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For large scale magnets wound with cable-in-conduit conductors, the safe operation of the joints is of paramount importance to guarantee adequate reliability and stability margin of the whole magnet. For this reason, during the R&D activities undertaken for the development of the ITER magnet system, several experimental campaigns were launched to study the AC and DC performance of the joint and limit the risk of thermal runaways at the joints during the tokamak operation. The joint electrical resistance must be limited below specified values to avoid excessive heating generated by the transport current. Moreover, in presence of time-varying fields, different type of losses arise at the joints, which can be associated to their superconducting and resistive parts. The relative importance of these losses depends on the joint manufacturing solution. The aim of this investigation is to analyse the performance at different working conditions of the joints for the connection of the conductors of the Poloidal Field (PF) coils of the ITER magnet system. This work presents, for the first time, a wide review of the test campaign performed from 2016 to 2021 on the PF joint samples during the three manufacturing phases, namely pre-qualification, qualification and production. The values of electrical resistances and losses under sinusoidal field variations are reported in the paper at different operating conditions, thus building a useful database to assess the joint performances during the machine operation. The data here collected show the impact of the manufacturing techniques on the joint performances and, furthermore, represent a useful tool for the validation of numerical and analytical models of joints.

ITER magnets are in the final phases of production and are preparing for the upcoming assembly challenges. The ITER magnet team has expressed the need for close-range support labs in order to perform qualification tests, procedure tuning, mockup testing, site acceptance tests, as well as training of assembly staff and logistics management. In the framework of its general support to the ITER project, CEA's Institute for Magnetic Fusion Research proposed a support structure in line with this need on CEA Cadarache premises. In July 2014, the MIFI agreement (Magnet Infrastructure Facilities for ITER) was signed between ITER organization and CEA. It led to the creation of four laboratories and a storage area dedicated to magnet instrumentation components. This paper gives a general description of the work organization in MIFI, and describes the main ongoing activities with specific focuses on critical qualification activities like high-voltage testing and diagnostic of Glass–Kapton–Glass resin impregnated insulation and the scale 1 mock-up and test of the intermediate outer intercoil structure assembly procedure.

The superconducting coils of the ITER magnet system have hundreds of electrical lap joints interconnecting superconducting cables. The joints operate in a magnetic field of up to 4 T, field derivatives of 0.5 T/s, and currents up to 70 kA. The acceptance tests for the toroidal field (TF), poloidal field (PF), and correction coil (CC) coils will be performed at 77 K, before they are assembled in the pit. Hence there will be no possibility to measure the resistance of the joints in the superconducting state before the whole magnet system is enclosed in the Tokamak cryostat. In addition, no reliable nondestructive method has been found to spot the joints with a failure at room temperature. Therefore, the production of the joints relies on the strict adhesion to established robust manufacturing procedures during the qualification phase. As additional quality monitoring, a periodic test of the joint samples manufactured in parallel with a coil fabrication is foreseen to control the reproducibility of the joint electrical performance. In order to qualify the manufacturing procedures, to establish a series production tools and worker teams, a comprehensive qualification program has been set up for manufacturers of the coils in Russia (Poloidal Coil 1, PF1), China (PF6, feeders, CC), Japan (TF), Europe (TF and PF), and USA (Central Solenoid, CS). This program includes a set of mockups manufactured according to the process to be used for the coils and submitted to different tests. They include mechanical testing of materials, electrical tests of full size joint samples, destructive microscopic examination of the joint mockups, and mechanical testing of the full size joint mockups. All tests are carried out in specialized laboratories qualified for this type of work. This paper describes the main items of the qualification program, the tests performed, and the acceptance criteria. The test results are reported and compared to the criteria.

This paper summarizes the work on the manufacturing and testing of conductor samples for ITER poloidal field 1&6 coils. The work was carried out by the IO and multiple institutions in the EU, Switzerland and Russia. Five samples were manufactured and tested in the SULTAN test facility. Analyses of dc performances, ac, and mechanical losses test results have been performed. All measurements were carried out before and after cyclic mechanical load created by the electromagnetic forces. DC test results of all samples tested demonstrated that the conductor performances are not affected by cycling mechanical load and meet the required value of current-sharing temperature T cs ≥ 5.8 K at magnetic field B max = 6.4 T and operating current I op = 48 kA. Measurements of the ac and mechanical losses were performed by gas flow calorimetric method. AC loss measurements were carried out at different operating currents and background magnetic fields. AC loss of tested samples showed no change under cycling mechanical load, but showed a correlation of ac loss versus background magnetic field. Mechanical loss, as had been expected, decreased significantly after first ten cycles then reaches a stabile value. The value of mechanical loss even in virgin condition does not affect workability of a magnet.

Hysteresis losses of five Nb 3 Sn International Thermonuclear Experimental Reactor reference strands were investigated by means of magnetization loop measurements in a vibrating sample magnetometer in a perpendicularly applied magnetic field. The magnetization loops were recorded while continuously sweeping the applied field between the extreme values ±B m , covering a wide range of maximum applied fields (0.2-10 T). In this paper, we compare the directly determined hysteresis losses based on the area of the smaller measured loops and the losses calculated by the integration of the width ΔM of the B m = 10 T magnetization loop. A suitable fitting function is proposed to describe the ΔM(B) dependence, which leads, for each strand, to an excellent agreement with the experimentally determined hysteresis losses, magnetization, and pinning force. Transport critical current measurements in a perpendicularly applied magnetic field were also performed for all the strands, and on the basis of the comparison with the transport measurements, the critical current densities were determined from the magnetization data.

Direct current tests performed in the past on the conductor samples of the toroidal field (TF) ITER coils revealed degradation of current sharing temperature, T cs. The degradation progresses with repetitive electromagnetic loading, and also with thermal cycles between 4.5 K and room temperature. This feature was observed on short samples in SULTAN test facility (EPFL-SPC, Switzerland) as well as in TF Insert Coil tests in CSMC test facility (Naka, Japan). We present three independent observations suggesting that initiation of sample quench followed by a fast current discharge, which normally complements every I c and T cs test in both SULTAN and CSMC, enhances the T cs degradation rate. The exact mechanism of this contribution to the degradation remains unidentified.

Four ITER main busbar (MB) conductor samples were tested in the SULTAN test facility (Centre de Recherches en Physique des Plasma, Ecole Polytechnique Fédérale de Lausanne, Switzerland) between 2011 and 2013. The MB conductors are NbTi-based cable-in-conduit conductors (CICCs), and they will become part of the feeder system of the ITER magnets. The measured dc performance of the four MB samples varied significantly, supposedly depending on the design of the bottom terminations. Two out of three samples with a U-bend box, made of a continuous conductor section, exhibit approximately 0.5 K lower current-sharing temperature Tcs than the sample with a solder-filled bottom joint, consisting of two straight conductor sections. We assess the theoretically expected Tcs performance of the MB conductor based on the characterization of individual NbTi strands, on the ITER NbTi scaling law, and on the magnetic field distribution across the cable cross section. The magnetic field in a SULTAN sample consists of three components, namely, the background SULTAN field, the self-field generated by the current in the conductor under test, and the magnetic field generated by the current in the return conductor. Taking into account all the three components, we calculate the average electric field in the cable and determine Tcs as in the experiment, namely, as the temperature at which the electric field reaches the critical value of Ec=0.1 μV/cm. The theoretically assessed Tcs confirms that the MB sample with solder-filled joint behaves as expected.

Some of the bronze-route Nb3Sn wires produced for the ITER toroidal field magnets have values of the Cu residual resistivity ratio (RRR) lower than the specification (RRR > 100) when reacted with the longest ITER heat treatment cycle (cycle A: high-temperature plateau 200 h at 650 ◦C). As the low RRR value was suspected to be due to the Cr plating, CERN has carried out extensive investigations on two different Cr-plated Nb3Sn wires in order to assess how the RRR is influenced by the Cr plating. Each type of wire was reacted with three heat treatments differing for the duration of the high-temperature plateau (from 100 to 200 h). The presence of Cr in the Cu stabilizer was investigated using a transmission electron microscope together with energy-dispersive X-ray spectrometry. In the most critical wire reacted for 200 h, CrS nanoparticles and Cr in solid solution (0.02 at.%) with Cu were found up to 13 μm from the Cr–Cu interface. At larger distance, we could not quantify the Cr concentrations because of the detector sensitivity limit (about 0.016 at.% Cr in Cu). We provide a model that can explain the measured RRR degradation purely in terms of the Cr contamination that we observed.

The ITER toroidal field (TF) strand procurement initiated the largest Nb3Sn superconducting strand production hitherto. The industrial-scale production started in Japan in 2008 and finished in summer 2015. Six ITER partners (so-called Domestic Agencies, or DAs) are in charge of the procurement and involved eight different strand suppliers all over the world, of which four are using the bronze route (BR) process and four the internal-tin (IT) process. In total more than 500 tons have been produced including excess material covering losses during the conductor manufacturing process, in particular the cabling. The procurement is based on a functional specification where the main strand requirements like critical current, hysteresis losses, Cu ratio and residual resistance ratio are specified but not the strand production process or layout. This paper presents the analysis on the data acquired during the quality control (QC) process that was carried out to ensure the same conductor performance requirements are met by the different strand suppliers regardless of strand design. The strand QC is based on 100% billet testing and on applying statistical process control (SPC) limits. Throughout the production, samples adjacent to the strand pieces tested by the suppliers are cross-checked ('verified') by their respective DAs reference labs. The level of verification was lowered from 100% at the beginning of the procurement progressively to approximately 25% during the final phase of production. Based on the complete dataset of the TF strand production, an analysis of the SPC limits of the critical strand parameters is made and the related process capability indices are calculated. In view of the large-scale production and costs, key manufacturing parameters such as billet yield, number of breakages and piece-length distribution are also discussed. The results are compared among all the strand suppliers, focusing on the difference between BR and IT processes. Following the completion of the largest Nb3Sn strand production, our experience gained from monitoring the execution of the QC activities and from auditing the results from the measurements is summarised for future superconducting strand material procurement activities.

The ITER machine will require approximately 275 tons of Nb-Ti strands that will be used in poloidal field (PF) coils, correction coils (CC) and feeder busbars. The performance of all these conductors for the ITER machine is qualified by a short full-size sample (4 m) current sharing temperature (T cs) test in the SULTAN facility at CRPP in Villigen, Switzerland, at the design operating current and peak field. Three ITER domestic agencies participated in PF conductor fabrication (China, the European Union, Russia) while the conductors for feeder busbars and correction coils are entirely produced by the Chinese domestic agency. Each conductor type was qualified by the ITER International Organization after reaching T cs values in excess of ITER specifications. This qualification enabled the launch of procurement and industrial production of the Nb-Ti cable-in-conduit conductors in each domestic agency. In this paper, we summarize the performance of the qualified Nb-Ti samples of the ITER Project, comparing strand performance with conductor performance. The details of the test results will be discussed in terms of dc performance, ac losses and minimum quench energies of each conductor type.

ITER cable-in-conduit conductor (CICC) used in the superconducting magnet system consists of a cable made of Nb3Sn or Nb-Ti strands inserted in a stainless steel tube (called jacket or conduit). Depending on the coil type, the jacket material is either made of a low carbon AISI 316LN (for toroidal field coil) and AISI 316L grade stainless steels (for poloidal field and correction coils) or a high Mn austenitic stainless steel developed for ITER called JK2LB (for the central solenoid coil). Mechanical properties of base material and weld joint need to be tested at room and/or cryogenic temperature under predefined mechanical deformation and heat treatment conditions. The amount of required steel requires a tight mechanical properties control to ensure homogeneity during production within the ITER defined requirements. This paper will compare test results such as the ultimate tensile strength (UTS), the yield strength (YS), and the elongation to failure (EL) measured by all ITER jacket supplier test laboratories both for seamless tubes and weld joints.

The ITER Conductor Procurement Arrangement requires that all the conductors have to be qualified before the production phase. The objects of the work here presented are the Poloidal Field Coils conductors, specifically the PF1/6 and the PF2 conductors. The realization of the samples comprised the jacketing, the compaction and the straightening of the CICC, together with the design and manufacturing of the bottom hairpin box and the upper termination. The instrumentation and the related jacket machining completed the preparation of the samples. The samples have been assembled according to the specification defined by ITER and have been shipped to SULTAN facility. This paper describes all the activities performed during the preparation, the features of the samples and all the related issues.

In the framework of the final design activities related to the ITER Toroidal Field (TF) coils, following the very good results obtained during the TF Prototype Conductors measurement campaign, Fusion For Energy, the European Domestic Agency for ITER, has launched the conductor performance qualification phase in order to confirm the final ITER TF conductor design. Six conductor lengths have been cabled by different strand types coming from different producers: LUVATA Pori, Oxford Instruments Superconducting Technology (OST), ALSTOM (for two different strand layouts) and European Advanced Superconductors (EAS) (for two different strand layouts). ENEA has been in charge of performing QA and monitoring activity during the conductor production at LUVATA Fornaci di Barga and TRATOS Cavi, and to make visual/destructive tests over some spare lengths of the samples in order to have a detailed characterization of the produced conductors. The conductor unit lengths have been successfully manufactured and all the different work phases are here described, along with a discussion of the encountered problems and the adopted solutions. Five samples have been successfully tested in the SULTAN facility during 2009.

The aim of this paper is to present an up to date review of the status of the European DIPOle (EDIPO) project whose objective is to build a new facility to perform both DC and AC tests of large superconductor samples in high magnetic background field (up to 12.5 T). EDIPO was designed by EFDA, is being built by BNG (Babcock-Noell) and, once completed, it shall be installed near the existing SULTAN facility at the Paul-Scherer-Institute, (CRPP, Villigen, Switzerland). The results presented focus on the manufacture of the dummy coil (which is a 1:1 model of one of the EDIPO poles) that was completed in spring 2009. Details are given about the dummy coil winding results, coil reaction heat treatment, impregnation and acceptance tests. Moreover, a progress status is provided about the manufacturing of the other parts of the EDIPO magnet assembly, in particular with respect to the manufacturing of the test-well, AC coils, yoke sheets cutting and assembly, the outer cylinder production and welding (316 LN steel, 35 mm thick) and the coils ends preparation. Some results regarding the R&D studies are also reported about the winding trials carried out to improve the winding accuracy and the numerical simulations made to support coil winding (e.g. conductor spring back).

The performance degradation under electro-magnetic, transverse load has grown to a key issue for the design of Nb3Sn cable-in-conduit conductors (CICC). Beside the tolerance to bending strain of the basic Nb3Sn strand and the void fraction of the CICC, a relevant parameter is thought to be the cable pattern. A sequence of ldquolongrdquo twist pitches in the early stages of a multi-stage cable is credited to mitigate the performance degradation compared to ldquoshortrdquo pitches. To assess quantitatively the effect of long/short pitches maintaining constant all other conductor parameters, a short length of four stages CICC is prepared, where the first half length has long pitches (83/140/192 mm) in the first three cable stages and the second half length has short pitches (34/95/139 mm). The last stage pitch is 213 mm for both lengths. The cable is made of Cr plated copper and Nb3Sn strands with a diameter of 0.81 mm. The conductor is assembled into a SULTAN hairpin sample where the two branches have respectively long and short pitches. The DC performance, AC loss and pressure drop are measured in both conductor sections and compared to former conductors with the same design. The results are reported and the balance of advantages and drawbacks of long vs. short pitches is discussed.

In the ITER TF Coils reference design the circular conductor is enclosed in the grooves of a stainless steel plate, the so called radial plate. In this paper, we present an alternative solution using a relatively thin walled rectangular conductor without an additional reinforcing structure. We describe the conductor and the layout of the winding pack and analyze the magnetic, thermo-hy-draulic and mechanical behavior of the proposed coil. Then a cost analysis is carried out to estimate the significant saving that can be achieved by the proposed solution.

Following the outcome of the conceptual design phase the EFDA dipole magnet will be made of rectangular cable-in-conduit conductors (CICC) jacketed in 316LN. In order to optimize the required amount of superconductor two different conductor types are used: a high-field (HF) conductor consisting of 144 strands and a low-field (LF) conductor with 108 strands. A high strand with a critical current density (at 4.2 K and 12 T) and an effective filament diameter of was selected. The first series of conductor prototype specimens was tested in summer 2006 but the conductor performances were lower than expected from the pre-prototype tests of 2005 and not fulfilling the design criteria. The conductor layouts were modified to increase the strand support inside the cable and the revised HF conductor design was qualified successfully end of 2006. A current sharing temperature 6 K was found at the dipole operating conditions (12.8 T, 17 kA) confirming the required temperature margin of more than 1 K. The HF conductor qualification process including the design modifications, analysis of the test results and comparison to the expectations are discussed.

A 12.5 T superconducting dipole magnet (European DIPOle, EDIPO) has been designed by EFDA and it is now being procured within the framework of the European Fusion Programme in order to be installed in CRPP-PSI. This saddle-shaped magnet is designed to reach 12.5 T in a 100 times 150 mm rectangular bore over a length of about 1.5 m in order to test full size conductor samples that shall be produced during the ITER magnets procurement. The magnet uses Cable In Conduit Conductor (CICC) technology and the cables are made of high Jc (about 2300 A/mm2 at 4.2 K, 12 T) superconducting strands. In this paper the main magnet parameters are given together with the key supporting electromagnetic, mechanical and thermal analyses. An update on the general status of the procurement of the strand, conductors, dipole magnet and facility is also given together with the key results of the on-going supporting R&D.

Recent studies have shown that the presently considered conductor for the poloidal field coil 6 (PF6) of the ITER machine is being designed at the limit of what the considered NbTi superconductor can deliver. Furthermore tests have shown that the performance of conductor prototypes remained under the specified 1.5 K temperature margin. As a result of these issues, ITER is considering a PF6 coil design using Nb3Sn superconductor. We have investigated a PF6 coil design based on Nb3Sn superconductor, which at the same time delivers the required temperature margin as well as the higher performance capability at a similar cost as the reference NbTi design. In addition the design analysis presented here discusses performance parameters such as hot spot temperature and quench propagation. Finally we also present possible coil design and manufacturing procedures.

The ITER superconducting magnet system will require approximately 650 tons of toroidal field and central solenoid Nb3Sn strands with different designs, and more than 250 tons of Nb–Ti strands. This called for a significant scale up of the worldwide production of Nb3Sn and Nb–Ti strands. Over the worldwide and mass productions, it is essential to accurately analyze and characterize the properties of ITER superconducting strands in terms of critical current $(I_{c})$ and ac loss, and thus determine the operational limits of the conductors, ultimately optimizing the operating scenarios of the Tokamak. In this paper, the proper scaling parametrization $I_{c}(B,T)$ and $n$ -value as a function of $I_{c}\ (n(I_{c}))$ are investigated for Nb–Ti strands throughout massive production. Despite the differences in ITER Nb–Ti strand's architecture and their composition, optimized Nb–Ti strand scaling parameterizations throughout production for each supplier have been reached with the minimal deviation ( $B, T$ window and even

In the cable-in-conduit-conductor (CICC) design of the toroidal field system for the international thermonuclear reactor (ITER) Nb3Sn is used as superconductor material. Considering the single strand performance, the crucial characteristic is the strain dependence of the critical current. Within this context, the performance of the CICC under strain is determined by the behaviour of the single strands and additional effects related to the manufacturing process. In the framework of the European fusion technology program a task has been started to investigate single strands as well as sub-size CICC performance using different cable layouts (9, 45 and 180 strands). For this systematic approach, parameters such as the void fraction, the number of pure copper strands, the void fraction or the cabling pattern have been varied. To examine the critical properties in detail, the available test facility, consisting of two experimental setups, is capable to measure the strain dependence in magnetic fields up to 14 T at 4.2 K, by applying an axial load to the samples. Measurements on such sub-size CICC samples are presented and compared to the expected performance.

The effect of transverse loads on strands has been pointed as a possible cause of the difference observed when scaling transport properties of single strands to those of cable-in-conduit conductors. Single multifilamentary strands inside cables are in fact subject to bending strain due to the electromagnetic forces at operating conditions and to the geometrical layout. Here the influence of pure bending strain, applied in combination with a longitudinal strain, on the critical current of advanced strands for ITER has been studied. The tested samples are single strands inserted inside a thin stainless steel jacket and wound on stainless steel barrels. After the heat treatment, a pure bending strain has been applied transferring the wires on different diameter mandrels, using ad-hoc developed and qualified techniques. Transport critical current has been measured on the single strands before and after the steel jacketing, as well as after the additional application of two different values of maximum bending strain: 0.5% and 0.25%. This was the best choice in order to verify experimentally whether the so-called long twist pitch condition can be applied for the selected strands. The distribution of the bending strain over the strand cross-section has been calculated with finite element numerical codes, and the expected critical current degradation in the limiting cases of short and long twist pitch has been computed and compared with experimental data.

Within the framework of ITER-related projects, new tasks have been recently launched by EFDA CSU Garching (European Fusion Development Agreement Close Support Unit Garching), for the definition and production on industrial scale of advanced Nb3Sn strands, to be used in the manufacturing of the ITER high field CS and TF magnets. We performed an extended characterization of the advanced Nb3Sn strands coming from different European companies, in terms of strand layout (diameter, thickness of Cr coating, Cu:non-Cu ratio), critical transport current, RRR, and hysteresis losses. The results of the measurement campaign show that the upgraded strands meet the latest ITER requirements, with an overall critical transport current of at least 200 A (at 12 T, 4.2 K), equivalent to a non-Cu Jc of about 800 A/mm2, a Cu:non-Cu ratio of about 1, a strand diameter of 0.81 mm, and with non-Cu hysteresis losses limited to less than 1000 kJ/m3 on a plusmn3 T field cycle at 4.2 K

In the scope of the design activities for a 12.5 T dipole magnet, a short length of cable-in-conduit conductor (CICC) with steel jacket was prepared in spring 2005 as a pre-prototype of the high field dipole conductor. The small, rectangular conductor uses for the first time "advanced" Nb3Sn strands, with Jc=1100 A/mm2 at 12 T, 4.2 K, also planned in the qualification samples for the ITER conductors. At CRPP, a hairpin sample for SULTAN has been prepared from the short conductor length, heat treated and instrumented. The test program included pressure drop, AC losses and full DC characterization before and after cyclic loading. At large, the test results indicate that the high performance of the advanced strands is preserved in the small size, steel jacketed CICC

The latest achievements in the development of high performance Nb3Sn strands are reported. In the field of high energy physics (HEP) critical current densities up to 3000 A mm−2 (at 12 T, 4.2 K and 10 µV m−1) can be achieved. Current actions are aimed at reducing the filament size to about 50 µm and decreasing production costs by using less expensive base materials. In order to take advantage of the progress made by the HEP R&D program, the fusion community has initiated several strand procurement actions worldwide to produce advanced Nb3Sn strand types fulfilling the updated strand specification of the International Thermonuclear Experimental Reactor project. Recently developed internal-tin strands reach critical current densities of more than 1100 A mm−2 and acceptable non-Cu hysteresis losses of about 700 kJ m−3. An important milestone towards higher production rate and yield was reached: a billet of about 100 kg was successfully drawn to the final strand diameter without problems. In powder-in-tube conductors, the composition and number of filaments have been modified to meet the European target specification. Although the bronze technique itself sets certain limitations on Jc, strands with critical current densities around 800 A mm−2 have been produced on an industrial scale.

One of the most important characteristics of Nb/sub 3/Sn strands is the strain dependence of the critical current density which has to be taken into account for every conductor design, especially for cable-in-conduit conductors (CICC) due to the additional influence of the jacket material. Therefore, a task has been launched in the framework of the European Fusion Technology Program to determine the residual thermal strain of single strands and different multi-stage conductors on absolute scale which is essential for the final optimization of the ITER TF conductor. For this purpose, an existing test rig for critical current measurements on single strands and multi-stage CICC as a function of strain and field, the FBI facility at the Forschungszentrum Karlsruhe, has been recently modified and modernized to improve the overall accuracy and resolution of the system. After a description of the facility, its capabilities and the recently performed upgrades, this paper explains the scheduled test program and the strategy to assess the impact of the cable pattern and size on the residual thermal strain. Results on Nb/sub 3/Sn strands jacketed in stainless steel are presented as well.

The tests of the toroidal field model coil (TFMC) were completed in 2002 in the TOSKA facility of Forschungszentrum Karlsruhe, Germany. Operation reached a combined 80 kA in the TFMC and 16 kA in the LCT coil, resulting in a peak electromechanical load very close to that expected in the full-size ITER TF coils (800 kN/m). Here we concentrate on the measurements of the current sharing temperature T/sub cs/ of the TFMC conductor, possibly the highlight of the whole test campaign. These tests were performed by increasing in steps the helium inlet temperature T/sub in/ in double pancake DP1, resulting in an increasing normal voltage V across the DP1.1 and DP1.2 conductors, and were repeated for several combinations of currents in the TFMC and in the LCT coil. The analysis of the V - T/sub in/ characteristic by means of the M&M code allows to self-consistently deriving an estimate of T/sub cs/, as well as an indirect assessment of the "average" strain state in the conductor. The TFMC isolated strand has also been very recently characterized at different applied uniaxial strain, and preliminary results indicate a stronger reduction of carrying capacity compared to the extrapolation from Summers scaling used in the analysis so far. As a consequence, the performance of the TFMC conductor, as preliminarily re-evaluated here, appears more in line with the strand performance than in previous analysis, although a BI-dependent "degradation" is still present.

The influence of neutron irradiation on the angular, field and temperature dependence of the critical transportcurrent density J c of YBa 2 Cu 3 O 7 d (Y-123) thick films, as well as on the irreversibility fields was investigated. Thespecimens were deposited by liquid phase epitaxy either directly on NdGaO 3 or on seeded (100)MgO substrates with afinal Y-123 layer thickness of 1 and 5–10 lm, respectively. XRD of the superconducting layers proves good in-planeand out-of-plane alignments with FWHMs below 2 . The specimens showed excellent irreversible properties withcritical current densities of up to 3 10 9 A/m 2 at zero field and 77 K and irreversibility fields above 7 T at 77 K for Hkcprior to neutron irradiation, which was performed in fast neutron fluence steps of 10 21 m 2 , corresponding to 5 10 21 defect cascades per m 3 per step. J c enhancements by 100% and an anisotropy reduction by a factor of 2 are achieved atthe optimum neutron fluence. 2003 Elsevier B.V. All rights reserved.

We report on the field and temperature dependence of the critical transport current density Jc, the angular dependence of the transport current at various external magnetic fields and the irreversibility fields in YBa2Cu3O7?? (Y-123) thick films prepared by liquid phase epitaxy (LPE). A comparison of the irreversible properties between specimens produced with and without silver additions to the melt is also presented. Transmission electron microscopy (TEM) was employed to obtain information on the correlation between the transport properties and the microstructure. The samples were deposited either directly on NdGaO3 (NGO) or on seeded (100) MgO substrates, where a 200 nm thin YBCO film deposited by pulsed laser deposition (PLD) acts as seed layer for the LPE process. The final thickness of the Y-123 layer is of the order of 1 ?m for the NGO and between 2 and 10 ?m for the MgO samples. The critical current densities reach 3 ? 109 A m?2 at zero field and 77 K in the best case.

Results on the angular and the field dependence of the critical transport current density as well as on the temperature dependence of the irreversibility fields in YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// coated conductors are presented. The films were deposited by pulsed laser deposition on (Y/sub 2/O/sub 3//YSZ/CeO/sub 2/) triple buffer structures. Either textured magnetic Ni-Fe alloys or recently developed nonmagnetic Ni-Cr-W alloys with good biaxial texture (8.4/spl deg/ and 9.4/spl deg/ for /spl omega/ and /spl phi/ scans) were used as substrates. The influence of artificial pinning centers, introduced by fast neutron irradiation, on the irreversible properties of the YBCO films is discussed.

High critical current densities (J/sub c/) are required for applications, but grain boundaries suppress the attainable J/sub c/ values. Because of the presence of 90/spl deg/ grain boundaries and because of poor structural and electrical properties, a-axis thin films are of great interest to investigate the role played by Y/sub 0.8/Ca/sub 0.2/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YCaBCO) cap layers in the transport properties. The transport critical current densities of the films under various magnetic fields and the angular dependence of J/sub c/ at fixed temperature were measured before and after irradiation. A thin YCaBCO cap layer deposited on top of a-axis oriented YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) films, is found to improve the electrical transport properties. Ca-doping decreases the resistivity and strongly increases J/sub c/ of a-axis oriented YBCO films. Furthermore, strong pinning by 90/spl deg/ grain boundaries was observed.

Coated conductors deposited by a novel high-rate pulsed laser deposition (PLD) technique on ion-beam-assisted-deposition (IBAD) layers of yttria-stabilized zirconia (YSZ) on non-magnetic steel ribbons were investigated. An XRD analysis of the superconducting layers showed in-plane alignments with a FWHM from 10 to 25/spl deg/ and an out-of-plane alignment of better than 2/spl deg/. We report on the field and temperature dependence of the critical transport current density in these YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// films of various thickness. In addition, the angular dependence of the transport current at various external magnetic fields is presented. Thirdly, artificial pinning centers were introduced into the superconductors by neutron irradiation. The influence of these defects on the critical current densities and on the anisotropy ratio are discussed.

The differences in thermal contraction of the composite materials in a cable in conduit conductor (CICC) for the International Thermonuclear Experimental Reactor (ITER), in combination with electromagnetic charging, cause axial, transverse contact and bending strains in the Nb3Sn filaments. These local loads cause distributed strain alterations, reducing the superconducting transport properties. The sensitivity of ITER strands to different strain loads is experimentally explored with dedicated probes. The starting point of the characterization is measurement of the critical current under axial compressive and tensile strain, determining the strain sensitivity and the irreversibility limit corresponding to the initiation of cracks in the Nb3Sn filaments for axial strain. The influence of spatial periodic bending and contact load is evaluated by using a wavelength of 5 mm. The strand axial tensile stress–strain characteristic is measured for comparison of the axial stiffness of the strands. Cyclic loading is applied for transverse loads following the evolution of the critical current, n-value and deformation. This involves a component representing a permanent (plastic) change and as well as a factor revealing reversible (elastic) behavior as a function of the applied load. The experimental results enable discrimination in performance reduction per specific load type and per strand type, which is in general different for each manufacturer involved. Metallographic filament fracture studies are compared to electromagnetic and mechanical load test results. A detailed multifilament strand model is applied to analyze the quantitative impact of strain sensitivity, intrastrand resistances and filament crack density on the performance reduction of strands and full-size ITER CICCs. Although a full-size conductor test is used for qualification of a strand manufacturer, the results presented here are part of the ITER strand verification program. In this paper, we present an overview of the results and comparisons.

The influence of the expected Lorentz loading and time dependent operating conditions of a magnet on the conductor AC loss is experimentally simulated by a cryogenic cable press that applies cyclic mechanical loading. A series of ITER conductor tests with the press have commenced and we report on the results from the first set of two TF conductors, which have the option-II cabling scheme but consist of NB3Sn strands from different manufacturers. With the press, we apply a transverse load of 578 kN/m and the load cycle is repeated up to 30,000 times. As a function of load cycles, we measure the cable mechanical stiffness, interstrand contact resistances, and the coupling loss. When compared with a previously measured option-II type conductor, the present conductors have higher initial losses. However, they showed greater cable displacement and larger increase in contact resistance with load cycles. This is due to the lower cable stiffness thought to be related to the lower axial strand stiffness, resulting in greater cable displacement than the previous cable. Consequently, the two conductors tested here have lower losses already within the first few cycles.

The world-wide procurement of Nb$_{3}$Sn and NbTi for the ITER superconducting magnet systems will involve eight to ten strand suppliers from six Domestic Agencies (DAs) on three continents. To ensure accurate and consistent measurement of the physical and superconducting properties of the composite strand, a strand test facility benchmarking effort was initiated in August 2008. The objectives of this effort are to assess and improve the superconducting strand test and sample preparation technologies at each DA and supplier, in preparation for the more than ten thousand samples that will be tested during ITER procurement. The present benchmarking includes tests for critical current (I-c), n-index, hysteresis loss (Q(hys)), residual resistivity ratio (RRR), strand diameter, Cu fraction, twist pitch, twist direction, and metal plating thickness (Cr or Ni). Nineteen participants from six parties (China, EU, Japan, South Korea, Russia, and the United States) have participated in the benchmarking. This round, conducted with a bronze-route Nb$_{3}$Sn strand, involved samples prepared by a common laboratory (CERN) and sent out to the participants ({''}IO-prepared samples{''}) and also samples prepared by each individual participant ({''}self-prepared samples{''}). I-c samples prepared and measured by CERN were found to have an average I-c of 188.7 A and a standard deviation of 1.8 A (1.0\%), while those same set of samples measured by all the participating labs in round-robin fashion were found to have a standard deviation of 2.1 A. Self-prepared samples had an average I-c of 188.1 A, and showed a lab-to-lab standard deviation of 3.5 A. The results demonstrate significant progress in the world-wide capability to accurately and reproducibly measure Nb$_{3}$Sn critical current over the past decade. Future benchmarking efforts will include an annual cross-check of supplier and DA facilities, and also a round of internal tin Nb$_{3}$Sn samples to assess each contributor's sample-preparation techniques. A separate round of NbTi benchmarking is also envisioned.

In this paper we report the main test results obtained on the Poloidal Field Conductor Insert coil (PFI) for the International Thermonuclear Experimental Reactor (ITER), built jointly by the EU and RF ITER parties, recently installed and tested in the CS Model Coil facility, at JAEA-Naka. During the test we (a) verified the DC and AC operating margin of the NbTi Cable-in-Conduit Conductor in conditions representative of the operation of the ITER PF coils, (b) measured the intermediate conductor joint resistance, margin and loss, and (c) measured the AC loss of the conductor and its changes once subjected to a significant number of Lorentz force cycles. We compare the results obtained to expectations from strand and cable characterization, which were studied extensively earlier. We finally discuss the implications for the ITER PF system.

Two short length samples have been prepared and tested in SULTAN to benchmark the performance of high current density, advanced Nb3Sn strands in the large cable-in-conduit conductors (CICC) for ITER. The cable pattern and jacket layout were identical to the toroidal field model coil conductor (TFMC), tested in 1999. The four conductor sections used strands from OST, EAS, OKSC and OCSI respectively. The Cu:non-Cu ratio was 1 for three of the new strands, compared to 1.5 in the TFMC strand. The conductors with OST and OKSC strands had one Cu wire for two Nb3Sn strands, as in TFMC. In the EAS and OCSI conductors, all the 1080 strands in the cable were Nb3Sn. A dc test under relevant load conditions and a thermal-hydraulic campaign was carried out in SULTAN. The CICC performance was strongly degraded compared to the strand for all the four conductors. The current sharing temperature at the ITER TF operating conditions (jop = 286 A/mm2, B = 11.15 T) was lower than requested by ITER.

The performance of the toroidal field (TF) magnet conductors for the ITER machine are qualified by a short full-size sample (4 m) current sharing temperature (T-cs) test in the SULTAN facility at CRPP in Villigen, Switzerland, using the operating current of 68 kA and the design peak field of 11.8 T. Several samples, including at least one from each of the six ITER Domestic Agencies participating in TF conductor fabrication (China, European Union, Japan, Russia, South Korea and the United States), have been qualified by the ITER Organization after achieving T-cs values of 6.0-6.9 K, after 700-1000 electromagnetic cycles. These T-cs values exceed the ITER specification and enabled the industrial production of these long-lead items for the ITER tokamak to begin in each Domestic Agency. Some of these samples did not pass the qualification test. In this paper, we summarize the performance of the qualified samples, analyze the effect of strand performance on conductor performance, and discuss the details of the test results.

Taking the relay of the large Hadron collider (LHC) at CERN, ITER has become the largest project in applied superconductivity. In addition to its technical complexity, ITER is also a management challenge as it relies on an unprecedented collaboration of seven partners, representing more than half of the world population, who provide 90% of the components as in-kind contributions. The ITER magnet system is one of the most sophisticated superconducting magnet systems ever designed, with an enormous stored energy of 51?GJ. It involves six of the ITER partners. The coils are wound from cable-in-conduit conductors (CICCs) made up of superconducting and copper strands assembled into a multistage cable, inserted into a conduit of butt-welded austenitic steel tubes. The conductors for the toroidal field (TF) and central solenoid (CS) coils require about 600?t of Nb3Sn strands while the poloidal field (PF) and correction coil (CC) and busbar conductors need around 275?t of Nb?Ti strands. The required amount of Nb3Sn strands far exceeds pre-existing industrial capacity and has called for a significant worldwide production scale up. The TF conductors are the first ITER components to be mass produced and are more than 50% complete. During its life time, the CS coil will have to sustain several tens of thousands of electromagnetic (EM) cycles to high current and field conditions, way beyond anything a large Nb3Sn coil has ever experienced. Following a comprehensive R&D program, a technical solution has been found for the CS conductor, which ensures stable performance versus EM and thermal cycling. Productions of PF, CC and busbar conductors are also underway. After an introduction to the ITER project and magnet system, we describe the ITER conductor procurements and the quality assurance/quality control programs that have been implemented to ensure production uniformity across numerous suppliers. Then, we provide examples of technical challenges that have been encountered and we present the status of ITER conductor production worldwide.

The ITER central solenoid (CS) must be capable of driving inductively 30 000 15 MA plasma pulses with a burn duration of 400 s. This implies that during the lifetime of the machine, the CS, comprised of six independently powered coil modules, will have to sustain severe and repeated electromagnetic cycles to high current and field conditions. The design of the CS calls for the use of cable-in-conduit conductors made up of Nb3Sn and pure copper strands, assembled in a five-stage, rope-type cable around a central cooling spiral that is inserted into a circle-in-square jacket made up of a special grade of high manganese stainless steel. Since Nb3Sn cable-in-conduit conductors are known to exhibit electromagnetic cycling degradation, prior to the launch of production, the conductor design and potential suppliers must be qualified through the successful testing of full-size conductor samples. These tests are carried out at the SULTAN test facility. In this paper, we report the results of the on-going CS conductor performance qualification and we present the options under consideration for the different modules constituting the CS coil.

For the ITER Central Solenoid (CS), with Nb3Sn CICCs that operate under fast ramping conditions, the selection of the twist pitch lengths can have a significant impact on the performance. The critical current and temperature margin are influenced by the thermal contraction of the composite materials, the transverse electromagnetic forces, and coupling currents. The numerical cable model JackPot-ACDC is developed to calculate the interstrand coupling loss for any time-dependent current and magnet field for all strand trajectories in a CICC. It was a priori predicted that the amount of coupling loss and critical current degradation is subject to interference due to different subcable twist pitches. Here test results are discussed of the ITER CS conductor sample, manufactured according to the proposed design, optimizing the transverse load degradation, the temperature margin, and the coupling loss.

The International Thermonuclear Experimental Reactor (ITER) cable-in-conduit conductor used in the superconducting magnet system consists of a cable made of 300 to 1440 strands housed in a stainless steel tube (called as jacket or conduit). There are circular, square, as well as circle-in-square jackets, made of either a very low carbon AISI 316LN and AISI 316L grade stainless steels, or a high Mn austenitic stainless steel developed for ITER called JK2LB. Selected mechanical properties of the base material and weld joint were tested at room temperature and/or cryogenic temperatures (

Superconductors for the ITER Poloidal Field Coils are large cable-in-conduit conductors (CICC) made of NbTi strands encased in a round-in-square stainless steel jacket. Three prototype conductor sections for poloidal field coils PF1/6, PF2/3/4 and PF5 have been fabricated in collaboration of the domestic RF, CN and EU agencies and tested in SULTAN Test Facility at the nominal operating conditions. The test aimed to characterize the DC and AC behavior of the conductors. The DC test was focused on the current sharing temperature (T-cs) at the nominal operating current and nominal operating background field. The take-off electric field at the nominal Helium mass flow rate was investigated versus the cable current density over a broad range of field and temperature. The AC loss measurement was performed before any electromagnetic loading and after a number of load cycles in order to define the impact of cyclic loads on the coupling currents constant of the cable. From the test results in SULTAN test facility, the margins in normal operation and the limits of the operation range of the ITER PF conductors are assessed.

The ITER magnet coils are wound from Cable-In-Conduit Conductors (CICC) made up of superconducting and copper strands assembled into a multistage, rope-type cable inserted into a conduit of butt-welded austenitic steel tubes. The conductors for the Toroidal Field (TF) and Central Solenoid (CS) coils require about 500 tons of Nb3Sn strands while the Poloidal Field (PF) and Correction Coil (CC) conductors need around 250 tons of Nb-Ti strands. The required amount of Nb3Sn strands far exceeds pre-existing industrial capacity and calls for a significant worldwide production scale up. After explaining the in-kind procurement sharing of the various conductor types among the six ITER Domestic Agencies (DA) involved: China, Europe, Japan, South Korea, Russia, and the United States, we detail the technical requirements defined by the ITER International Fusion Energy Organization (IO), and we present a brief status of ongoing productions. The most advanced production is that for the TF conductors, where all six DAs have qualified suppliers and about 50% of the required strands have been produced and registered into the web-based conductor database developed by the IO.

We analyzed the ITER TFEU5 cable-in-conduit conductor (CICC) after the full SULTAN conductor qualification test in order to explore whether Lorentz force induced strand movement inside the CICC produces any fracture of the brittle Nb3Sn filaments. Metallographic image analysis was used to quantify the change in void fraction of each sub-cable (petal); strands move in the direction of the Lorentz force, increasing the void space on the low force side of the CICC and producing a densification on the high force side. Adjacent strand counting shows that local increases in void space result in lower local strand–strand support. Extensive metallographic sampling unambiguously confirms that Nb3Sn filament fracture occurred in the TFEU5 CICC, but the filament fracture was highly localized to strand sections with high local curvature (likely produced during cabling, where strands are pivoted around each other). More than 95% of the straighter strand sections were free of filament cracks, while less than 60% of the bent strand sections were crack free. The high concentration of filament fractures on the tensile side of the strand–strand pivot points indicates that these pivot points are responsible for the vast majority of filament fracture. Much lower crack densities were observed in CICC sections extracted from a lower, gradient-field region of the SULTAN-tested cable. We conclude that localized filament fracture is induced by high Lorentz forces during SULTAN testing of this prototype toroidal field CICC and that the strand sections with the most damage are located at the petal corners of the high field zone.

NbTi strands to be used in four of the six ITER poloidal field (PF) coils, all the correction coils (CC) and all the superconducting feeder busbars are being produced in China. Short full-size qualification conductor (cabled and jacketed) samples have been developed at ASIPP and tested at CRPP. Single pinning mechanism parametrization for this Chinese strand (type S2) has been obtained using the Bottura scaling law. The determination of the scaling parameters using a Kramer-type regression method will be described. A comparison between the critical temperature at the operating current and field of a single strand as determined by the parametrization and the current sharing temperature (TCS) of a few conductor samples tested at the SULTAN facility will be made. The validity and limitation of the estimation will be discussed. The estimated TCS dependence on various (superconducting critical as well as geometric and volumetric) parameters will be assessed using the modelled critical surface. Errors propagated from critical current (Ic) measurements of the strands and parameter fitting, and other uncertainties, will be quantified.

Results on the field and temperature dependence of the critical transport current density in YBa 2 Cu 3 O 7−δ coated conductors of various thickness are presented. The conductors were deposited by liquid phase epitaxy on RE-123 buffer layers on MgO substrates. The analysis of the remanent fields in magnetization loops with different maximum external field provides information about the inter- and intragrain current distribution. From the reserve leg of the magnetization loops the average grain size in the YBCO layer can be estimated. The results of these magnetic measurements are compared to those obtained from magneto-optical imaging. Modifications of the defect structure by neutron irradiation are reported as well. The rssulting defects are either collision cascades, originating from fast neutrons (E > 0.1 MeV), or point defects, originating from neutrons with lower energies. Their influence on J c will be discussed.

The PFCI is a single-layer solenoid wound from a 45 m long ITER-type NbTi dual-channel cable-in-conduit conductor, designed to be representative of the one currently proposed for the ITER PF1&6 coils. The PFCI, installed in the bore of the ITER central solenoid model coil (CSMC) at JAEA Naka, Japan, and well instrumented from both the thermal hydraulic and the electromagnetic points of view, has been successfully tested in June-August 2008. The test concentrated on DC performance (current sharing temperature and critical current measurements) and AC loss measurements. The results of the analysis of those measurements are reported in the paper, with particular attention to the comparison with the PFCI short sample, which was previously tested in the SULTAN facility. The evolution of the DC performance of the CSMC is also discussed.