G. de Rijk

Advanced Accelerator Magnets for Upgrading the LHC

The Large Hadron Collider is working at about half its design value, limited by the defective splices of the magnet interconnections. While the full energy will be attained after the splice consolidation in 2014, CERN is preparing a plan for a Luminosity upgrade (High Luminosity LHC) around 2020 and has launched a pre-study for exploring an Energy upgrade (High Energy LHC) around 2030. Both upgrades strongly rely on advanced accelerator magnet technology, requiring dipoles and quadrupoles of accelerator quality and operating fields in the 11-13 T range for the luminosity upgrade and 16-20 T range for the energy upgrade. The paper will review the last ten year of Nb3Sn accelerator magnet R&D and compare it to the needs of the upgrades and will critically assess the results of the Nb3Sn and HTS technology and the planned R&D programs also based on the inputs of first year of LHC operation.

Accelerator-Quality HTS Dipole Magnet Demonstrator Designs for the EuCARD-2 5-T 40-mm Clear Aperture Magnet

Future high-energy accelerators will need very high magnetic fields in the range of 20 T. The Enhanced European Coordination for Accelerator Research and Development (EuCARD-2) Work Package 10 is a collaborative push to take high-temperature superconductor (HTS) materials into an accelerator-quality demonstrator magnet. The demonstrator will produce 5 T stand alone and between 17 and 20 T when inserted into the 100-mm aperture of a Fresca-2 high-field outsert magnet. The HTS magnet will demonstrate the field strength and the field quality that can be achieved. An effective quench detection and protection system will have to be developed to operate with the HTS superconducting materials. This paper presents a ReBCO magnet design using a multistrand Roebel cable that develops a stand-alone field of 5 T in a 40-mm clear aperture and discusses the challenges associated with a good field quality using this type of material. A selection of magnet designs is presented as the result of the first phase of development.

The EuCARD-2 Future Magnets European Collaboration for Accelerator-Quality HTS Magnets

EuCARD-2 is a project supported by FP7-European Commission that includes, inter alia, a work-package (WP10) called “Future Magnets.” This project is part of the long term development that CERN is launching to explore magnet technology at 16 T to 20 T dipole operating field, within the scope of a study on Future Circular Colliders. The EuCARD2 collaboration is closely liaising with similar programs for high field accelerator magnets in the USA and Japan. The main focus of EuCARD2 WP10 is the development of a 10 kA-class superconducting, high current density cable suitable for accelerator magnets, The cable will be used to wind a stand-alone magnet 500 mm long and with an aperture of 40 mm. This magnet should yield 5 T, when stand-alone, and will enable to reach a 15 to 18 T dipole field by placing it in a large bore background dipole of 12-15 T. REBCO based Roebel cables is the baseline. Various magnet configurations with HTS tapes are under investigation and also use of Bi-2212 round wire based cables is considered. The paper presents the structure of the collaboration and describes the main choices made in the first year of the program, which has a breadth of five to six years of which four are covered by the FP7 frame.

The EuCARD High Field Magnet Project

In the EC FP7 program EuCARD (European Coordination for Accelerator Research & Development) the subject of work-package 7 is High Field Magnets. The aim of the HFM work-package is to develop technologies for magnets in the range 13 T-20 T for accelerator applications like HL-LHC and HE-LHC. The work-package foresees construction of a 13 T dipole with an aperture of 100 mm, an HTS dipole insert with ΔB = 6 T, an HTS current link and a helical undulator. The work-package has 12 European partners and the funding is shared between the EC and the partners. This contribution describes the aims of the work-package, the status of the work and the structure of the collaboration.

Design of a high-field twin aperture superconducting dipole model

An initial design for the Large Hadron Collider to be installed in the tunnel of the CERN LEP machine is based on the so-called two-in-one concept, originally proposed and tested at Brookhaven National Laboratory for fields up to approximately 5 T. The dipole magnets of the two rings have a common magnetic circuit and are located in one cryostat. Two major lines of development are followed for the construction of models of high-field magnets: (1) using Nb/sub 3/Sn superconducting cables at 4.5 K, and (2) using NbTi alloy superconducting cables at 2 K. The models discussed are 1 m long twin aperture dipoles. They employ NbTi alloy superconducting cables at 2 K but, apart from the coils, the mechanical design concepts are equally valid for magnets using using Nb/sub 3/Sn cables. A role of the models is to test the various components and procedures of fabrication in order to obtain the best performances at lowest cost. Possible variants in the design are mentioned. >

Design of the EuCARD High Field Model Dipole Magnet FRESCA2

This paper reports on the design of FRESCA2, a dipole magnet model wound with Nb3Sn Rutherford cable. This magnet is one of the deliverables of the High Field Magnets work package of the European FP7-EuCARD project. The nominal magnetic flux density of 13 Tesla in a 100 mm bore will make it suitable for upgrading the FRESCA cable test facility at CERN. The magnetic layout is based on a block coil, with four layers per pole. The mechanical structure is designed to provide adequate pre-stress, through the use of bladders, keys and an aluminum alloy shrinking cylinder.

Performance of a

Future upgrades of the Large Hadron Collider (LHC) will require large aperture and high gradient quadrupoles. Nb3Sn is the most viable option for this application but is also known for its strain sensitivity. In high field magnets, with magnetic fields above 12 T, the Lorentz forces will generate mechanical stresses that may exceed 200 MPa in the windings. The existing measurements of critical current versus strain of Nb3Sn strands or cables are not easily applicable to magnets. In order to investigate the impact of high mechanical stress on the quench performance, a series of tests was carried out within a LBNL/CERN collaboration using the magnet TQS03 (a LHC Accelerator Research Program (LARP) 1-meter long, 90-mm aperture Nb3Sn quadrupole). The magnet was tested four times at CERN under various pre-stress conditions. The average mechanical compressive azimuthal pre-stress on the coil at 4.2 K ranged from 120 MPa to 200 MPa. This paper reports on the magnet performance during the four tests focusing on the relation between pre-stress conditions and the training plateau.

{\rm Nb}_{3}{\rm Sn}

A design study is presented for the coil layout of the EUCARD-2 Five Tesla HTS Research (FeaTHeR) magnet. The angular dependence of the critical current in the used ReBCO Roebel cable is taken into account. This leads to a new coil layout named aligned block. This layout makes optimal use of the anisotropy of the ReBCO coated conductor, by aligning all tapes with the magnetic field lines. Both two-dimensional cross sections and three-dimensional coil layouts are presented. In the layouts the magnetic field angle is highest at the edges of the cable causing a large variation of the critical current over its width. Different approaches to the calculation of the critical current, with and without current sharing in and between the tapes, are presented. The values are compared to the values found using a non-linear network model of the cable, in which the electrical properties of the elements are calculated as a function of magnetic field and magnetic field angle. The model also includes electrical contact between the strands using additional network elements.

Quadrupole Under High Stress

For the high-energy LHC, a study of a 33 TeV center of mass collider in the LHC tunnel, main dipoles of 20 T operational field are needed. In this paper, we first review the conceptual design based on block coil proposed in the Malta workshop, addressing the issues related to coil fabrication and assembly. We then propose successive simplifications of this design, associating a cost estimate of the conductor. We then analyze a block layout for a 15 T magnet. Finally, we consider two layouts based on the D20 and HD2 short models built by LBL. A first analysis of the aspects related to protection of these challenging magnets is given.

Study of a 5 T Research Dipole Insert-Magnet Using an Anisotropic ReBCO Roebel Cable

The Short Model Coil (SMC) assembly has been designed, as test bench for short racetrack coils wound with cable. The mechanical structure comprises an iron yoke surrounded by a 20 mm thick aluminum alloy shell, and includes four loading pads that transmit the required pre-compression from the outer shell into the two coils. The outer shell is pre-tensioned with mechanical keys that are inserted with the help of pressurized bladders and two 30 mm diameter aluminum alloy rods provide the axial loading to the coil ends. The outer shell, the axial rods, and the coils are instrumented with strain gauges, which allow precise monitoring of the loading conditions during the assembly and at cryogenic temperature during the magnet test. Two SMC assemblies have been completed and cold tested in the frame of a European collaboration between CEA (FR), CERN and STFC (UK) and with the technical support from LBNL (US). This paper describes the main features of the SMC assembly, the experience from the dummy assemblies, the fabrication of the coils, and discusses the test results of the cold tests showing a peak field of 12.5 T at 1.9 K after training.