G. Willering

Feasibility Study of

Feasibility study of Cu stabilized Nb3Al strand and Rutherford cable for the application to high field accelerator magnets are being done at Fermilab in collaboration with NIMS. The Nb3Al strand, which was developed and manufactured at NIMS in Japan, has a non-copper Jc of about 844 A/mm2 at 15 Tesla at 4.2 K, a copper content of 50%, and filament size of about 50 microns. Rutherford cables with 27 Nb3Al strands of 1.03 mm diameter were fabricated and tested. Quench tests on a short cable were done to study its stability with only its self field, utilizing a high current transformer. A pair of 2 meter long Nb3Al cables was tested extensively at CERN at 4.3 and 1.9 K up to 11 Tesla including its self field with a high transport current of 20.2 kA. In the low field test we observed instability near splices and in the central region. This is related to the flux-jump like behavior, because of excessive amount of Nb in the Nb3Al strand. There is possibility that the Nb in Nb3Al can cause instability below 2 Tesla field regions. We need further investigation on this problem. Above 8 Tesla, we observed quenches near the critical surface at fast ramp rate from 1000 to 3000 A/sec, with quench velocity over 100 m/sec. A small racetrack magnet was made using a 14 m of Rutherford cable and successfully tested up to 21.8 kA, corresponding to 8.7 T.

{\rm Nb}_{3}{\rm Al}

Future application of high-temperature superconductors in large volume, high field magnets and in magnet current distribution systems requires cabling of RE-Ba2Cu3O7 − δ coated conductor tapes. The substantial aspect ratio of RE-Ba2Cu3O7 − δ coated conductors and the highly resistive buffer layers in these tapes make the development of compact and homogeneous cable terminals complex. The contact resistance between individual tapes and the cable terminations of two types of high-temperature superconducting cables was determined at 77 K at relatively low current ramp rates using a non-destructive method. The current distribution between tapes in the cables caused by a variation in contact resistance was calculated with a simple model, which was validated using different experimental methods. The results show that the current distribution at low current ramp rates in cables made from RE-Ba2Cu3O7 − δ coated conductors is mainly dictated by the variations in contact resistances between tapes in the cable and the cable terminals. Development of practical cable terminals that minimize the variations in contact resistances is therefore instrumental for the successful application of high-temperature superconducting cables in magnets.

Rutherford Cable for High Field Accelerator Magnet Application

GSI, Darmstadt is preparing to build FAIR (Facility for Antiproton and Ion Research) which include SIS 300, a 300T - m fast-ramping heavy ion synchrotron. Dipoles for this ring will be 2.9 m long, producing 6 T over a 100 mm coil aperture and ramped at 1 T/s. The cable for these dipoles must have low losses and produce acceptable field distortions during the fast ramp. We plan to achieve this objective by using fine (~ 3 mum) filaments of NbTi in a wire with an interfilamentary matrix of CuMn to reduce proximity coupling and increase the transverse resistivity. The Rutherford cable have a thin stainless steel core and the wires will be coated with SnAg solder which has been oxidized, using a recipe similar to that developed at CERN, to increase the adjacent strand resistance Ra. Measurements of crossover strand resistance Re and Ra in cored cable with oxidized SnAg coating will be presented, together with data on critical current, persistent current magnetization and eddy current coupling in a wire with ultra fine filaments and a CuMn matrix in the interfilamentary region of the wire. These data will be used to predict losses and field distortion in the SIS 300 dipole and optimize the final design of cable for FAIR.

Effect of variations in terminal contact resistances on the current distribution in high-temperature superconducting cables

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.

Cable Design for FAIR SIS 300

The test of the first LARP (LHC Accelerator Research Program) Long Quadrupole is a significant milestone toward the development of Nb3Sn quadrupoles for LHC (Large Hadron Collider) Luminosity Upgrades. These 3.7-m long magnets, scaled from the 1-m long Technological Quadrupoles, are used to develop our capabilities to fabricate and assemble Nb3Sn coils and structures with lengths comparable to accelerator magnet dimensions. The long quadruples have a target gradient of 200 T/m in a 90-mm aperture. Pre-stress and support are provided by an Al-shell-based structure pre-loaded using bladders and keys. The coils were fabricated at BNL and FNAL, the shell-based structure was designed and assembled at LBNL, the test is performed at FNAL. In this paper we present the final steps of the development of the first model (LQS01), several upgrades to the test facility, the test results of witness cables, and the short sample limit.

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

The coupling-loss-induced quench (CLIQ) is an innovative system for the protection of superconducting magnets. Its energy-deposition mechanism, based on coupling loss generated directly in the superconductor, is fundamentally faster than heat diffusion, upon which traditional quench-heater-based systems rely. CLIQ electrical design relies on simple and robust components, i.e., easy to install and be replaced in case of damage. After being successfully tested on model magnets of different geometries and types of superconductor, CLIQ is now applied for the first time for the protection of a full-scale dipole magnet. For this purpose, a 14-m-long LHC twin-aperture dipole magnet is equipped with CLIQ terminals and two 80-mF 500-V CLIQ units are connected to its windings. Experimental results obtained under various operating conditions convincingly show that a CLIQ-based quench protection system can effectively protect large-scale magnets by quickly and homogeneously transferring to the normal-state voluminous regions of the winding packs. A developed dedicated simulation code correctly reproduces the complex electrothermal transient occurring during a CLIQ discharge. The successful test completes the development program of CLIQ quench protection systems, which has convincingly demonstrated the maturity and readiness of the system for application in large-scale magnet systems.

Final Development and Test Preparation of the First 3.7 m Long Nb3Sn Quadrupole by LARP

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.

First Implementation of the CLIQ Quench Protection System on a 14-m-Long Full-Scale LHC Dipole Magnet

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.

First Implementation of the CLIQ Quench Protection System on a Full-Scale Accelerator Quadrupole Magnet

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.

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

Over the last five years, the model MQXC quadruple, a 120-mm aperture, 120 T/m, 1.8 m long, Nb-Ti version of the LHC insertion upgrade (due in 2021), has been developed at CERN. The magnet incorporates several novel concepts to extract high levels of heat flux and provide high quality field harmonics throughout the full operating current range. Existing LHC-dipole cable with new, open cable and ground insulation was used. Two, nominally identical 1.8-m-long magnets were built and tested at 1.8 K at the CERN SM18 test facility. This paper compares in detail the two magnet tests and presents: quench performance, internal stresses, heat extraction simulating radiation loading in the superconducting coils, and quench protection measurements. The first set of tests highlighted the conflict between high magnet cooling capability and quench protection. The second magnet had additional instrumentation to investigate further this phenomenon. Finally, we present test results from a new type of superconducting magnet protection system.