S. Izquierdo Bermudez

Magnet Design of the 150 mm Aperture Low-

The high luminosity LHC (HL-LHC) project is aimed at studying and implementing the necessary changes in the LHC to increase its luminosity by a factor of five. Among the magnets that will be upgraded are the 16 superconducting low-β quadrupoles placed around the two high luminosity interaction regions (ATLAS and CMS experiments). In the current baseline scenario, these quadrupole magnets will have to generate a gradient of 140 T/m in a coil aperture of 150 mm. The resulting conductor peak field of more than 12 T will require the use of Nb3Sn superconducting coils. We present in this paper the HL-LHC low-β quadrupole design, based on the experience gathered by the US LARP program, and, in particular, we describe the support structure components to pre-load the coils, withstand the electro-magnetic forces, provide alignment and LHe containment, and integrate the cold mass in the LHC IRs.

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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 magnets conceptual design finalized, the MQXF project, a joint effort between the U.S. LHC Accelerator Research Program and the Conseil Européen pour la Recherche Nucléaire (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.

Quadrupoles for the High Luminosity LHC

The upgrade of the Large Hadron Collider (LHC) collimation system includes additional collimators in the LHC lattice. The longitudinal space for these collimators will be created by replacing some of the LHC main dipoles with shorter but stronger dipoles compatible with the LHC lattice and main systems. The project plan comprises the construction of two cryoassemblies containing each of the two 11-T dipoles of 5.5-m length for possible installation on either side of interaction point 2 of LHC in the years 2018-2019 for ion operation, and the installation of two cryoassemblies on either side of interaction point 7 of LHC in the years 2023-2024 for proton operation. The development program conducted in conjunction between the Fermilab and CERN magnet groups is progressing well. The development activities carried out on the side of Fermilab were concluded in the middle of 2015 with the fabrication and test of a 1-m-long two-in-one model and those on the CERN side are ramping up with the construction of 2-m-long models and the preparation of the tooling for the fabrication of the first full-length prototype. The engineering design of the cryomagnet is well advanced, including the definition of the various interfaces, e.g., with the collimator, powering, protection, and vacuum systems. Several practice coils of 5.5-m length have been already fabricated. This paper describes the overall progress of the project, the final design of the cryomagnet, and the performance of the most recent models. The overall plan toward the fabrication of the series magnets for the two phases of the upgrade of the LHC collimation system is also presented.

Development of MQXF: The Nb 3 Sn Low-

As part of the Large Hadron Collider (LHC) Luminosity upgrade program, the U.S.-LHC Accelerator Research Program collaboration and CERN are working together to design and build a 150-mm aperture Nb3Sn quadrupole for the LHC interaction regions. A first series of 1.5-m-long coils was fabricated and assembled in a first short model. A detailed visual inspection of the coils was carried out to investigate cable dimensional changes during heat treatment and the position of the windings in the coil straight section and in the end region. The analyses allow identifying a set of design changes which, combined with a fine tune of the cable geometry and a field quality optimization, were implemented in a new second-generation coil design. In this paper, we review the main characteristics of the first generation coils, describe the modification in coil layout and discuss their impact on parts design and magnet analysis.

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The luminosity upgrade of the large hadron collider (HL-LHC) requires the development of new type of superconducting cables based on advanced Nb3Sn 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 Nb3Sn conductor modeling.

Quadrupole for the HiLumi LHC

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.

The 11 T Dipole for HL-LHC: Status and Plan

In high field magnet applications, Nb3Sn coils undergo a heat treatment step after winding. During this stage, coils radially expand and longitudinally contract due to the Nb3Sn phase change. In order to prevent residual strain from altering superconducting performances, the tooling must provide the adequate space for these dimensional changes. The aim of this paper is to understand the behavior of cable dimensions during heat treatment and to provide estimates of the space to be accommodated in the tooling for coil expansion and contraction. This paper summarizes measurements of dimensional changes on strands, single Rutherford cables, cable stacks, and coils performed between 2013 and 2015. These samples and coils have been performed within a collaboration between CERN and the U.S. LHC Accelerator Research Program to develop Nb3Sn quadrupole magnets for the HiLumi LHC. The results are also compared with other high field magnet projects.

Second Generation Coil Design of the Nb3Sn low-beta Quadrupole for the High Luminosity LHC

As part of the Large Hadron Collider Luminosity upgrade (HiLumi) program, the US LARP collaboration and CERN are working together to design and build a 150 mm aperture Nb3Sn quadrupole magnet that aims at providing a nominal gradient of 140 T/m. In this paper we describe the optimization process yielding the selected 2D coil cross-section and the 3D coil ends design. For the 2D optimization a sector-coil model that allows fast computation of field harmonics is used to identify, among a large number of cases, those cross-sections that provide an acceptable field quality. A more detailed analysis of these solutions is then performed and it led to the selection of an optimized cross-section from which a real coil is built by approximating sectors with blocks of cable. A 3D design of the coil ends is then realized with the Roxie software. Optimization constraints are set on the integrated multipoles, the peak field, and the coil head length.

Quench Analysis of High-Current-Density Nb 3 Sn Conductors in Racetrack Coil Configuration

Magnetic field changes in superconducting filaments are shielded by the so-called persistent currents. These currents produce field errors, which are particularly important at low energy level, where the effect is stronger compared to the field generated by the transport current. For the next generation of particle accelerators, magnetic fields higher than 10 T are needed, requiring the use of Nb3Sn as superconductor. Due to the larger filament size (typically ten times larger than in the Nb-Ti Large Hadron Collider dipoles) and higher current density, strand magnetization effects shall be carefully studied. This paper presents an analysis on the expected field errors due to persistent currents in a 16-T dipole magnet. We briefly discuss the method and compare with experimental results on the 11-T Nb3Sn dipole for the Hi-Luminosity upgrade. The model is then used to predict the persistent-current effects for different coil layouts and strand characteristics in a 16-T dipole.

Magnetic Measurements and Analysis of the First 11-T Nb 3 Sn Dipole Models Developed at CERN for HL-LHC

The upgrade of the LHC collimation system includes additional collimators in the LHC lattice. The longitudinal space for the collimators can be obtained by replacing some LHC main dipoles with shorter but stronger dipoles compatible with the LHC lattice and the existing powering circuits, cryogenics, and beam vacuum. A joint development programme aiming at building a 5.5 m long two-in-one aperture Nb3Sn dipole prototype suitable for installation in the LHC is being conducted by FNAL and CERN. As part of the first phase of the programme, 1 m and 2 m long single aperture models are being built and tested. Later on, the collared coils from these models will be assembled and tested in a two-in-one aperture configuration in both laboratories. A 2 m long practice model made of a single coil wound with Nb3Sn cable, MBHSM101, was developed and constructed at CERN. It has been completed, and tested at both 4.3 K and 1.9 K. This practice model features collared coils based on removable pole concept, S2-glass cable insulation braided over a mica layer, and coil end spacers made of sintered stainless steel with springy legs. The paper describes the main features of this practice model, the main manufacturing steps and the results of the cold tests.