Poloidal Field Conductor Research Articles

Parametric thermal-hydraulic analysis of the DEMO PF coils designed by CEA during the breakdown phase

EU-DEMO - the European DEMOnstration Fusion Power Plant, is being designed as an intermediate stage between the ITER experimental reactor and the future fusion power plant. The fully superconducting magnet system of the DEMO tokamak includes six Poloidal Field (PF) coils. Two different concepts of the DEMO PF winding packs are being developed by the EPFL-SPC (Switzerland) and CEA IRFM (France) teams. The current cycle of the PF coils consists of the following phases: Premagnetization (10 s), Plasma Current Ramp-Up (PCRU, 80 s), plasma burn (7200 s) and dwell (600 s). The PCRU phase starts with the fast breakdown (0.8 s), during which the operating current and magnetic field change very rapidly causing significant heat generation due to AC losses. Thus, the breakdown is potentially the most critical phase of normal operation of PF conductors regarding the temperature margin. This study was focused on the thermal-hydraulic analysis of the PF coils designed by CEA during breakdown. According to the CEA design, based on the 2018 DEMO reference, the PF coils are double-pancake wound with square NbTi Cable-in-Conduit Conductors with a central cooling channel. Operation of conductors designed for each PF coil was simulated with the THEA code by Cryosoft. Since the detailed reference current breakdown scenario for the PF coils is not available yet, the analysis was performed parametrically. We took into account the external or internal magnetic field profile along each PF conductor as well as heat generation due to the AC hysteresis losses and coupling losses (for the trial values of the parameter nτ = 100, 200 and 300 ms). The study was aimed at the estimation of the minimum temperature margin in each PF conductor during the breakdown and verification if it fulfills the performance criterion min(ΔTmarg) ≥ 1.5 K. The results of analysis should help in further improvements and optimization of the PF winding packs design.

DC performance and AC loss of sub-size MgB2 CICC conductor for fusion magnet application

Given the low price and relatively high transition temperature (39 K) of MgB2 conductor, MgB2-based superconductors are a potential candidate for the lower field fusion coils, such as poloidal field (PF) coils, correction coils and feeders. However, to date, the application of MgB2 is limited to demonstrators in a low magnetic field of up to 5 T and at temperatures of up to 10 to 20 K, relying on cryogen-free, helium gas or liquid hydrogen cooling, which significantly reduce the cost of cryogenic systems. To demonstrate the feasibility and performance verification of large size MgB2 PF conductors based on ITER and CFETR requirements, a 4th-stage subsize MgB2 cable-in-conduit conductor (CICC) cable sample is made at the Institute of Plasma Physics, Chinese Academy of Sciences. The CICC contains 96 in situ MgB2 superconducting wires, manufactured by Western Superconducting Technology Ltd. and 48 copper wires. The critical current of the sub-size cables and MgB2 witness wires are examined with different background magnetic fields at 4.2 K. In addition, the AC loss is measured utilizing magnetization and calorimetric methods. To further clarify the influence of electromagnetic force on the AC loss performance, the cable sample is pressed transversely at room temperature and then inserted into a dipole magnet for AC loss measurement at 4.2 K. The critical current at 4.2 K of the subsize MgB2 CICC cable shows 20% degradation compared to the witness wires at 2 T background magnetic field. However, no further critical current degradation is visible during ramping up and down the magnetic field. The coupling loss time constant for 1 T background magnetic field amounts to 480 ms. No significant effect of the applied transverse stress on the coupling loss is observed between 0 and 10 MPa.

Summary of NbTi Strand Performance for ITER PF Conductors in China

China, who was in charge of most of the poloidal field (PF) conductor unit lengths manufacturing for the ITER Project, has supplied all the required PF conductors and finished the PF package successfully. Around 130 t of NbTi strands were supplied and used for the PF conductors. During the package execution process, the Chinese Domestic Agency (CNDA) has organized the verification for the NbTi strand acceptance tests based on the requirements of the ITER procurement arrangement (PA). The verification tasks for NbTi strands were undertaken by the Institute of Plasma Physics Chinese Academy of Sciences (ASIPP). In total, about 550 billets of NbTi strands have been verified and supplied for the PF conductors. The verification levels fit the ITER PA requirements. The test results showed that the quality of the NbTi strands has been well controlled. In addition, in order to supply essential data for the conductor performance prediction and analysis, a full critical current I c characterization of the NbTi strands, which requires a measurement of I c versus field, B and temperature, T, has been obtained using the test facilities from the National Institute of Standards and Technology (NIST) and ASIPP. The parameterization using the Bottura scaling law for the strand with I c close to the average value from the verification results has been analyzed. The test methods and strand performances are reviewed and summarized in this article.

Overview of verification tests on AC loss, contact resistance and mechanical properties of ITER conductors with transverse loading up to 30 000 cycles

The ITER magnet system uses cable-in-conduit conductor (CICC) technology with individual strands twisted in several stages resulting in a rope-type cable, which is inserted into a stainless steel conduit. The combination of high current (up to 68 kA) and background magnetic field (up to 13 T) results in large transverse Lorentz forces exerted on the conductors during magnet system operation. The high transverse forces, accompanied with the cyclic nature of the load, have a strong influence on the conductor properties. The Twente Cryogenic Cable Press is used to simulate the effect of the Lorentz forces on a conductor comparable to the ITER magnet operating conditions. An overview is presented of the AC coupling and hysteresis loss, mechanical deformation characteristics and inter-strand contact resistance measurement results obtained on full-size ITER CICCs measured in the Twente Cryogenic Cable Press. The aim of this work is to characterize conductors’ electromagnetic and mechanical properties during cycling of the load up to 30 000 cycles. The evolution of the magnetization (AC coupling loss time constant nτ), mechanical properties and inter-strand resistance Rc between selected strands is presented along with loading history. The Rc between first triplet strands is also measured as a function of applied load. It is shown that transverse load cycling has a strong influence on the CICC properties. An overview of the results for eight toroidal field conductors, two central solenoid conductors, three poloidal field conductors of different types (PF1&6, PF4, PF5), one main bus-bar and one correction coil conductor is presented.

Design updates of magnet system for Korean fusion demonstration reactor, K-DEMO

A conceptual design study for a steady-state Korean fusion demonstration reactor (K-DEMO) was initiated in 2012. The superconducting magnet system is one of the key components of the K-DEMO and the preliminary study on superconducting magnet system has been done in 2015. The superconducting magnet system of K-DEMO consists of 16 TF (Toroidal Field), 8 CS (Central Solenoid) and 12 PF (Poloidal Field) coils. All of the TF, CS and PF coils use internally-cooled Cable-In-Conduit Conductors (CICC). A high performance Nb3Sn-based conductor, an ITER-type Nb3Sn and NbTi conductor will be used for the magnet. Based on the preliminary design, continuous design study is being performed and the K-DEMO magnet system has been evolved mainly for solving engineering issues. Two issues were considered for the TF, the reinforcement of the low field TF CICC structure and the fabrication stability of the high field TF CICC. In the CS magnet, reducing the number of Helium feed-throughs was a main issue. A small change has been made for the PF magnets for keeping a void fraction of the PF conductor. In this work, the major parameters and design updates of the K-DEMO magnet system is presented.

Self-Field Effects on JC(B,T) Measurements of Nb–Ti Strands in High Magnetic Fields

We have investigated the applied magnetic field, temperature, and self-field dependence of the critical current density of a Nb–Ti strand produced for the ITER poloidal field conductor. Measurements were made on a standard ITER barrel in magnetic fields from 4.0 to 8.0 T and temperatures from 3.5 to 6.0 K. We investigated the effect of self-field by changing the direction of the transport current, resulting in an inward or outward Lorentz force acting on the strand. At 4.2 K and 5 T, the difference in the measured $J_{{\text{C}}}$ between the two Lorentz force polarities was about 1%. From this low value, we conclude that self-field effects in the Nb–Ti strand are about 40% of those expected using standard self-field calculations for fully transposed multifilamentary strands, consistent with theoretical considerations for annular filament transposition.

Design, Manufacture, and Test of an 80 kA-Class Nb3Sn Cable-In-Conduit Conductor With Rectangular Geometry and Distributed Pressure Relief Channels

Within the frame of the R&D activities carried out in Europe for the toroidal field coils of the nuclear fusion device DEMO, a fundamental milestone was considered to be the demonstration of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn cable-in-conduit conductors (CICC) performance in the demanding range of interest for DEMO. Among the different technological solutions envisaged, the present paper deals with a wind & react CICC solution, with rectangular geometry, thick steel jacket, and distributed pressure relief channel, designed to operate at 82 kA in a magnetic field of 13 T and with a current sharing temperature T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cs</sub> > 6.5 ,K. The main manufacturing steps of the prototype conductor are described in the present paper, carried out within industrial environment, partly using the facilities and procedures available for the manufacture of ITER conductors. A sample was designed for the EDIPO facility at the Swiss Plasma Center, Switzerland, in the configuration usually adopted for the test of ITER poloidal field conductors, where the two straight conductor legs are part of the same cable length, with a continuous transition through a bottom hairpin-type joint, thus avoiding any resistive connection. The conductor has been characterized in terms of dc performance at relevant operating conditions and the absence of any performance degradation with electro-magnetic load cycles has been verified, thus, qualifying the proposed technological solution. AC losses and thermo-hydraulic tests have also been carried out, providing relevant information for further coil design.

Review of Scientific Results Obtained During Production of ITER TF and PF Conductors in Russia

The Russian Scientific R&D Cable Institute was obliged to produce and to deliver 20% toroidal field (TF) conductors and 20% poloidal field (PF) cables for the ITER magnet system. The task has been completed in October 2015. This work was a great technological challenge in the development of tricky technologies. Development of manufacturing processes was accompanied by extensive scientific studies directed to an improvement of technologies and a search of causes resulting in change of cables and conductors properties during manufacturing and testing. In this paper, we present the extended review and summary of results obtained for Russian superconductors that permitted improving technologies and finding out the reasons of changes of properties of cables and conductors during manufacturing and testing. The study of rotation and untwisting of TF cables during insertions into jackets allowed developing the model describing the rotation process and suggesting the device which keeps untwisting within demanded levels. Analyses of mandatory SULTAN high-field electromagnetic tests of PF and TF conductors from Russia confirmed good quality and adequacy of manufacturing processes. Micrographic studies of Nb 3Sn strands before and after SULTAN tests permitted qualitatively understanding the reason for the stability of RF TF conductor behavior during multiple electromagnetic and thermal cycling. Statistical studies of residual resistance ratio (RRR) of Nb 3Sn and NbTi strands permitted tracking of its degradation during manufacturing processes and SULTAN tests. The optimal heat treatment process for Nb 3Sn conductors has been confirmed after RRR statistical study. The hydraulic performance has been measured in the short samples of TF conductor. By using the parameters obtained, it is possible to predict hydraulic performance of helium flow in a conductor at any temperature. These results led to deeper understanding and to improving manufacturing processes and quality of TF and PF conductors for the ITER magnet system and could be useful for suppliers of superconducting cables and conductors using ITER-like technologies.

Status of the ITER Conductors in China

The ITER magnet system is made up of four main subsystems: the 18 toroidal field (TF) coils, the central solenoid, the six poloidal field (PF) coils, and the correction coils (CCs). The feeder system, with its main busbar (MB) and CC busbar (CB), represents one of the main magnet components as well. All coils and busbars with different dimensions used cable-in-conduit conductors. China has signed three conductor packages, which are the so-called procurement arrangements, between ITER and the Chinese Domestic Agency (CN DA): a TF conductor package, a PF conductor package, and a CC and feeder conductor package. They include 7.5% of the TF conductors (11); all the PF2 (12), PF3 (16), PF4 (16), and PF5 (16) conductors; all the CC (18) conductors; and the MB (3) and CB (2) conductors for the feeders. Complex technologies have been developed by ASIPP for the serial production of all ITER conductors, in terms of cabling parameter design, welding, and elaboration of cable insertion, compaction, and winding processes. China has finished all qualification phases and is well into the main series production. All conductor samples required for quality control have successfully passed the SULTAN tests with good performance. The status of the production of ITER conductors in China is described in this paper.

Performance analysis of the Nb−Ti conductor qualification samples for the ITER project

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 (Tcs) 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 Tcs 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.

Stability analysis of the conductors for CFETR poloidal field coils

China Fusion Engineering Testing Reactor (CFETR) is a new tokamak device. The poloidal field (PF) system of CFETR plays an important role in controlling the location and shape of the plasma. The stable operation of the PF system is largely based on the superconductor stability. Therefore, the stability analysis of the PF1, PF4 and DC2 conductors of PF system has been performed. For stability analysis, a mechanical disturbance and an electromagnetic disturbance are applied, respectively. The calculation results of stability margin, current sharing temperature, temperature margin and distribution of conductor temperature are presented and discussed in this paper. The analysis results illustrates that the present design of CFETR PF conductor can satisfy the requirement of safety margin.

Progress on TF and PF Conductor for ITER in China

China International Nuclear Fusion Energy Program Execution Center (ITER China), the China Domestic Agency for ITER, is in charge of about 7.51% of the Toroidal Field (TF) conductors, 65.15% of the Poloidal Field (PF) conductors, namely 11 TF conductors and 60 PF conductors. All those conductors are supplied in-kind by ITER China. Following success in qualification test on full-scale short conductor samples, ITER China is stepping into the series production of the TF and PF super conductors. Several tons of Nb3Sn and NbTi strands were manufactured, tested, and delivered from strands supplier to assembling supplier under the supervision of the ITER China and all the conductor manufacture processes complied with ITER quality assurance requirements. This article reports the progress on TF and PF conductor of ITER China up to December 2012.

Status of the Procurement of the European Superconductors for the ITER Magnets

The European share of the ITER magnet superconductors includes the supply of around 20 km of Toroidal Field and 7 km of Poloidal Field conductors. This represents 20% and 11% of the total conductor amounts respectively needed for all ITER Toroidal Field (TF) and Poloidal Field (PF) coils. For TF conductor, around 97 tons of superconducting niobium tin strand and 60 tons of copper wire are needed to be purchased, cabled and inserted within a stainless steel jacket tube to form the Cable-In-Conduit-Conductor (CICC). For PF conductor, about 45 tons of superconducting multifilamentary niobium titanium wire need to be cabled and jacketed. Altogether, Fusion for Energy (F4E) placed 4 supply contracts for the European TF and PF coil conductors (one for copper strand, two for niobium tin strand and a single contract for cabling and jacketing). In addition, the PF conductor procurement is based on a bi-lateral agreement with the Russian Federation Domestic Agency (RF DA) where NbTi cables are supplied by RF DA and their jacketing is provided by F4E. In this article, the procurement strategy and the current procurement status are reported for the European contribution of TF and PF conductors.

NbTi Strands Verification for ITER PF CICC Process Qualification of CNDA

China is in charge of most of Poloidal Field (PF) conductors production for the International Thermonuclear Experimental Reactor (ITER). The execution for PF conductors shall be in three main phases. According to ITER Procurement Arrangement (PA), the Domestic Agency (DA) shall be required to verify the room and low temperature acceptance tests carried out by the strand suppliers. As the reference laboratory of Chinese DA (CNDA), the superconducting strands test laboratory of Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP) was undertaking the task of strands verification for ITER conductors. The verification test includes: diameter, Nickel plating thickness, copper-to-non-copper volume ratio, twist pitch direction and length, standard critical current (IC) and resistive transition index (n), residual resistance ration (RRR), and hysteresis loss. 48 NbTi strands with 7 billets were supplied for the PF Cable-In-Conduit Conductor (CICC) process qualification. In total, 54 samples were measured. The verification level for PF CICC process qualification was 100%. The test method, facility and results of each item are described in detail in this publication.

Challenges and status of ITER conductor production

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.

ITER and JT-60SA Conductor Production at ICAS

The European and the Korean contributions to the ITER Project in terms of cable-in-conduit conductors have been assigned to the Italian Consortium for Applied Superconductivity (ICAS). ICAS' charter members are: ENEA, with the role of general supervision and monitoring, CRIOTEC Impianti S.r.l., in charge of jacketing activities, and TRATOS Cavi S.p.A., with the duty of cable manufacturing. The ITER production consists of about 40 km of Nb3Sn toroidal field (TF) conductor and 22 km of NbTi poloidal field (PF) conductor, for coils 1 and 6. As part of the EU contribution to the construction of the JT-60SA Japanese Tokamak Reactor, ICAS is also in charge of producing about 28 km of NbTi TF conductor, required for the whole TF magnet system. Thanks to the high level and unique expertise available at ICAS, the consortium has been also assigned to manufacture the ITER Central Solenoid conductor samples to be tested in SULTAN for the final, positive, assessment of the updated design. In the present paper, the steps leading to the successful qualification of the main production processes will be described. What is more, the manufacturing of the first conductor unit lengths of the “production phase,” i.e., the ones that will be wound to operate in the actual ITER TF and JT-60SA TF superconducting magnets, will be reported. The fabrication of the ITER CS conductor samples will be illustrated as well.

$J_{\rm c}(B, T)$ Characterization of Commercial NbTi Strands for the ITER Poloidal Field Coils by Transport and Magnetization Methods

Commercial multifilamentary NbTi strands produced by the Russian Federation Domestic Agency for the ITER poloidal field (PF) coils PF1 and PF6 were characterized regarding by magnetization and transport measurements. The results of critical current measurements at magnetic fields in the wide range from 2 to 10 T and at temperatures from 3.5 to 7.5 K are presented. The experimental results were obtained at two laboratories; the two sets of data show a good match and demonstrate the high reproducibility of data. The current sharing temperature of the strands is between 5.9 and 6 K at 6.4 T at an operating current of 33 A. These results enable finding strand characteristic parameters for fits and lead to a database, which is essential for the future analysis of experimental data for the PF conductor performance.

Research on the Necking Process of ITER PF Conductor

As for the outer-square and inter-circle structure of poloidal field(PF) conductor,the necking process is studied using four rollers by the ABAQUS software. The forming principle for necking deformation,the metal flowing rules and the force situation are analyzed. The four times forming process is designed and the material of conductor is 316L stainless steel whose stress-strain curve is obtained by tensile test. The simulation results show that the distribution of stress and strain in the necking process is uneven and the maximum stress occurred at the back of contact position and the strain is concentrated in the four corner parts which are in accordance with the law of least resistance. The cross-sectional dimension of the deformed conductor is measured in the analysis software and the dimension is within the specified range. The experiment is done on the self-designed molding machine and there are five different cross-sections whose size are measured and averaged. The results are similar with the simulation results which meet the requirement of the conductor size and verify the feasibility of the forming process.

Performance evaluation and analysis of ITER poloidal field conductors

The performance evaluation and analysis of PF5 conductor of the ITER Project in China have been performed using the Gandalf code (Bottura [1]). This study focuses on the Tcs and MQE of PF5 conductor with Cu–non Cu ratio of 2.3 NbTi strands from WST. The PF5 conductor samples have been measured in SULTAN at CRPP for evaluating the performance successfully. The measurement results are also presented with the evaluation results in the paper. The evaluation results related to Tcs and MQE are agreed well with the measurement results. The simulation with Gandalf code can predict the performance of PF5 conductor effectively and provide the helpful method for ITER conductor design and analysis.

Fatigue tests on the ITER PF jacket

This paper focuses on fatigue tests on the ITER Poloidal Field (PF) jacket made of 316L stainless steel material. During manufacture, the conductor will be compacted and spooled after cable insertion. Therefore, sample jackets were prepared under compaction, bending and straightening in order to simulate the status of PF conductor during manufacturing and winding. The fatigue properties of materials were measured at T<7K, including S–N and fatigue crack growth rate (FCGR). The testing results show that the present Chinese PF jacket has good fatigue properties, which conclude that the results are accordant with the requirements of ITER.