H. Bajas

Cold Test Results of the LARP HQ

The high gradient quadrupole magnet is a 120-mm-aperture, 1-m-long Nb3Sn quadrupole developed by the LHC Accelerator Research Program collaboration in support of the High-Luminosity LHC project. Several tests were performed at Lawrence Berkeley National Laboratory in 2010-2011 achieving a maximum gradient of 170 T/m at 4.4 K. As a next step in the program, the latest model (HQ01e) was sent to CERN for testing at 1.9 K. As part of this test campaign, the magnet training has been done up to a maximum current of 16.2 kA corresponding to 85% of the short sample limit. The ramp rate dependence of the quench current is also identified. The efficiency of the quench heaters is then studied at 4.2 K and at 1.9 K. The analyses of the magnet resistance evolution during fast current discharge showed evidence of quench whereas high energy quenches have been successfully achieved and sustained with no dump resistor.

\hbox{Nb}_{3} \hbox{Sn}

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.

Quadrupole Magnet at 1.9 K

A new protection system for superconducting magnets called coupling-loss induced quench system (CLIQ) has been recently developed at CERN. Recent tests on Nb-Ti coils have shown that CLIQ is a valid, efficient, and promising method for the protection of high-magnetic-field superconducting magnets. However, the protection of new-generation Nb3Sn accelerator magnets is even more challenging due to the much higher stored energy per unit volume and to the significantly larger enthalpy needed to initiate and propagate a normal zone in such coils. Now, the CLIQ system is tested for the first time on a Nb3Sn magnet in the CERN magnet test facility in order to investigate its performance in practice, thereby validating the method for this type of superconducting magnets as well. Furthermore, we successfully reproduced the electrothermal transients during a CLIQ discharge. Finally, the implementation of various CLIQ-based protection schemes for the full-scale Nb3Sn quadrupole magnet for the LHC high luminosity upgrade is discussed. The impact of key system parameters on CLIQ performance and the advantages and drawbacks of using multiple CLIQ units on a single magnet are discussed.

Development of MQXF: The Nb 3 Sn Low-

A key challenge for a future circular collider (FCC) with centre-of-mass energy of 100 TeV and a circumference in the range of 100 km is the development of high-field superconducting accelerator magnets, capable of providing a 16 T dipolar field of accelerator quality in a 50 mm aperture. This paper summarizes the strategy and actions being undertaken in the framework of the FCC 16 T Magnet Technology Program and the Work Package 5 of the EuroCirCol.

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FNAL and CERN are performing an R&D program with the goal of developing a 5.5 m long twin-aperture 11 T Nb3Sn dipole suitable for installation in the Large Hadron Collider (LHC). An important part of the program is the development and test of a series of short single-aperture and twin-aperture models with a nominal field of 11 T at the LHC nominal current of 11.85 kA and 20% margin. This paper describes design and fabrication features, and test results of a 1 m long single-aperture Nb3Sn dipole model tested at FNAL.

Quadrupole for the HiLumi LHC

The HQ magnet is a 120-mm aperture, 1-m-long Nb3Sn quadrupole developed by the LARP collaboration in the framework of the High-Luminosity LHC project. A first series of coils was assembled and tested in five assemblies of the HQ01 series. The HQ01e model achieved a maximum gradient of 170 T/m at 4.5 K at LBNL in 2010-2011 and reached 184 T/m at 1.9 K at CERN in 2012. A new series of coils incorporating major design changes was fabricated for the HQ02 series. The first model, HQ02a, was tested at Fermilab where it reached 98% of the short sample limit at 4.5 K with a gradient of 182 T/m in 2013. However, the full training of the coils at 1.9 K could not be performed due to a current limit of 15 kA. Following this test, the azimuthal coil pre-load was increased by about 30 MPa and an additional current lead was installed at the electrical center of the magnet for quench protection studies. The test name of this magnet changed to HQ02b. In 2014, HQ02b was then shipped to CERN as the first opportunity for full training at 1.9 K. In this paper, we present a comprehensive summary of the HQ02 test results including: magnet training at 1.9 K with increased preload; quench origin and propagation; and ramp rate dependence. A series of powering tests was also performed to assess changes in magnet performance with a gradual increase of the MIITs. We also present the results of quench protection studies using different setting for detection, heater coverage, energy extraction and the coupling-loss induced quench (CLIQ) system.

Protecting a full-scale Nb3Sn magnet with CLIQ, the new coupling-loss-induced quench system

The design, fabrication, and tests of the new generation of superconducting magnets for the High Luminosity upgrade of the Large Hadron Collider (HL-LHC) require the support of an adequate sensing technology able to assure the integrity of the strain-sensitive and brittle superconducting cables through the whole service life of the magnet: assembly up to 150 MPa, cool down to 1.9 K, and powering up to about 16 kA. A precise temperature monitoring is also needed, in order to guarantee the safe working condition of the superconducting cables in the power transmission lines (SC-Link) designed to feed the magnet over long distance. Fiber Bragg Grating-based temperature and strain monitoring systems have been implemented in the first SC-Link prototype and in two subscale dipole magnets and tested in the cryogenic test facility at CERN, at 30 K, 77 K, and 1.9 K.

The 16 T Dipole Development Program for FCC

The coupling-loss induced quench system (CLIQ) is an innovative method for the protection of high-field superconducting magnets. With respect to the conventional method based on quench heaters, it offers significant advantages in terms of electrical robustness and energy-deposition velocity. Its effective intrawire heating mechanism targets a fast and homogeneous transition to the normal state of the winding pack, hence assuring a quick magnet discharge and avoiding overheating of the coils hot spot. Furthermore, it is possible to implement CLIQ as a time- and cost-effective repair solution for the protection of existing magnets with broken quench heaters. After being successfully tested on model magnets of different geometries and made of different types of superconductor, CLIQ is now applied for the first time for the protection of a full-scale quadrupole magnet at the CERN magnet test facility. One aperture of a 3.4-m-long LHC matching quadrupole magnet is equipped with dedicated terminals to allow the connection of a CLIQ system. Experimental results convincingly show that CLIQ can protect this coil over the entire range of operating conditions. The complex electrothermal transients during a CLIQ discharge are successfully reproduced by means of a 2-D model. The test is part of the R&D program of CLIQ quench protection systems, which has convincingly demonstrated the maturity of this technology and its effectiveness also for large-scale magnet systems. The proposed CLIQ-based solution for the quench protection of the LHC matching quadrupole magnet is now ready to be implemented in the LHC machine if needed.

Quench Performance of a 1 m Long Single-Aperture 11 T

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.

\hbox{Nb}_{3}\hbox{Sn}

As part of the European project EuCARD, the aim of the short model coil (SMC) dipole magnet is to perform R&D on the Nb3Sn coil fabrication technology while testing Nb3Sn superconducting cables. The baseline design features two double-layer racetrack coils, within a support structure based on bladders and keys technology and surrounded by an aluminum shell. The last magnet assembled up to now of the SMC series (SMC3a) was tested in 2011 and it reached a peak field of 12.5 T in the coil, corresponding to approximately 90 % of the short sample limit. Following the successful test of SMC3a, modifications were implemented in the design of the coil parts and support structure in order to accommodate wider cables. While making a valid contribution to the development of the Nb3Sn magnets technology, the final goal of the high field magnet project is to design, build, and test the FRESCA 2 magnet. Based on the SMC structure, the racetrack model coil represents an upgrade of the SMC in order to test a FRESCA 2 cable. The first part of this paper describes the status of activities on the SMC project, the design changes for the future SMC, and their predicted magnet parameters. The second part is dedicated to the description of the magnetic and mechanical design of the racetrack model coil.