Among the components to be upgraded in LHC interaction regions for the HiLumi-LHC projects are the inner triplet (or low-β) quadrupole magnets, denoted as Q1, Q2a, Q2b, and Q3. The new quadrupole magnets, called MQXF, are based on Nb3Sn superconducting magnet technology and operate at a gradient of 132.6 T/m, with a conductor peak field of 11.4 T. Q1 and Q3 are composed of magnets (called MQXFA) fabricated by the U.S. Accelerator Upgrade Project (AUP), with a magnetic length of 4.2 m. Q2a and Q2b consist of magnets (called MQXFB) fabricated by CERN, with a magnetic length of 7.15 m. After a series of short models, constructed in close collaboration by the US and CERN, the development program is now entering in the prototyping phase, with CERN on one side and BNL, FNAL, and LBNL on the other side assembling and testing their first long magnets We provide in this paper a description of the status of the MQXF program, with a summary of the short model test results, including quench performance, and mechanics, and an update on the fabrication, assembly, and test of the long prototypes.

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The HL-LHC Low-β Quadrupole Magnet MQXF: From Short Models to Long Prototypes

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.

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Progress on HL-LHC Nb3Sn Magnets

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.

Tests of the FRESCA2 100 mm bore Nb$_3$Sn block-coil magnet to a record field of 14.6 T

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.

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Progress in the Development of the Nb 3 Sn MQXFB Quadrupole for the HiLumi Upgrade of the LHC

In the framework of the EuroCirCol project, the development of a conceptual design of the Future Circular Collider main dipole is performed. Here is described the status of the study focusing on the block coil layout. An electromagnetic investigation carried out in a double aperture configuration is reported. This study is followed by a preliminary investigation of the magnet performance in three-dimensional (3-D) taking into consideration the integrated field quality as well as the peak field in the coil ends. Finally, a 2-D mechanical analysis of the bladder and key structure is detailed. This study clearly shows that the design reaches the limit of the material properties.

Design of a Nb 3 Sn 16 T Block Dipole for the Future Circular Collider

The production of superconducting magnets for particle accelerators involves high-precision assemblies and tight tolerances, in order to achieve the requirements for their appropriate performance. It is therefore essential to have a strict control and traceability over the geometry of each component of the system, and also to be able to compensate possible inherent deviations coming from the production process. The objective of this paper is to present the experience from systematic geometrical measurements performed during the ongoing production of model magnets for the high luminosity–Large Hadron Collider upgrade. First, the methodology for the data acquisition and its ulterior analysis is described. Then, the results obtained in terms of coil geometry are explained with the goal of identifying the principal factors causing systematic and unexpected dimensional deviations. Finally, the integrated effect of assembly operations, cool down, and powering of the magnet is investigated looking at measurements before and after cold tests.

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Applied Metrology in the Production of Superconducting Model Magnets for Particle Accelerators

The Nb3Sn dipole magnet FRESCA2 has been developed and manufactured within the framework of a collaboration between the Centre for Atomic Energy (CEA Saclay) and the European Organization for Nuclear Research (CERN). The aim of the magnet is to upgrade the superconducting cable test station at CERN with a Nb3Sn dipole providing a 13-T magnetic field in a 100-mm aperture. The magnet is composed of four coils in a block-type configuration with flared ends, 1.6 m long, housed in a bladder, and key-type mechanical structure. A first assembly of the magnet, FRESCA2a, has been done at CERN at the end of 2016 with the four first Nb3Sn coils fabricated at CEA Saclay and CERN. The coils were dimensionally measured, and tailored shims were fabricated and inserted in the coil pack to improve the contact between layers. In addition, tests with pressure sensitive films were carried out to verify the uniformity of the loading. The magnet was cold tested in February 2017 and dismounted to replace one coil. A second assembly, FRESCA2b, was produced and tested in August 2017. In this paper, we provide a detailed description of the various steps of the assembly from the impregnated coils to the delivery of the dipole to the test facility, with a particular emphasis on the procedures followed and the tooling developed.

Assembly of the Nb 3 Sn Dipole Magnet FRESCA2

In order to predict volume changes of Nb
 3
Sn coils during reaction heat treatment (RHT), coefficients of thermal expansion (CTE) of the involved materials need to be known. While the CTEs of bulk materials are widely available, there is a lack of knowledge about the dimensional changes of the Nb
 3
Sn Rutherford cables. By extensometry we have measured the length changes of 11 T dipole Nb3Sn Rutherford cables during the RHT. Similar to the length change behavior of multifilament wires, after an initial cable length increase the cable length decreases when the temperature exceeds about 150 °C, due to relaxation of the Nb subelements when the Cu matrix is annealed. At higher temperatures the cable length increase is determined by the CTE of the unreacted Nb, before the onset of Nb
 3
Sn formation leads to an increased expansion rate. During cooling, the thermal contraction of fully reacted Nb
 3
Sn cable is similar to that of Nb
 3
Sn bulk.

Length Changes of Unconfined Nb3Sn Rutherford Cables During Reaction Heat Treatment

Future applications requiring high magnetic fields, such as the proposed Future Circular Collider, demand a substantially higher critical current density, Jc, at fields ≥16 T than is presently available in any commercial strand, so there is a strong effort to develop new routes to higher Jc Nb3Sn As a consequence, evaluating the irreversibility field (Hirr) of any new conductor to ensure reliable performance at these higher magnetic fields becomes essential To predict the irreversibility field for Nb3Sn wires, critical current measurements, Ic, are commonly performed in the 12-15 T range and the Kramer extrapolation is used to predict higher field properties The Kramer extrapolation typically models the contribution only for sparse grain boundary pinning, yet Nb3Sn wires rely on a high density of grain boundaries to provide the flux pinning that enables their high critical current density However, whole-field range VSM measurements up to 30 T recently showed for Nb3Sn RRP® wires that the field dependence of the pinning force curve significantly deviates from the typical grain boundary shape, leading to a 1-2 T overestimation of Hirr when extrapolated from the typical mid-field data taken only up to about 15 T In this work we characterized a variety of both RRP® and PIT Nb3Sn wires by transport measurements up to 29 T at the Laboratoire National des Champs Magnetiques Intenses (LNCMI), part of the European Magnetic Field Laboratory in Grenoble, to verify whether or not such overestimation is related to the measurement technique and whether or not it is a common feature across different designs Indeed we also found that when measured in transport the 12-15 T Kramer extrapolation overestimates the actual Hirr in both types of conductor with an inaccuracy of up to 16 T, confirming that high field characterization is a necessary tool to evaluate the actual high field performance of each Nb3Sn wire

Alejandro Carlon Zurita

Papers

Applied Metrology in the Production of Superconducting Model Magnets for Particle Accelerators

Assembly of the Nb 3 Sn Dipole Magnet FRESCA2

Length Changes of Unconfined Nb3Sn Rutherford Cables During Reaction Heat Treatment

Evidence of Kramer extrapolation inaccuracy for predicting high field Nb3Sn properties