Dummy Conductor Research Articles

Inter-Layer Joint for the TF Coils of DEMO—Design and Test Results

In summer 2015, a new reference baseline is issued for the DEMO EUROfusion tokamak. Thereafter, the toroidal field (TF) coils have been updated with the new layout of the react-and-wind conductor proposed by the Swiss Plasma Center (SPC). Each TF coil consists of 12 single layers of graded Nb3Sn conductors connected in series by “invisible” inter-layer joints fully embedded in the winding pack. The high-grade Nb3Sn conductor operates at 63 kA, 12.2 T with Tcs > 6.5 K. The new prototype of the high-grade cable had been manufactured by ENEA at TRATOS, Italy, and delivered to SPC: 20 m of Nb3Sn and about 10 m of copper dummy conductor. The SULTAN conductor sample consisting of two identical, beforehand heat-treated conductors was prepared and tested at SPC in May–June 2017. Upon the completion of comprehensive conductor tests, the one of those conductors was used for manufacturing of the designed at SPC potentially applicable inter-layer TF joint. The conductor was cut, and then, joined again in accordance with the developed at SPC design of inter-layer joint. The SULTAN sample comprising the earlier tested non-destroyed conductor and the earlier tested, but then cut and joined conductor was re-assembled and tested in the SULTAN test facility at SPC in August 2017.

Using the HELIOS facility for assessment of bundle-jacket thermal coupling in a CICC

In a Cable In Conduit Conductor (CICC) cooled by forced circulation of supercritical helium, the heat exchange in the bundle region can play a significant role for conductor safe operation, while remaining a quite uncertain parameter. Heat exchange between bundle and jacket depends on the relative contributions of convective heat transfer due to the helium flow inside the bundle and of thermal resistance due to the wrappings between the cable and the conduit.In order to qualify this thermal coupling at realistic operating conditions, a dedicated experiment on a 1.2m sample of ITER Toroidal Field (TF) dummy conductor was designed and performed in the HELIOS test facility at CEA Grenoble. Several methods were envisaged, and the choice was made to assess bundle-jacket heat transfer coefficient by measuring the temperature of a solid copper cylinder inserted over the conductor jacket and submitted to heat deposition on its outer surface.The mock-up was manufactured and tested in spring 2015. Bundle-jacket heat transfer coefficient was found in the range 300–500Wm−2K−1. Results analysis suggests that the order of magnitude of convective heat transfer coefficient inside bundle is closer to Colburn–Reynolds analogy than to Dittus–Boelter correlation, and that bundle-jacket thermal coupling is mainly limited by thermal resistance due to wrappings. A model based on an equivalent layer of stagnant helium between wraps and jacket was proposed and showed a good consistency with the experiment, with relevant values for the helium layer thickness.

Investigation of ITER TF Conductor Hydraulic Resistance

Conductors for winding ITER toroidal field magnets are cable-in-conduit type where multi-stage petals are twisted around central cooling spiral inserted into stainless steel jacket. Every conductor unit length and its components pass various tests during manufacturing and the final acceptance test consists of 760 m or 415 m long conductor global leak test and an investigation of conductor hydrodynamic performances. The hydraulic resistance is one of the main TF CICC's parameter to provide sufficient cool down and stable work of the magnet system during operation. Starting from 2011, when the first copper Dummy conductor was completed, 22 conductors in total were tested in a test facility that was designed and manufactured at National Research Centre “Kurchatov Institute.” A scheme of the pressure drop test facility, measurements technique and results of mass flow rate measurements depending on inlet and outlet pressure difference are presented in this work. Measurements were carried out by flowing nitrogen gas through conductor at room temperature. However, in accordance with the theory of turbulent self-similarity it is possible to develop a simple criterion for estimation of hydraulic resistance at any temperature. The model of two parallel independent flows has been used. The simple explanation of mass flow rate non-linearity at small pressure difference has been provided.

Overview of Conductor Production for ITER Toroidal Field Magnet in Korea

The ITER toroidal field (TF) conductor is made up of superconducting Nb3Sn and copper strands assembled into a multistage, rope-type cable inserted into a conduit of butt-welded stainless steel jacket sections. For the ITER Project, the Korean Domestic Agency (KODA) took the responsibility of the procurement of 27 superconducting conductors for the ITER TF magnets. After concluding the Procurement Arrangement (PA) with the ITER International Organization in May 2008, KODA has been implementing the PA through four major industrial contracts: (1) Nb3Sn strand, (2) cable, (3) stainless steel jacket sections, and (4) jacketing. Prior to the production of conductors required for the TF coils, one 760-m-long copper dummy conductor and one 100-m-long superconducting conductor were fabricated for manufacturing process qualification. As of June 2013, 16 TF conductors were successfully manufactured. The full-size conductor performance tests in the SULTAN facility yielded very high performance. This article describes the technical requirements of the TF conductor and how KODA has been manufacturing the conductors with a high-level quality assurance/quality control system. It also presents the results of acceptance tests, including those of the SULTAN test.

Mass Production of ${\rm Nb}_{3}{\rm Sn}$ Conductors for ITER Toroidal Field Coils in Japan

Japan Atomic Energy Agency (JAEA) was the first to start the mass production of the TF conductors (jacketing) in March 2010 among the 6 parties who are procuring TF conductors in the ITER project. A 760-m Cu dummy conductor was successfully fabricated prior to the manufacture of the actual Nb3Sn conductors. Suitable manufacturing techniques for the long TF conductors were established during fabrication of the dummy conductor. This paper summarizes the technical developments including a high-level quality assurance, leading to the first successful mass production of ITER TF conductor. Approximately 63 tons of strands were manufactured by the two suppliers by August 2011. This amount corresponds to approximately 60% of the total contribution from Japan. Five sDP conductors (415 m) and six rDP conductors (760 m) to be used in the TF coils were completed as of February 2011. This amount corresponds to approximately 25% of the total contribution (rDP: 24, sDP: 9) from Japan. JAEA is manufacturing one conductor per month under a contract with two Japanese companies for strands, one company for cabling and one company for jacketing. This progress is a significant step in the construction of the ITER machine.

Preparation for the ITER Central Solenoid Conductor Manufacturing

The ITER Central Solenoid (CS) conductor is composed of 576 superconducting strands and 288 Cu strands assembled together into a multistage cable and protected by a circle-in-square jacket with the outer dimension of 49 mm × 49 mm. In R&D to prepare for the ITER CS conductor manufacturing, mechanical tests of jacket, welding tests and manufacturing of 181-m long dummy conductor have been performed. In this paper, the R&D activities are presented, showing that as a result of this R&D, the CS conductor manufacturing technologies have been preliminary defined to start the procurement of the CS conductor.

First qualification of ITER Toroidal Field Coil conductor jacketing

The Japan Atomic Energy Agency (JAEA) has the responsibility to procure 25% of the ITER Toroidal Field Coil conductors as the Japanese Domestic Agency (JADA) in the ITER project. The TF conductor is a circular shaped, cable-in-conduit conductor, composed of a cable and a stainless steel conduit (jacket). The outer diameter and maximum length of the TF conductor are 43.7 mm and 760 m, respectively. JAEA started to produce strand, cables and jacket sections and to construct a conductor manufacturing (jacketing) facility in 2008. Following preparation in December 2009 of the jacketing facility, the dummy cable, the jacket sections and fabrication procedures, such as welding, cable insertion, compaction and spooling, JAEA manufactured a 760 m long Cu dummy conductor for process qualification. Into the 760 m long Cu dummy conductor jacketing, JAEA successfully inserted the cable with a maximum force of 32 kN. The outer diameter of the cross section of the spooled conductor was 43.7 ± 0.15 mm, which complies with the ITER target requirement of 43.7 ± 0.3 mm. Following qualification of all manufacturing processes, JAEA has started to fabricate superconducting conductors for the TF coils.

New Technology Complex for ITER TF and PF Cables and TF Conductors Production

Complex through technology, developed in JSC VNIIKP for serial production of ITER PF and TF cables and TF conductors, is described. This technology was developed as a result of performing some RD (2) Cabling workshop; (3) Jacketing line equipped with full production machinery and test devices. First dummy conductor with copper strands cable was jacketed and tested this year on modified jacketing line.

Axial compression test on ITER-TFMC conductors at room temperature

The superconductor used for the ITER TF Model Coil (TFMC) consists of 720 twisted Nb 3Sn-strands and 360 copper strands, which are cabled around a central spiral and are surrounded by a stainless steel jacket. The conductor is heat-treated at 650 °C in a stainless steel mould. After cool down to room temperature the conductor is found to be elongated by 0.45 mm/m, which can be attributed to the lower shrinkage of Nb 3Sn. This means that the jacket is under tensile stress and the cable under longitudinal compression after cool down. This effect aggravates upon cooling further to the operating temperature. The cable and the jacket can be interpreted as a double spring system, the jacket being the expansion spring and the cable being the compression spring. In order to evaluate and possibly predict such elongations, it is necessary to know the Young's modulus of the cable. Therefore, we investigated samples cut from a dummy conductor, a not heat-treated and a heat-treated TFMC conductor by mechanically compressing them. From the stress–strain diagram the elastic modulus of the cables was determined and found to be about one-tenth of that calculated assuming all components of the cable being fully bonded. The stiffness of the cables turned out to be fairly independent of the state of the cable as all three cables show almost the same modulus. The mechanical compression of the cable is about 8.5 times larger than the measured elongation of the jacket.

Manufacturing of the ITER TF Full Size Prototype Conductor

The experience gained in the past for the ITER toroidal field model coil conductor and the results obtained so far have led to the definition of an upgraded full size prototype conductor, based on advanced Nb3Sn strand, and entirely manufactured in the European Union (EU). Samples for the characterization in the Sultan facility have been prepared by Luvata (Italy) following the conductor layout defined by ITER. ENEA was responsible for conductor fabrication. Since the conductor layout was new, a full size copper dummy conductor has been preventively produced for the setting of the cabling and jacketing tools. Then, a total of four full size superconducting cables have been prepared by using Nb3Sn advanced strands produced by Oxford Instruments (OST) and European advanced superconductors (EAS), by internal tin and bronze technology, respectively. The details of manufacturing procedures will be described in this paper.

Indirect measurement of current distributions in Bi-2223/Ag tapes by pickup coil method

We develop a quantitative measurement method to evaluate current distributions in high T/sub c/ superconducting tapes. In our measurement the current distributions are indirectly found from the results of the self-field distributions measured by a group of coils, which is composed of nine small pickup coils, located around the tape. It is confirmed that the accuracy and the sensitivity of this measurement method, using a group of pickup coils, are good enough to measure the current distributions for the high T/sub c/ superconducting tapes by adopting it to a dummy conductor composed of insulated copper wires. Using this method, we have measured the current distributions in multifilamentary Ag-sheathed Bi-2223 tape at 77 K. Measured and numerical self-field distributions are compared. The comparison between the experimental and numerical results shows that the current distribution in the tape is not uniform and that distribution changes as the amplitude of the transport current increases. In addition, no frequency dependence of the current distributions is observed. Further, we investigate the influence of the external magnetic field on the current distributions. On the basis of the obtained results the characteristics of the current distributions in the high T/sub c/ superconducting tape are discussed.

Winding technique for HT-7U Tokamak magnet coils

On-site winding of magnet coils of HT-7U started in September 1998, using a numerically controlled winding machine. Magnet coils are all forced-flow superconducting coils, consisting of superconducting Toroidal Field (TF) coils and superconducting Poloidal Field (PF) coils. All of TF and PF coils will use NbTi Cable-in-Conduit Conductor (CICC) cooled with supercritical helium. This paper reports on the set up of a new winding facility with unique capabilities for continuous winding of long length CICC. An analytical method used to predict conduit springback before winding is presented and the results are compared to the ones obtained during winding. Research and development of winding of the TF Dummy Coil (TFDC) has been carried out and a winding method has been developed by winding trials. Windings of the TFDC and CS Model Coil (CSMC) have been successfully completed by using the developed technique. In R&D, long dummy conductor and short sample jackets have been used to demonstrate a plastic deformation characteristic of CICC during the winding process. The developed winding method can be applied to fabricate all TF and PF coils.

Current distributions at steady-state of superconducting cable conductors for AC use

In order to obtain information towards achieving uniformity of current distribution in superconducting cable conductors for AC use, the authors have developed a compact measuring system that enables them to quantitatively evaluate the current distribution. In this system, current distribution is found from the observed results of the self-field measured by a group of coils, each of which is composed of six small pick-up coils, located around the sample conductor. It is confirmed that the accuracy and the sensitivity of this system are good enough to measure the current distributions for the superconducting cable conductors for AC use by adopting it to a dummy conductor composed of insulated copper wires. Using this system, they experimentally investigated how the localized transport current, in the steady-not quenched state, is made uniform in the superconducting cable conductor composed of strands without insulation for AC use. By this experiment, they observed how the localized current transfers among the noninsulated strands. At the same time, the frequency dependence of the change of the current distribution can also be observed as well. These measured results are discussed using approximate analysis relating to the inter-strand coupling effect.

Design, fabrication and testing of a production line for TPX demonstration conductor

A complete production line for fabricating CICC dummy conductor has been designed, constructed and tested for TPX program. The line is capable of producing up to 500 meters of CICC conductor and is scaleable up to 1.5 km in length. The line consists of a payoff spool for feeding cable inside the tubes, a welding station, a drawing station which includes dies and a bull-block, and a takeup spool. We have demonstrated the fabrication of the conductors. Two types of conductor, one for TF and the other one for PF coil have been produced The conduit materials are Incoloy 908, which are formed from strips. The cables are made from OFHC copper strands. The detail design, fabrication and testing of each component of the system is presented in this paper.

Flow visualization of coolant in cable-in-conduit conductor

Flow visualization to observe the flow behavior of a coolant in a cable-in-conduit conductor was performed by nuclear magnetic resonance (NMR) imaging. A dummy conductor in which many vinyl strings were inserted into an acrylic tube was used for visualization. Regions of both high and very low flow velocity, in comparison with the average velocity, existed in a cross section of the conductor. This fact shows that the flow behavior of the coolant in the cable-in-conduit conductor is not homogeneous but heterogeneous. Thus it is not possible to directly apply a numerical code based on homogeneous flow behavior to stability analysis of the cable-in-conduit conductor.

Feasibility test of a shrink-fit assembly of a large-diameter superconducting solenoid for a colliding beam detector

A dummy solenoid coil of 3 m in diameter and 0.65 m in length was fabricated in order to establish a shrink-fit assembly procedure for a thin, large superconducting solenoid magnet without a permanent inner bobbin for a colliding beam detector. A dummy conductor of aluminum and actual insulation materials were used for the dummy coil. The coil thickness was 20 mm. After an outer support cylinder of aluminum alloy of 16 mm in thickness was shrink-fifted upon the coil, the temporary mandrel used for coil winding was removed. With this arrangement the solenoid can be built substantially thinner in terms of radiation thickness compared with those built with the conventional method. The interference between the coil and support cylinder to provide adequate pre-stress was determined. Various mechanical properties of the solenoid components required in the present method were studied.