating 4.5 T along a magnetic length of 7.78 m and a bore diameter of 105 mm. Its development plan foresees the construction of a short model 1.6 m long, followed by a prototype and by the series of 6 magnets. The magnet design was carried out at INFN Genova in a collaboration framework with CERN and the construction is ongoing in the industry (ASG Superconductors, Italy). The short model activities have just been accomplished after a successful power test performed at CERN, while the prototype is in its construction phase. In this contribution, the main features of the D2 magnet will be described, underlining the improvements implemented in the prototype with respect to the short model design. Then, the main results of the power test will be presented, focusing on training performance, protection scheme effectiveness and magnetic measurements.

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.

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.

MQXF is the Nb$_3$Sn Low-β Quadrupole magnet that the HL-LHC project is planning to install in the LHC interaction regions in 2026 as part of an upgrade to increase the LHC inte-grated luminosity by about a factor of ten. The magnet will be fab-ricated in two different lengths: 4.2 m for MQXFA, built in the US by the Accelerator Upgrade Project (AUP), and 7.15 m for MQXFB, fabricated by CERN. In order to qualify the magnet de-sign and characterize its performance with different conductors, cable geometries and pre-load configuration, five short model magnets, called MQXFS, were fabricated, assembled and tested. The latest model, MQXFS6, uses a new powder-in-tube (PIT) su-perconducting wire, featuring a bundle barrier surrounding the filaments. The coil and the support structure were equipped with strain gauges and optical fibres to monitor strain during assembly, cool-down and excitation. In this paper we further develop the conventional azimuthal preload analysis and introduce a new set of tools for MQXF coil pack characterization which we use to an-alyse the behaviour of MQXFS6 room temperature preload and to reanalyse all the short models tested at CERN. A comparison is made between all the studied magnets revealing new characteriz-ing preload parameters. MQXF is the Nb$_3$Sn Low-β Quadrupole magnet that the HL-LHC project is planning to install in the LHC interaction regions in 2026 as part of an upgrade to increase the LHC integrated luminosity by about a factor of ten. The magnet will be fabricated in two different lengths: 4.2 m for MQXFA, built in the US by the Accelerator Upgrade Project (AUP), and 7.15 m for MQXFB, fabricated by CERN. In order to qualify the magnet design and characterize its performance with different conductors, cable geometries and pre-load configuration, five short model magnets, called MQXFS, were fabricated, assembled and tested. The latest model, MQXFS6, uses a new powder-in-tube (PIT) superconducting wire, featuring a bundle barrier surrounding the filaments. The coil and the support structure were equipped with strain gauges and optical fibres to monitor strain during assembly, cool-down and excitation. In this paper we further develop the conventional azimuthal preload analysis and introduce a new set of tools for MQXF coil pack characterization which we use to analyse the behaviour of MQXFS6 room temperature preload and to reanalyse all the short models tested at CERN. A comparison is made between all the studied magnets revealing new characterizing preload parameters.

In the context of CERN's high-luminosity upgrade project (HL-LHC) for the Large Hadron Collider (LHC), a new double aperture beam orbit corrector magnets will be installed near the recombination dipole (D2). These 2.2 m long NbTi dipoles are built with the canted cosine theta (CCT) technique. The two independently powered apertures are oriented such that their field vectors are perpendicular to each other and to the direction of the beams. A full-length double aperture prototype was built and tested at CERN in the SM18 test facility. Here we present the results of powering tests at 1.9 and 4.5 K: training of each aperture, magnetic field quality and cross-talk effects, quench detection system effectiveness, quench protection performance and quench-back with several energy extraction systems.

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

This paper discusses the simulated and experimentally observed quench behavior of the first HL-LHC Twin Aperture Orbit Corrector Prototype, also known as the MCBRDp1 magnet. This superconducting magnet features two independently powered apertures. Each aperture comprises two concentric canted-cosine-theta-type Nb-Ti/Cu coils that together generate a dipolar magnetic field over the bore. These coils are held in place by conductive aluminum-alloy formers. The circuit is protected by a combination of energy extraction and quench-back in the coils. When the coils are discharged over an energy extractor, eddy currents are generated in the formers, and the resulting heat quickly and efficiently brings the Nb-Ti/Cu strands above their current sharing temperature, provided that the resistive voltage over the energy extractor is sufficiently large. This paper compares simulations and experimental observations. It is shown that with the BBQ tool, the initial voltage development after a training quench is correctly reproduced. The ProteCCT simulation tool is shown to be consistent with experimentally observed discharges of the MCBRDp1 prototype for different bath temperatures, energy extractor types, and initial operating currents. The baseline energy extractor resistor value of 1.5 Ω and the non-linear varistor option both give worst-case hotspot temperatures below the 200 K hotspot temperature limit. At ultimate current, the resulting hotspot temperatures are 143 and 167 K, and the peak voltages-to-ground are 590 and 440 V, respectively.

The HL-LHC project is an upgrade of the LHC that requires double aperture dipole correctors. In 2015, CERN selected Canted Cos-Theta (CCT) magnet, and the development of the MCBRD magnets followed. Since then, a prototype (P1) has been built and measured at CERN, and quench results agree with simulations. In 2017, China joined the programme with in-kind prototypes and twelve (four spares) series magnets with WST, IMP, IHEP, and BAMA efforts. The first MCBRD magnet built in China is the second prototype (P2) of the programme. This magnet was subjected to triggered energy extractions at CERN magnet test facility. The energy extraction is used for magnet protection, keeping voltage-to-ground and hot-spot temperature below 560 V and 250 K, respectively. A high magnetic-field change rate during the magnet discharge from high currents causes a substantial quench-back due to the heat generated by the eddy currents in the magnet formers. A specialized software, ProteCCT, was developed as part of the STEAM project to simulate such a unique quench behaviour. This contribution focuses on comparing measured results between the P1 and P2 magnets, and P2 quench measurement and simulation results. The correction factors available in the ProteCCT software allow matching the measured results. We identify measurements needed to refine simulation inputs further and improve the simulation predictive capability.

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.

In the frame of the high-luminosity upgrade project for the large hadron collider, new twin aperture beam orbit corrector magnets will be installed near the recombination dipole (D2). These magnets are 2.2 m long canted cosine theta NbTi dipoles, with two independently powered apertures oriented such that their field vectors are perpendicular to each other and to the direction of the beams. A 0.5 m model magnet in single and double aperture configuration and a full-length double aperture prototype were built and tested at CERN. In this paper, the performance of these magnets at 1.9 K in terms of training behavior, quench detection and protection, and other tests is discussed. In addition, the thermal response of the magnet to a hypothetical beam discharge is simulated and analyzed.

In the frame of the compact linear collider project, a high-field short-period superconducting damping wigglers will be required to reduce the emittance of the electron and positron beams. The use of Nb 3 Sn as superconducting material is being investigated, as a valid option for its smaller size and increased working margin. At CERN, Geneva, Switzerland, a second Nb 3 Sn damping wiggler short model has been developed, assembled, and tested. In this paper, the cold power test of that magnet is discussed in terms of training, quench detection, protection, endurance, and other tests.

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.

The National Institute for Nuclear Physics (INFN) is developing, at the Laboratory of Accelerators and Applied Superconductivity (LASA Milan, Italy), five families of corrector magnets, from skew quadrupole up to dodecapole order, which will be installed in the interaction regions of the High-Luminosity Large Hadron Collider (LHC). These magnets are based on a superferric design, which allows a relatively simple, compact and easy-to-construct magnets. This activity takes place within the framework of a collaboration agreement between CERN and INFN. The magnets have been designed and prototype units have been built and tested for the sextupole, octupole, and decapole orders. Magnetic measurements have been performed in order to characterize the field quality, and to validate the design and construction. This paper presents the instruments and the approach for the magnetic measurements on the prototype magnets. Moreover, the results of measurements at cryogenic temperature, up to the nominal field level, are reported. The magnetic field quality, in terms of transfer function and field multipoles, is analysed as function of the excitation level. The iron saturation effects, which are a major concern of the selected design, are compared with the 3-D magnetic calculations and discussed in view of the construction of the series magnets to be integrated in the corrector package assembly.

The fast thermal and electromagnetic transients that occur in a superconducting magnet in case of a quench have the potential of generating large mechanical stresses both in the superconducting coils and in the magnet structure. While the investigation of such quench loads should generally be conducted to ensure a safe operation of the system, its importance is greatly enlarged in the case of high-field magnets based on strain sensitive superconductors. For these, a rigorous analysis of the magnet mechanics during a quench becomes critical. The scope of this work is hence to bring, for the first time, a detailed understanding of the three-dimensional mechanical behavior of a Nb3Sn accelerator magnet during a quench discharge. The study relies on the use of finite element models, where various multi-domain simulations are employed together to solve the coupled physics of the problem. Our analysis elaborates on the case study of the new MQXF quadrupole magnet, currently being developed for the high-luminosity upgrade of the LHC. Notably, we could find a very good agreement between the results of the simulation and experimental data from full-scale magnet tests. The validated model confirms the appearance of new peak stresses in the superconducting coils. An increase in the most relevant transverse coil stresses of 20–40 MPa with respect to the values after magnet cool-down has been found for the examined case. Peer Reviewed "Article signat per 13 autors/es: J Ferradas Troitino, H Bajas, L Bianchi, B Castaldo, P Ferracin, M Guinchard, S Izquierdo, J V Lorenzo, F Mangiarotti, J C Perez, E Takala, G Vallone and C Senatore"

The HL-LHC interaction region magnet triplets (Q1,Q2, and Q3) will be composed of superconducting Nb3Sn quadru-poles. The MQXF quadrupole protection system is based on CLIQ (Coupling-Loss Induced Quench system) and outer layer quench heaters.This paper reports a summary of quench heaters to coil high voltage tests performed on MQXF short and long coils in air after fabrication, and in air and He gas after magnet training. Breakdown voltage values demonstrate good marginwith respect to the Electrical design criteria for the HL-LHC inner triplet mag-nets. A modification in thequench heater installation-with an ex-tra layer of fiber glass between the coil and the quench heater trace-has been proposed and tested in a mirror magnet to further increase electrical margins. Results demonstrated improvements of high voltage margin at the expense of a clear increase of hot spot temperature.Thebaseline heater to coil insulation was assessed to be able to guarantee safe operation for the Nb3Sn quadrupole mag-nets for the interaction regions of HL-LHC. The HL-LHC interaction region magnet triplets (Q1, Q2, and Q3) will be composed of superconducting Nb3Sn quadrupoles. The MQXF quadrupole protection system is based on CLIQ (Coupling-Loss Induced Quench system) and outer layer quench heaters. This paper reports a summary of quench heaters to coil high voltage tests performed on MQXF short and long coils in air after fabrication, and in air and He gas after magnet training. Breakdown voltage values demonstrate good margin with respect to the Electrical design criteria for the HL-LHC inner triplet magnets. A modification in the quench heater installation- with an extra layer of fiber glass between the coil and the quench heater trace- has been proposed and tested in a mirror magnet to further increase electrical margins. Results demonstrated improvements of high voltage margin at the expense of a clear increase of hot spot temperature. The baseline heater to coil insulation was assessed to be able to guarantee safe operation for the Nb3Sn quadrupole magnets for the interaction regions of HL-LHC.

Achieving the targets of the High Luminosity LHC project requires the installation of new inner triplet magnet circuits for the final focusing of the particle beams on each side of the two main interaction points. Each of the four circuits will include six 150 mm aperture, 132.2 T/m gradient, Nb $_3$ Sn quadrupole magnets to be installed in the LHC tunnel. The recently updated circuit topology is such that the protection of each magnet can be studied from a single magnet point-of-view. To limit the hot-spot temperature and the peak voltage-to-ground, a protection system was designed that quickly and reliably transfers voluminous parts of the coil to the normal-conducting state, hence distributing more homogeneously the magnets stored energy in the windings. This system is based on two elements: quench heaters attached to the outer layers of the magnet coils and CLIQ (Coupling-Loss Induced Quench). The performance of the protection system is investigated by simulating the electro-magnetic and thermal transients occurring after a quench with the program STEAM-LEDET, and by conducting dedicated experiments at the CERN and FNAL magnet test facilities. The effectiveness of the quench protection system is assessed at all representative operating current levels. Furthermore, the coils hot-spot temperature and peak voltage to ground are analyzed for various failure cases, conductor parameters, and parameter distribution among the four coils. It is concluded that the proposed design assures an effective, reliable, and fully redundant quench protection system.

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 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.

New high-field accelerator magnets based on Nb3Sn superconductors are pushing the boundaries of magnet design and quench protection towards new limits. While their large stored energies and current densities result in a very challenging scenario for magnet protection, their great electromagnetic forces create also new requirements in terms of magnet design and stress management techniques. Furthermore, the strain sensitivity of Nb3Sn cables turns the electro-mechanical limits of the conductor into a parameter of the highest importance, where conductor degradation becomes a critical aspect in magnet operation. The coupling of all the above-mentioned considerations during a quench is a case of special interest that adds further complexity to the design of Nb3Sn magnets. The objective of this paper is to provide a complete two-dimensional investigation of the coil and magnet structure mechanics during a quench event. The analysis is performed using a combination of finite element codes that provide the necessary input for the mechanical study. The core of the modelling strategy relies on a thermal-electric model, whose results are directly used as loads in the mechanical simulation. In doing so, the stress evolution during and after a quench is obtained. We focus for this time in the analysis of a quench heater protected magnet, where electro-magnetic dynamic effects are of less importance in contrast to other protection systems like CLIQ.

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.

A study was performed to understand the quench behavior and ensure adequate quench protection of the canted cosine theta (CCT) twin aperture orbit corrector magnet, a superconducting magnet under development as part of the high-luminosity upgrade of the Large Hadron Collider (HL-LHC). The cosine theta geometry features canted superconducting coils, which together produce a magnetic dipole field. The NbTi/Cu strands are placed in slots inside formers that maintain the shape of the coils. The presence of these formers affects the quench behavior of the magnet by preventing direct thermal contact between adjacent groups of strands. At the same time, a discharge of the stored energy over an external resistor results in significant eddy current heating inside the formers, which quickly brings the entire superconducting magnet to a normal state. A calculation model was developed that describes the electrical and thermal behavior of this type of magnet, and the results of this model are compared to experimental observations on a 0.5 m CCT model coil. It is found that the calculation results and experimental observations are generally consistent, although the simplified manner in which the eddy current heating is described in the model leads to a modest overestimation of the hotspot temperature. The calculation results indicate that a proposed quench protection configuration, featuring a discharge over a 0.7 $\Omega$ energy extractor and a 0.05 $\Omega$ crowbar, is sufficient to protect both the 0.5 m CCT model magnet and the 2.2 m CCT prototype magnet, resulting in hotspot temperatures of 63 and 193 K, and peak voltages to ground of 300 and 310 V, respectively.

The large hadron collider (LHC) upgrade, called high-luminosity LHC is planned for the next decade. A wide range of magnets and new technologies are currently under development. One of these systems will be a set of twin aperture beam orbit correctors positioned on the approaches to the ATLAS and CMS experiments. This twin aperture magnet system comprising 16 magnets, approximately 2 m long, with large 105-mm clear aperture coils. Each aperture will independently deliver 5-T⋅m integral field, between apertures the field vectors are rotated by 90° from each other, and individually powered. This paper presents the sequence of component developments to produce a cost-effective canted cosine theta model magnet. We describe the challenges encountered during the manufacture of the coil formers with their helical canted coil winding process which places a number of insulated wires into the 2-mm-wide 5-mm-deep slot. We describe the: pressurized impregnation process, multiple jointing to connect inner and outer sets of wires within the confines of the coil assembly, and magnet assembly into support structure and yoke. Finally, we present the quench performance and initial test results of this novel coil configuration.

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.

International audience; The Nb$_3$Sn block coil dipole magnet FRESCA2 was developed within the framework of a collaboration between CEA Saclay and CERN, in the continuity of the European project EuCARD and EuCARD2. With an aperture of 100 mm and a target bore field of 13 T at 10.6 kA, the magnet is required for a new FRESCA2 cable test facility at CERN. In 2017, the magnet was pre-loaded to retain the forces while the magnet was powered to achieve 13.3 T in the magnet bore. Results of these tests were published. In 2018, the loading of the magnet has been increased for powering to higher current. In this paper, the updated results of the cold powering tests are discussed in terms of training, memory, and stable operation. The loading of the magnet and the mechanical measurements during cooldown are shown and compared to the earlier loading steps. The protection of the magnet is further reviewed and measured results are compared to the model simulations.

In the frame of the high-luminosity upgrade project (HL-LHC) at CERN, a double aperture, independently powered, family of beam orbit corrector magnets will be installed close to the two main LHC experiments Atlas and CMS. These 2.6 T magnets, built using a canted cos-theta design. This paper describes the development of the prototype, full size 2-m-long magnets. We first focus on design and assembly techniques: from coil winding using a CNC machined aluminium former to impregnation, layer-jump, quench protection, and yoke assembly. We then present the power test results at 1.9 K: training, field quality and protection.

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.

With the first tokamak designed for full nuclear operation now well into final assembly (ITER), and a major new research tokamak starting commissioning (JT60SA), nuclear fusion is becoming a mainstream potential energy source for the future. A critical part of the viability of magnetic confinement for fusion is superconductor technology. The experience gained and lessons learned in the application of this technology to ITER and JT60SA, together with new and improved superconducting materials, is opening multiple routes to commercial fusion reactors. The objective of this roadmap is, through a series of short articles, to outline some of these routes and the materials/technologies that go with them.