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The 11 T magnet and other high-field magnets in the framework of the High-Luminosity Large Hadron Collider project are based on epoxy resin impregnated Nb 3 Sn coils. This paper presents the development of an experimental measurement setup, and associated methodology for the determination of the mechanical properties in compression of epoxy resin impregnated Nb 3 Sn coils. The challenges addressed include the nonlinear compliance correction in complex compression testing. The application of these techniques is demonstrated by the experimental characterization of the compression behavior of 11 T coils.

The development of new generation superconducting magnets for fusion research, such as the ITER experiment, is largely based on coils wound from so-called “Cable-In-Conduit” Conductors (CICCs). CICCs consist of various types of stainless steel jackets, densely filled with compacted superconducting strands, which are cooled by supercritical helium. The design of the various magnet systems, and in particular the ITER Poloidal Field (PF) coils, imposes the use of electrical joints to connect unit lengths of the CICCs. The electrical joints are delicate, electrical resistive components, carefully designed to provide efficient high current transfer while avoiding heat generation. The PF joints are subjected to fast varying magnetic fields that induce currents which, combined with the Joule heating in the resistive joints due to transport current, increase the temperature of the helium. Various characteristics, including electrical performance and mechanical behavior, have been addressed in the past in order to optimize manufacturing for satisfactory joint operation. Here an extensive post-mortem characterization of pre-qualification full-size PF joints is reported. Void fraction, twist pitch, and the current path connection are investigated in order to understand their effect on electrical performance and tune the manufacturing processes.

The correction coils (CCs) are a system of 18 superconducting coils composed of top CCs, side CCs, and bottom CCs, and are required to correct asymmetries and reduce magnetic error fields detrimental to the plasma performance of the ITER machine. The CC terminals will be connected with those of the superconducting busbars with twin-box joints. Qualification of the manufacturing procedure of the coil terminals is achieved by performing electrical tests of prototype joints in relevant conditions of current, temperature, and background field (4.5 K, 10 kA, and 2.5 T). In order to control the dc resistance and ac loss in series production, special tooling and manufacturing processes were developed. The main technological steps are conductor dejacketing, nickel removal, and silvering of the cable in the terminal, followed by cable compaction and tungsten inert gas welding of the termination box. To this end, two joint samples were manufactured in 2015 and 2016. The performance of the joint samples was verified before and after 1000 electromagnetic cycles. The qualification tests were carried out in the SULTAN facility and ASIPP facility. The test results show the dc resistance below 5- $\text{n}\Omega $ criterion and ac losses below 7 J/cycle.

International audience; The ITER magnet system is based on the “cable-in-conduit” conductor concept, which consists of various types of stainless steel jackets filled with superconducting strands. The jackets provide high strength and fracture toughness to counteract the high stress imposed by, amongst others, electromagnetic loads at cryogenic temperature. Material properties of austenitic stainless steel at cryogenic temperature are known to some extent, but only partial information is available for their welds, particularly in combination with weld fillers envisaged for cryogenic service. When a full inspection of the welded components is not possible, it becomes of special interest an assessment of its fracture toughness under close-to-service conditions if a fracture mechanics’ design approach is to be adopted. In absence of defects, brittle secondary phases are generally held responsible of the loss of ductility and toughness which is to be expected after postweld heat treatments. Their quantification becomes thus essential in order to explain the negative impact in fracture toughness after unavoidable thermal treatments. This paper investigates fracture toughness behavior at 7 K of AISI 316L and AISI 316LN tungsten inert gas welds using two fillers adapted to cryogenic service, EN 1.4453 and JK2LB. Additionally, the effect of such an aforementioned heat treatment, here the Nb$_3$Sn reaction heat treatment (650° for 200 h) on fracture toughness of the welds is evaluated. A correlation between the evolution of properties and the quantity of secondary phases as a result of the above treatment is provided.

The ITER central solenoid (CS) components are currently being manufactured. This Nb3Sn superconducting magnet will provide the magnetic flux swing required to induce up to 15 MA as plasma current. It includes six identical coils, called modules, stacked on top of each other to form a solenoid, enclosed inside a structure split into nine subsets, to provide vertical precompression and mechanical support. High mechanical stresses in materials and high voltages call for the use of structural materials with high strength and toughness and high dielectric strength insulating materials, respectively. The pulsed operation imposes materials with high fatigue strength at cryogenic temperatures. Unlike for the structure, where large existing manufacturing tools were usable, the modules required the construction of a dedicated manufacturing line. A comprehensive qualification programme is performed at the manufacturers before applying procedures for the production of the CS components. The main characteristics of the CS components, their manufacturing routes and the different elements of the qualification programme are described. The overall plan for the manufacture is reported. The status of the first series production components manufactured is presented as well as the planned delivery schedule to the ITER site.

The central solenoid (CS) is the backbone of the ITER magnet system. It consists of six independent coils held together by a vertical precompression structure that must react to tensile loads and provide sufficient preload to maintain coil-to-coil contact during all stages of plasma operation. Material selection and specifications applicable to the structural components of the precompression structure are particularly demanding. These include large forgings manufactured from a high-strength austenitic stainless steel (FXM-19) with a stringent specification in terms of microstructure and maximum allowed magnetic permeability. Stringent requirements are also imposed on all welded joints. In particular, the attachment welds of the cooling pipes to the structure are subject to challenging restrictions in terms of weld imperfections and geometry. They must induce limited distortion of the components and are submitted to inspections carried out in accordance with the most severe acceptance levels of applicable national and international standards. The results of a campaign of tests covering nondestructive and destructive evaluation of samples from different components and welds are summarized in this paper, particularly focusing on the quality achieved at the microstructural and macrostructural level. The influence of the microstructure on the final properties and on the inspectability of the material by nondestructive examinations is also discussed.

The ITER magnet system is based on the “cable-in-conduit” conductor (CICC) concept, which consists of stainless steel jackets filled with superconducting strands. The jackets provide high strength, limited fatigue crack growth rate and fracture toughness properties to counteract the high stress imposed by, among others, electromagnetic loads at cryogenic temperature. Austenitic nitrogen-strengthened stainless steels have been chosen as base material for the jackets of the central solenoid and the toroidal field system, for which an extensive set of cryogenic mechanical property data are readily available. However, little is published for their welded joints, and their specific performance when considering different combinations of parent and filler metals. Moreover, the impact of post-weld heat treatments that are required for Nb3Sn formation is not extensively treated. Welds are frequently responsible for cracks initiated and propagated by fatigue during service, causing structural failure. It becomes thus essential to select the most suitable combination of parent and filler material and to assess their performance in terms of strength and crack propagation at operation conditions. An extensive test campaign has been conducted at 7 K comparing tungsten inert gas (TIG) welds using two fillers adapted to cryogenic service, EN 1.4453 and JK2LB, applied to two different base metals, AISI 316L and 316LN. A large set of fracture toughness data are presented, and the detrimental effect on fracture toughness of post-weld heat treatments (unavoidable for some of the components) is demonstrated. In this study, austenitic stainless steel TIG welds with various filler metals have undergone a comprehensive fracture mechanics characterization at 7 K. These results are directly exploitable and contribute to the cryogenic fracture mechanics properties database of the ITER magnet system. Additionally, a correlation between the impact in fracture toughness and microstructure resulting from the above treatment is provided.

The material characterization of the gaseous electron multiplier (GEM) detectors is presented in this article; mechanical tests were performed to examine the mechanical performance of polyimide (Kapton) and GEM foil. Bearing in mind the mechanical stresses imposed during the assembly and subsequently during operation at Compact Muon Solenoid (CMS). The detectors will run for many years in the harsh radiation and environmental conditions at the CMS. In view of this a series of the tensile tests were performed by using different samples which were prepared accordingly, i.e. a set of the samples was exposed to neutron and another exposed to gamma radiation, and then these were made dry and wet to assess the effects of radiation and humidity on their tensile properties. In this paper the experimental setup, testing procedures, sampling and detailed results are presented. In general, due to the radiation and environment variation conditions the degradation of the GEM material is not significant with respect to its toughness, but the Young's modulus was effected.

The ITER Correction Coils (CCs) consist of three sets of six coils, Bottom (BCC), Side (SCC) and Top Correction Coils (TCC), respectively. The CCs rely on 10 kA NbTi cable-in-conduit conductor. Each CC winding pack is enclosed inside a 20 mm thick 316LN stainless steel case, providing structural reinforcement against the electromagnetic loads arising in the winding pack. The material of coil cases shall feature ready weldability both by laser and conventional techniques, high strength and toughness at 4 K. Material production involves not only hot rolled plates of different thicknesses, but also heavy gauge extrusions. An adapted steelmaking route including Electroslag Remelting (ESR) combined with a hot transformation step involving redundant multidirectional forging of the slabs and of the billets was adopted. This route allowed the stringent material specification to be fulfilled and permitted to achieve cleanliness, fineness of the structure and homogeneity on the final products. An extensive follow-up of the steel manufacturing through systematic non-destructive and destructive examinations was carried out in order to ascertain the soundness and the homogeneity of the final products. The progresses accomplished throughout the steel manufacturing are highlighted. The properties achieved on the final products that were enabled by the selected manufacturing route are discussed.

The twofold goals of this work are (i) to reduce the lack of experimental data in literature concerning the effects of temperature and mechanical strain on magnetic properties of the Ni-Fe alloy Cryophy, and (ii) to validate its use as magnetic shield material for last-generation cryomodule prototypes of crab cavities used in particle accelerators for transverse deflection. The relative magnetic permeability was measured at room and cryogenic temperature, and its lowest value at 4 K fits the minimum design criteria of 100,000 for the crab cavities experiment at CERN. Permeability after uniaxial plastic deformation between 0% and 3% was also measured by means of an Epstein frame. Results show that deformation induces a significant decrease in the magnetic performances, underlining that particular care must be taken during all stages of handling and operation. Finally, the attenuation inside the magnetic shields was tested for the prototype Super Proton Synchrotron at CERN. Results highlight that at 150 mm from the opening, the magnetic field is shielded as required.

Proceeding of: 27th International Cryogenic Engineering Conference and International Cryogenic Materials Conference 2018 (ICEC-ICMC 2018), September 3-7, 2018, Oxford, United Kingdom The ITER Central Solenoid (CS) consists of six independent wound modules. The cooling of the cable-in-conduit conductor is assured by a forced flow of supercritical He at 4.5 K supplied by He inlets located at the innermost radius of the coil. The inlets consist of a racetrack-shaped boss welded to the outer conduit wall through a full penetration Tungsten Inert Gas (TIG) weld. They are critical structural elements submitted to severe cyclic stresses due to the electro-magnetic forces acting on the coils. The weld contour is shape-optimised and locally processed by Ultrasonic Shot Peening (USP), conferring large compressive residual stresses on a subsurface layer of several millimetres thickness to improve fatigue strength. The distribution of the residual stresses and the effect of USP on microstructure and mechanical properties is assessed, with reference to the results of a cryogenic fatigue test campaign, performed on peened and as-welded inlets for comparison.

The ITER central solenoid (CS) consists of six independent coils kept together by a precompression support structure that must react vertical tensile loads and provide sufficient preload to maintain coil-to-coil contact when the solenoid is energized. The CS precompression system includes tie plates, lower and upper key blocks, load distribution and isolation plates and other attachment, support and insulating hardware. The tie plates operating at 4 K are manufactured starting from forgings in a high-strength austenitic stainless steel (FXM-19) with a stringent specification. Moreover, forged components for the lower and upper key blocks have to be provided in the same FXM-19 grade with comparably strict requirements. FXM-19 is a high-nitrogen austenitic stainless steel, featuring high strength and toughness, ready weldability, and forgeability. It features as well higher integral thermal contraction down to 4 K compared with the very high Mn steel grade selected for the CS coil jackets, hence providing an additional precompression of the CS modules during cooling down. The results of an extensive characterization of the physical properties of FXM-19, extended down to 4 K or lower, are reported here. The provided results allow requirements for the file for new materials described in French code RCC-MRx to be fulfilled, as well as to complete the material properties in the ITER database, where measurements were reported for room temperature and above only.

The central solenoid (CS) is a key component of the ITER magnet system to provide the magnetic flux swing required to drive induced plasma current up to 15 MA. The manufacture of its different subcomponents has now started, following completion of the design analyses and achievement of the qualification of the manufacturing procedures. A comprehensive set of analyses has been produced to demonstrate that the CS final design meets all requirements. This includes in particular structural analyses carried out with different finite-element models and addressing normal and fault conditions. Following the Final Design Review, held in November 2013, and the subsequent design modifications, the analyses were updated for consistency with the final design details and provide evidence that the Magnet Structural Design Criteria are fully met. Before starting any manufacturing activity of a CS component, a corresponding dedicated qualification program has been carried out. This includes manufacture of mockups using the real manufacturing tools to be tested in relevant conditions. Acceptance criteria have been established for materials and components, winding including joints, cooling inlets and outlets, insulation, precompression, and support structure elements.

Gas Electron Multiplier (GEM) technology is being used in various applications, particularly in high energy physics experiments. The GEM is known as a reliable detector in high radiation environment which can maintain high temporal and position resolution. GEM foil is the basic part of the detector which consists of a composite material (polyimide and copper). Large size GEM foil has complex mechanical structure and asymmetries which mainly arises due to formation of the HV sectors in the foil. These asymmetries become very relevant when large size foils are stretched to build a detector. In this article asymmetry affects are presented that define the tensile properties of a large size segmented GEM foil.

The ITER correction coils (CCs) consist of three sets of six coils located in between the toroidal (TF) and poloidal field (PF) magnets. The CCs rely on a Cable-in-Conduit Conductor (CICC), whose supercritical cooling at 4.5 K is provided by helium inlets and outlets. The assembly of the nozzles to the stainless steel conductor conduit includes fillet welds requiring full penetration through the thickness of the nozzle. Static and cyclic stresses have to be sustained by the inlet welds during operation. The entire volume of helium inlet and outlet welds, that are submitted to the most stringent quality levels of imperfections according to standards in force, is virtually uninspectable with sufficient resolution by conventional or computed radiography or by Ultrasonic Testing. On the other hand, X-ray computed tomography (CT) was successfully applied to inspect the full weld volume of several dozens of helium inlet qualification samples. The extensive use of CT techniques allowed a significant progress in the weld quality of the CC inlets. CT is also a promising technique for inspection of qualification welds of helium inlets of the TF magnets, by far more complex to examine due to their larger dimensions.

The ITER Central Solenoid (CS), positioned in the center of the ITER tokamak, will provide a magnetic field, contributing to the confinement of the plasma. The 13 m high CS consists of a vertical stack of 6 independently driven modules, dynamically activated. Resulting opposing currents can lead to high separation forces. A pre-compression structure is implemented to counteract these opposing forces, by realising a continuous 180 MN coil-to-coil contact loading. Preload is applied by mechanical fastening via 9 subunits, positioned along the coil stack, each consisting of 2 outer and 1 inner tie plate. The tie plates therefore need to feature outstanding mechanical behaviour in a large temperature range. High strength, Nitronic (R)-50 type F XM-19 austenitic stainless steel is selected as candidate material. The linearised stress distribution reaches approximately 250 MPa, leading to a required yield strength of 380 MPa at room temperature. Two different manufacturing methods are being studied for the procurement of these 15 m long tie plates. A welded solution originates from individual head- and slab-forgings, welded together by Gas Metal Arc Welding (GMAW). In parallel, a single piece forged solution is proven feasible, impressively forged in one piece by applying successive open die forging steps, followed by final machining. Maximum internal stress is experienced during cool-down to 4K as a result of a large difference in thermal contraction between the support system and the coils. Furthermore, the varying magnetic fields in the independently driven coils introduce cyclic loading. Therefore, assessment of the two manufacturing solutions, in terms of both static and dynamic mechanical behaviour, is performed at ambient as well as cryogenic temperature. An extensive characterisation including microstructural and mechanical examination is conducted, evaluating the comparative performance of both solutions, reporting, amongst others, yield strength reaching the requirement for both solutions. (C) 2015 Elsevier B.V. All rights reserved.

Cable-in-conduit conductors will be extensively implemented in the large superconducting magnet coils foreseen to confine the plasma in the ITER experiment. The design of the various magnet systems imposes the use of electrical joints to connect unit lengths of superconducting coils by inter-pancake coupling. These twin-box lap type joints, produced by compacting each cable end in into a copper - stainless steel bimetallic box, are required to be highly performing in terms of electrical and mechanical prop- erties. To ascertain the suitability of the first copper-clad plates, recently produced, the performance of several plates is studied. Validation of the bonded interface is carried out by determining microstructural, tensile and shear characteristics. These measure- ments confirm the suitability of explosion bonded copper-clad plates for an overall joint application. Additionally, an extensive study is conducted on the suitability of certain copper purity grades for the various joint types.

International audience; Cable-in-conduit conductors of the ITER magnet system are directly cooled by supercritical helium. Insulation breaks are required in the liquid helium feed pipes to isolate the high voltage system of the magnet windings from the electrically grounded helium coolant supply line. They are submitted to high voltages and significant internal helium pressure and will experience mechanical forces resulting from differential thermal contraction and electro-mechanical loads. Insulation breaks consist essentially of stainless steel tubes overwrapped by an outer glass – fiber reinforced composite and bonded to an inner composite tube at each end of the stainless steel fittings. For some types of insulator breaks Glass – Kapton – Glass insulation layers are interleaved in the outer composite. Following an extensive mechanical testing campaign at cryogenic temperature combined with leak tightness tests, the present paper investigates through non-destructive and destructive techniques the physical and microstructural characteristics of the low temperature high voltage insulation breaks and of their individual components, thus allowing to correlate the structure and properties of the constituents to their overall performance. For all the tests performed, consistent and reproducible results were obtained within the range of the strict acceptance criteria defined for safe operation of the insulation breaks.

The GEM detectors will be installed at the Compact Muon Solenoid (CMS) experiment during Long Shutdown II of the LHC in 2018. The GEM foil is a basic part of the detector which consists of a composite material, i.e. polyimide coated with copper and perforated with a high density of micro holes. In this paper the results of the GEM foil material characterization are reported, and a campaign of tensile and holes deformation tests is performed. During the tests, the complex radiation environment at CMS is taken into account and samples are prepared accordingly to see the impacts of the radiation on the GEM foil, i.e. non-irradiated samples are used as the reference and compared with neutrons- and gamma- irradiated. These studies provide the information necessary to optimize the stress level without damaging the foil and holes during the detector assembly in which the GEM foils stack is stretched simultaneously to maintain the uniform gap among the foils in order to get the designed performance of the detector. Finally, an estimate of the Young's modulus of the GEM foil is provided by using the tensile test data. © CERN 2016.

EuCARD-2 is a project partly supported by FP7 European Commission aiming at exploring accelerator magnet technology for 20-T dipole operating field. The EuCARD-2 collaboration is liaising with similar programs for high-field magnets in the U.S. and Japan. EuCARD-2 focuses, through the work package 10 “future magnets,” on the development of a 10-kA-class superconducting high-current-density cable suitable for accelerator magnets, for a 5-T stand-alone dipole of 40-mm bore and about 1-m length. After stand-alone testing, the magnet will be inserted in a large bore background dipole, 10–18 T. This paper reports on the design and development of models, which are called Feather0, wound with REBCO Roebel cable. Based on aligned block design to take advantage of the anisotropy of the REBCO tapes, Feather0 is a precursor of Feather2, which should reach the project goals in 2016. Feather0 is planned to be tested both in stand alone and as an insert mounted in the CERN Fresca facility providing 10-T background field. The progress of other designs pursued in the collaboration, one based on classical $\cos\vartheta$ layout with Roebel cable and the other based on coil block with stacked tape cable, will be also reported.

The development of new generation superconducting magnets for fusion research, such as the ITER experiment, is largely based on coils wound with so-called cable-in-conduit conductors. The concept of the cable-in-conduit conductor is based on a direct cooling principle, by supercritical helium, flowing through the central region of the conductor, in close contact with the superconducting strands. Consequently, a direct connection exists between the electrically grounded helium coolant supply line and the highly energised magnet windings. Various insulated regions, constructed out of high-voltage insulation breaks, are put in place to isolate sectors with different electrical potential. In addition to high voltages and significant internal helium pressure, the insulation breaks will experience various mechanical forces resulting from differential thermal contraction phenomena and electro-magnetic loads. Special test equipment was designed, prepared and employed to assess the mechanical reliability of the insulation breaks. A binary test setup is proposed, where mechanical failure is assumed when leak rate of gaseous helium exceeds 10−9P a · m3 /s. The test consists of a load-to-failure insulation break charging, in tension, while immersed in liquid nitrogen at the temperature of 77 K. Leak tightness during the test is monitored by measuring the leak rate of the gaseous helium, directly surrounding the insulation break, with respect to the existing vacuum inside the insulation break. The experimental setup is proven effective, and various insulation breaks performed beyond expectations. Advances in Cryogenic Engineering ‐ Materials: Proceedings of the International Cryogenic Materials Conference (ICMC) 2015 28 June to 2 July 2015, Tucson, AZ, USA

Mechanical testing of materials at low temperatures is one of the cornerstones of the Mechanical and Materials Engineering (MME) group at CERN. A long tradition of more than 20 years and a unique know - how of such tests has been developed with an 18 kN double-walled cryostat. Large campaigns of material qualification have been carried out and the mechanical behaviour of materials at 4 K has been vastly studied in sub - size samples for projects like LEP, LHC and its experiments. With the aim of assessing the mechanical properties of materials of higher strength and/or issued from heavy gauge products for which testing standardized specimens of larger cross section might be more adapted, a new 100 kN cryostat capable of hosting different shapes of normalized samples has been carefully designed and fabricated inhouse together with the associated tooling and measurement instrumentation. It has been conceived to be able to adapt to different test frames both dynamic and static, which will be of paramount importance for future studies of fracture mechanics at low temperatures. The cryostat features a double-walled vessel consisting of a central cylindrical section with a convex lower end and a flat top end closure. The transmission of the load is guaranteed by a 4 column system and its precise monitoring is assured by an internal load cell positioned next to the sample in the load train. This innovative approach will be discussed together with other nonconventional instrumentation solutions. A validation of the whole system has been carried out, where bending efforts on instrumented samples have been measured. Additionally, dedicated tooling has been fabricated for the device's optimization. The preliminary results obtained confirm an excellent performance of the system and enhance the analysis of materials under extreme conditions with state of the art instrumentation.

The stability of high-current superconductors is challenging in the design of superconducting magnets. When the stability requirements are fulfilled, the protection against a quench must still be considered. A main factor in the design of quench protection systems is the resistance growth rate in the magnet following a quench. The usual method for determining the resistance growth in impregnated coils is to calculate the longitudinal velocity with which the normal zone propagates in the conductor along the coil windings. Here, we present a 2D numerical model for predicting the normal zone propagation velocity in Al stabilized Rutherford NbTi cables with large cross section. By solving two coupled differential equations under adiabatic conditions, the model takes into account the thermal diffusion and the current redistribution process following a quench. Both the temperature and magnetic field dependencies of the superconductor and the metal cladding materials properties are included. Unlike common normal zone propagation analyses, we study the influence of the thickness of the cladding on the propagation velocity for varying operating current and magnetic field. To assist in the comprehension of the numerical results, we also introduce an analytical formula for the longitudinal normal zone propagation. The analysis distinguishes between low-current and high-current regimes of normal zone propagation, depending on the ratio between the characteristic times of thermal and magnetic diffusion. We show that above a certain thickness, the cladding acts as a heat sink with a limited contribution to the acceleration of the propagation velocity with respect to the cladding geometry. Both numerical and analytical results show good agreement with experimental data., Comment: To be published in Physics Procedia (ICEC 25 conference special issue)

Future detector magnets call for the development of next-generation large-sized Al stabilized Nb-Ti superconducting cables exhibiting high yield strength for coping with the large stress in wide bore magnets with peak magnetic fields up to 6 T, while avoiding significant degradation in residual resistivity ratio. Precipitation type alloys obtained by dilute-alloying of high purity Al with suitable additives like Ni, subjected to partial annealing following cold drawing, can feature a yield strength up to 110 MPa at 4.2 K. For the ATLAS central solenoid, a Nb-Ti cable has been plated with a precipitation-type Al-0.1wt%Ni alloy. However, this conductor with a critical current of 20 kA at 5 T, features a cross-section of only 130 mm2. Here, a first step in process scale-up to a 60 kA at 5 T class conductor is described. For the first time, a continuous co-extruded Al-0.1wt %Ni stabilized conductor has been produced with a cross-section as large as 700 mm2. Sections of the conductor are work hardened in order to increase the mechanical properties of the as-extruded temper. The mechanical and transport characteristics as a function of the amount of work hardening have been assessed by removing samples after every subsequent step.