Mechanical Analysis of the Collaring Process of the 11 T Dipole Magnet

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For the Large Hadron Collider luminosity upgrade, two of the NbTi 8.3 T main bending dipoles will be replaced by two shorter Nb 3 Sn 11.2 T dipoles to create space for the installation of collimators in the dispersion suppression region. With the aim to verify the design features, several 2 m long 11 T models have been constructed and tested at CERN. During the fabrication and assembly of, so far, seven single and two double aperture short model magnets, several challenges were identified and tackled. These include reproducibility in coil fabrication and assembly procedure, as well as control of mechanical stresses in the conductor and surrounding structure. In order to limit coil over-compression and improve reproducibility, the cable insulation was re-optimized. In addition, a review of the collaring procedure of the 11 T magnet was launched with the goal of reducing the risk of conductor degradation due to excessive stress. In this paper, the main fabrication and assembly steps are described, including the description of the actions taken to resolve the identified weakness.

Section: Magnet Designmentioning

confidence: 73%

Section: Collaring Proceduresmentioning

confidence: 94%

Section: Magnet Designmentioning

confidence: 99%

Section: Collaring Proceduresmentioning

confidence: 99%

“…Figure23. The result of the strain gauge measurements during powering are summarized for magnets SP106, SP107 and the two apertures of DP102.…”

mentioning

confidence: 99%

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Mechanical analysis of the Nb₃Sn 11 T dipole short models for the High Luminosity Large Hadron Collider

Bermúdez

Nilsson

Bottura

et al. 2019

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Metallographic analysis of 11 T dipole coils for High Luminosity-Large Hadron Collider (HL-LHC)

Balachandran¹,

Cooper

Oss

et al. 2021

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For next-generation accelerator magnets for fields beyond those achievable using Nb-Ti, Nb 3 Sn is the most viable superconductor. The high luminosity upgrade for the Large Hadron Collider (HL-LHC) marks an important milestone as it will be the first project where Nb 3 Sn magnets will be installed in an accelerator. Nb 3 Sn is a brittle intermetallic, so magnet coils are typically wound from composite strands containing ductile precursors before heat treating the wire components to form Nb 3 Sn. However, some mechanical assembly is still required after the coils have been heat-treated. In this paper, we present direct evidence of cracking of the brittle Nb 3 Sn filaments in a prototype dipole that resulted in degraded magnet performance. The cracking of the Nb 3 Sn, in this case, can be attributed to an issue with the collaring process that is required in the assembly of dipole accelerator magnets. Metallographic procedures were developed to visualize cracks present in the cables, along with quantitative image analysis for location-based crack analysis. We show that the stresses experienced in the damaged coil are above the critical damage stress of Nb 3 Sn conductor, as evidenced by a measured Cu stabilizer hardness of 85 HV 0.1 , which is higher than the Cu stabilizer hardness in a reference Nb 3 Sn cable ten-stack that was subjected to a 210 MPa transverse compression. We also show that once the collaring procedure issue was rectified in a subsequent dipole, the Nb 3 Sn filaments were found to be undamaged, and the Cu stabilizer hardness values were reduced to the expected levels. This paper provides a post-mortem verification pathway to analyze the damage, provides strand level mechanical properties, which could be beneficial for improving model prediction capabilities. This method could be applied beyond Nb 3 Sn magnets to composite designs involving high work hardening materials.

Optimizing the use of pressurized bladders for the assembly of HL-LHC MQXFB magnets

Troitino

Ambrosio

Bourcey

et al. 2023

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The use of pressurized bladders for stress control of superconducting magnets was firstly proposed at Lawrence Berkeley National Laboratory (LBNL) in the early 2000s. Since then, the so-called “bladders and keys” procedure has become one of the reference techniques for the assembly of high-field accelerator magnets and demonstrators. Exploiting the advantages of this method is today of critical importance for Nb3Sn-based accelerator magnets, whose production requires the preservation of tight stress targets in the superconducting coils to limit the effects of the strain sensitivity and brittleness of the conductor. The present manuscript reports on the results of an experimental campaign focused on the optimization of the “bladders and keys” assembly process in the MQXFB quadrupoles. These 7.2 m. long magnets shall be among the first Nb3Sn cryomagnets to be installed in a particle accelerator as a part of the High Luminosity upgrade of the LHC. One of the main practical implications of the bladders technique, especially important when applied to long magnets like MQXFB, is that to insert the loading keys, the opening of a certain clearance in the support structure is required. The procedure used so far for MQXF magnets involved an overstress in the coils during bladder inflation. The work presented here shows that such an overshoot can be eliminated thanks to additional bladders properly positioned in the structure. This optimized method was validated in a short model magnet and in a full-length mechanical model, becoming the new baseline for the series production at CERN. Furthermore, the results are supported by numerical predictions using Finite Element models.