The High-Luminosity project (HL-LHC) of the CERN Large Hadron Collider (LHC), requires low $\beta$ * quadrupole magnets in Nb $_\text{3}$ Sn technology that will be installed on each side of the ATLAS and CMS experiments. After a successful short-model magnet manufacture and test campaign, the project has advanced with the production, assembly, and test of full-size 7.15-m-long magnets. In the last two years, two CERN-built prototypes (MQXFBP1 and MQXFBP2) have been tested and magnetically measured at the CERN SM18 test facility. These are the longest accelerator magnets based on Nb $_\text{3}$ Sn technology built and tested to date. In this paper, we present the test and analysis results of these two magnets, with emphasis on quenches and training, voltage-current measurements and the quench localization with voltage taps and a new quench antenna.

-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.

The High Luminosity Large Hadron Collider (LHC) Project target is to reach an integrated luminosity of the LHC of 3000 fb(-1), corresponding to a factor 10 increase in collisions with respect to the current accelerator. One of the main components is the superconducting quadrupole called MQXF. It is based on Nb3Sn technology and has a 150 mm single aperture with a field gradient of 132.6 T m(-1). The MQXF magnets are currently in preseries production in a joint collaboration between CERN and the US-LHC Accelerator Upgrade Project. The first prototype magnet based on 7 m-long coils (MQXFBP1) was assembled and preloaded in 2019. The testing and disassembly was done in 2020. The coils were equipped with optical fiber Bragg grating (FBG) sensors. Some of the FBG values exhibited strain jumps during preload and their signals were lost during cool-down. The magnet did not reach the nominal current, quenching at 15.15 kA. At the time of writing three MQXFB magnets have been preloaded from which the second prototype is soon to be tested. The objective of this paper is to analyze the mechanical behavior of MQXFB magnets in the light of current knowledge, thanks to earlier short model experiments, as well as describe in detail the process of assembly and preload. We synthesize the mechanical theory of preload and present new considerations on symmetric/asymmetric bladder-key operations and their effects. An extensive comparison between long and short model magnets is presented. In our analysis and FE modeling we take into account the measured coil sizes that vary over position. We introduce a novel technique that allows preload homogenization over the length of the magnet after preload has been done. We analyze the role of the stainless steel shell and show its mechanical behavior. Finally, the relevant data at cold is presented and analyzed.

For a delivery system using high OH polyimide coated all-silica multimode fibers the photo degradation and recovery were investigated for 355nm pulsed laser radiation at a cryogenic temperature of 77K (liquid nitrogen) and at room temperature, as reference. For comparison, UV-induced fiber losses generated during light transportation of a deuterium lamp were measured. For these quite different light sources the efficiency of defect generation including UV-induced optical absorption at 214nm and at 265nm is significantly different at both temperatures used in these studies. At 77K, the UV-induced optical loss at 214nm is significantly reduced, while the same loss at 265nm is increased, for the tested high OH fibers. In addition, the values of both losses are rapidly changed when the fiber is warmed up from 77K to room temperature. The observed temperature dependent growth kinetics of UV-induced absorption at 214nm is considered to be explained by at least two different mechanisms of defect generation.

Magnetothermal instability may affect the performance of high critical current density Nb 3 Sn strands by quenching the superconductor at current values lower than its critical current. This phenomenon has important implications in the design of next-generation superconducting magnets for accelerators. According to the theory, the strand magnetothermal stability can be improved by changing the current distribution within the strand. In this paper, a technique to improve the conductor stability is presented. It consists in partially cutting the strand's outer superconducting subelements in the region where the wire is soldered to the copper block that feeds the current. The special cut forces the current to enter into the inner subelements without degrading the current capacity. As a consequence, the stability is increased, which avoids changing the strand critical current or its residual resistivity ratio because current redistribution during an external perturbation is strongly reduced. The experimental results obtained by applying this technique on a 0.8-mm restacked-rod process strand are presented and discussed.

The magnetothermal instability (MTI) is an issue in the high-J(c) Nb3Sn superconducting strands for accelerator magnets. The first kind of MTI is magnetization instability, which may be significantly reduced by using small filaments and with a high copper residual resistivity ratio (RRR) >= 100. However, theoretical and experimental evidence suggests that there is a second kind of MTI, which is self-field instability, which seems to be highly sensitive to external perturbations and less sensitive to RRRs. Recently at the European Organization for Nuclear Research (CERN), for studying this effect, a new measurement setup based on a Q-switched laser has been developed. By using the setup, it is possible to provide a local and controlled perturbation in nanosecond timescale from 0.5 to 26 mu J, which is absorbed by the strand at cryogenic temperature. In this paper, we report the results obtained by testing high-J(c) Nb3Sn strands with different RRRs. It is shown that the self-field instability is sensitive to the trigger energy in moderately high magnetic fields. Therefore, the stability is highly dependent on the perturbation spectrum, i.e., the conventional stability measurements might not be fully representative of the strand stability in magnets wound with high-J(c) Nb3Sn strands.

The CERN Large Hadron Collider (LHC) is envisioned to be upgraded in 2020 to increase the luminosity of the machine. The major upgrade will consist in replacing the NbTi quadrupole magnets of the interaction regions with larger aperture magnets. The Nb 3 Sn technology is the preferred option for this upgrade. The critical current density J c of Nb 3 Sn strands have reached sufficiently high values (in excess of 3000 at 12 T and 4.2 K) allowing larger aperture/stronger field magnets. Nevertheless, such large J c values may cause magneto-thermal instabilities that can drastically reduce the conductor performance by quenching the superconductor prematurely. In Nb 3 Sn magnets, a relevant parameter for preventing premature quenches induced by magneto-thermal instabilities is the Residual Resistivity Ratio (RRR) of the conductor stabilizing copper. An experimental and theoretical study was carried out to investigate how much the value of the RRR affects the magnet stability and to identify the proper conductor specifications. In this paper the main results are presented and discussed.

Magnetothermal instability may affect high critical current density superconducting strands that can quench even though the transport current is low compared to the critical current with important implications in the design of next generation superconducting magnets. The instability is initiated by a small perturbation energy which is considerably lower than the minimum quench energy (MQE). At CERN, a new experimental setup was developed to measure the smallest perturbation energy [minimum trigger energy (MTE)] which is able to trigger the magnetothermal instability in superconducting -strands. The setup is based on Q-switched laser technology which is able to provide a localized perturbation in nanosecond time scale. Using this technique the energy deposition into the strand is well defined and reliable. The laser is located outside the cryostat at room temperature. The beam is guided from room temperature on to the superconducting strand by using a UV-enhanced fused silica fiber. The strand is mounted on a VAMAS barrel. A part of the beam's energy is absorbed into the strand acting as the trigger energy for the magnetothermal instability. In this paper the experimental setup and the calibration of the absorbed energy is presented.

In the framework of the EuCARD program, CERN is participating in the development of a 13 T 100-mm-aperture dipole magnet to upgrade the superconducting cable test facility FRESCA at CERN. The conductor candidates for building this magnet are two 1-mm Nb3Sn strands: the Powder In Tube (PIT) produced by Bruker-EAS and the 132/169 RRP by Oxford Superconducting Technology (OST). Recently the PIT strand has been extensively characterized by CERN in collaboration with the University of Geneva (UniGe). The critical current dependence on the magnetic field and on the axial strain has been measured at different temperatures. Furthermore, the strand magnetization has been measured at different temperature using a vibrating sample magnetometer. Finally the magneto-thermal stability of this strand was studied by measuring the quench current between 0 T and 12 T at 1.9 K and 4.3 K. The experimental results are compared with an optimized scaling law for the critical current of Nb3Sn strands. In this paper the results obtained for the PIT strand are summarized and discussed.

A controlled UV-light delivery system is envisioned to be built in order to study the stability properties of superconducting strands. The application requires a wave guide from room temperature to cryogenic temperatures. Hydrogen loaded and unloaded polyimide buffered silica-silica 100 mu m core fibres were tested at cryogenic temperatures. A thermal stress test was done at 1.9 K and at 4.2 K which shows that the minimal mechanical bending radius for the fibre can be 10 mm for testing (transmission was not measured). The cryogenic transmission loss was measured for one fibre to assess the magnitude of the transmission decrease due to microbending that takes place during cooldown. UV-irradiation degradation measurements were done for bent fibres at 4.2 K with a deuterium lamp and 355 nm pulsed lasers. The irradiation tests show that the fibres have transmission degradation only for wavelengths smaller than 330 nm due to the two photon absorption. The test demonstrates that the fibres are suitable for the cryogenic UV applications with 355 nm and 70 mu J pulsed lasers. (C) 2011 Elsevier Ltd. All rights reserved.

In the framework of future LHC upgrades using Nb3Sn magnets, supported in part by the US LHC Accelerator Research Program (LARP) and the European EuCARD program, CERN is intensifying its research on Nb3Sn Rutherford cables. In the FRESCA cable test facility at CERN, two new Nb3Sn cable samples were investigated concerning their quench and critical current as well as stability performance. The two samples are based on RRP type strands with comparable layout. They have 27 strands with 0.7 mm diameter while the strands have 54 or 108 superconducting sub-elements. The cables are 10 mm wide and have a transposition pitch of about 75 mm. The cables are used in magnets built for LARP. Their performance is measured as a function of magnetic field up to 10 T with ramp rates of 10 to 1000 A/s and temperature of 1.9 and 4.3 K. A hall probe array is present to study the current distribution. Point heaters are used to study cable stability and current redistribution.

In the framework of future LHC upgrades using Nb 3 Sn magnets, supported in part by the US LHC Accelerator Research Program (LARP) and the European EuCARD program, CERN is intensifying its research on Nb 3 Sn Rutherford cables. In the FRESCA cable test facility at CERN, two new Nb 3 Sn cable samples were investigated concerning their quench and critical current as well as stability performance. The two samples are based on RRP type strands with comparable layout. They have 27 strands with 0.7 mm diameter while the strands have 54 or 108 superconducting sub-elements. The cables are 10 mm wide and have a transposition pitch of about 75 mm. The cables are used in magnets built for LARP. Their performance is measured as a function of magnetic field up to 10 T with ramp rates of 10 to 1000 A/s and temperature of 1.9 and 4.3 K. A hall probe array is present to study the current distribution. Point heaters are used to study cable stability and current redistribution.

In the framework of future LHC upgrades using Nb3Sn magnets, supported in part by the US LHC Accelerator Research Program (LARP) and the European EuCARD program, CERN is intensifying its research on Nb3Sn Rutherford cables. In the FRESCA cable test facility at CERN, two new Nb3Sn cable samples were investigated concerning their quench and critical current as well as stability performance. The two samples are based on RRP type strands with comparable layout. They have 27 strands with 0.7 mm diameter while the strands have 54 or 108 superconducting sub-elements. The cables are 10 mm wide and have a transposition pitch of about 75 mm. The cables are used in magnets built for LARP. Their performance is measured as a function of magnetic field up to 10 T with ramp rates of 10 to 1000 A/s and temperature of 1.9 and 4.3 K. A hall probe array is present to study the current distribution. Point heaters are used to study cable stability and current redistribution.

In the framework of the US LHC Accelerator Program (LARP), three US laboratories BNL, FNAL and LBNL are developing Nb 3 Sn quadrupole magnets for the Large Hadron Collider (LHC) luminosity upgrade. At present CERN is supporting this activity by testing some of the LARP 1 m long 90 mm aperture magnets. Recently two magnets using a shell based key and bladder technology (TQS) have been tested at CERN. These magnets (TQS02c, TQS03a) share the same mechanical structure and use a 27 strand Rutherford cable based on the 0.7 mm RRP strand. The main difference between the two magnets is the strand sub-element layout (54/61 in TQS02c versus 108/127 in TQS03a) and the strand critical current. The TQS03a wire has a lower (18%) critical current, a larger amount of copper stabilizer, and a larger number of superconducting sub-elements with respect to the TQS02c strand. The tests show that TQS02c was stable between 4.3 K and 2.7 K while it was limited by the self-field instability at lower temperatures. TQS03a was not limited by magneto-thermal instabilities and reached 93% of the short sample limit both at 4.3 K and 1.9 K. In this paper the results are summarized and compared with the stability measurements performed at CERN on individual strands.