Subsize Conductor Research Articles

Design and analysis of HTS subsize-conductors for quench investigations towards future HTS fusion magnets

High Temperature Superconductors (HTS) are promising materials for future fusion magnets as they allow operating magnets at higher fields and/or higher temperatures and can offer significantly higher temperature margins compared to state-of-the-art low-temperature-superconductor solutions.The high critical temperature of the HTS results on the other hand in drastically different behavior of the superconductor in case of quench, e.g. a slower quench propagation velocity. Additionally, the promising HTS material Rare Earth Barium Copper Oxide (REBCO) is only available as flat tape in long length, and many high-field high-current cable concepts for fusion magnets base on the formation of so-called macro-strands, which are subsequently cabled to the final fusion conductor. Thus, the number and size of strands are additional quantities, which are significantly differing between LTS and HTS.In this work, the design of HTS sub-size conductors for quench investigations is presented. The samples are designed for a critical current of approx. 15 kA at operating conditions (T ≥ 4.5 K, B ~ 12 T) with forced flow supercritical Helium cooling. Such critical currents can be achieved for example by a triplet of HTS CroCo macro-strands. Different designs of such triplets are discussed and analyzed by modeling with respect to the temperature and voltage evolution in the conductor in case of quench. The total stabilizer cross-section in the conductor, different thermal and electrical resistances between the individual macro-strands, helium and the jacket, and different layouts of the individual macro-strands are considered. The software THEA is used to calculate the quench performance of these designs under relevant operating conditions.

An Analysis of AC Loss of Bi-2212 CICC

With the development of magnetic confinement fusion research, higher magnetic fields are needed. Commonly used is Nb 3 Sn superconductor in high-field magnets, which does not have much potential for improvement. It is necessary to develop new conductor with new superconducting materials. Bi 2 Sr 2 CaCu 2 Ox is such a promising material. One sub-size conductor with 42 wires has been designed and manufactured at Institute of Plasma Physics Chinese Academy of Sciences (ASIPP) and its ac loss has been tested in the standard magnetization magnet for transverse magnetic fields at the University of Twente. The phenomenological model was used to analyze the test results. And the results showing that the coupling loss of Bi-2212 conductor is greater than that of Nb 3 Sn conductor under the same condition. This result may due to the fact that the silver matrix used in the Bi-2212 strands has a lower electrical resistivity than the copper matrix of the Nb 3 Sn strands.

First AC loss test and analysis of a Bi2212 cable-in-conduit conductor for fusion application

The main goal of the Chinese fusion engineering test reactor (CFETR) is to build a fusion engineering tokamak reactor with a fusion power of 50–200 MW, and plan to test the breeding tritium during the fusion reaction. This may require a maximum magnetic field of the central solenoid and toroidal field coils up to 15 T. New magnet technologies should be developed for the next generation of fusion reactors with higher requirements. Bi2Sr2CaCu2Ox (Bi2212) is considered as a potential and promising superconductor for the magnets in the CFETR. R&D activities are ongoing at the Institute of Plasma Physics, Chinese Academy of Sciences for demonstration of the feasibility of a CICC based on Bi2212 round wire. One sub-size conductor cabled with 42 wires was designed, manufactured and tested with limited strand indentation during cabling and good transport performance. In this paper, the first test results and analysis on the AC loss of Bi2212 round wires and cabled conductor samples are presented. Furthermore, the impact of mechanical load on the AC loss of the sub-size conductor is investigated to represent the operation conditions with electromagnetic loads. The first tests provide an essential basis for the validation of Bi2212 CICC and its application in fusion magnets.

Experiment of joints resistance and critical current of Bi2212 conductor

Bi2Sr2CaCu2Ox is a potential material for the superconducting magnets of the next generation of Fusion reactors. A R&D activity based on Bi2212 wire is running at ASIPP for the feasibility demonstration of CICC. One sub-size conductor cabled with 42 wires was designed and manufactured. A test facility was designed and set-up to measure the resistance of joints and the critical current of the Bi2212 CICC in liquid helium. A superconducting transformer, which consisted of two concentric layer-wound superconducting solenoids, was used to provide the current of the conductor sample. Both of the primary and secondary coils were immersed in liquid helium during the experiments. The highest current of the secondary loop was up to around 14kA. The critical current of the conductor was 13.2kA with criterion of 1μV/cm and the joints resistances were lower than 50nΩ.

Manufacture and Test of Bi-2212 Cable-in-Conduit Conductor

The CFETR, “China Fusion Engineering Test Reactor,” is a new tokamak device. Its magnet system includes the toroidal field (TF), central solenoid (CS), and poloidal field coils. The main goal of this study is to build a fusion engineering tokamak reactor with fusion power of 50-200 MW and self-sufficiency by blanket. The maximum field of CS and TF will get around 15 T, which is much higher than that of other reactors. New materials could be used to develop the magnet technology for the next generation of fusion reactors. Bi 2 Sr 2 CaCu 2 O x is considered as a potential material for the superconducting magnets. An R&D activity is running at ASIPP for the feasibility demonstration of cable-in-conduit conductor based on the Bi2212 wire. One subsize conductor cabled with 42 wires was designed and manufactured. In this paper, the manufacturing procedures and first test results of Bi-2212 conductor samples are described in details, including wire specifications, cabling and compaction process, conductor heat treatment, as well as test results on ac loss and critical current.

Mechanical-Electrical Modeling of Stretching Experiment on 45 ${\rm Nb}_{3}{\rm Sn}$ Strands CICCs

Cable-In-Conduit Conductors made with Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands will be used in ITER magnets. The current carrying capability of these Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands is known to be highly dependant on the strain state resulting from mechanical loading. The intricate cabling pattern of CICC, added to the thermal differential shrinkage between conductor jacket and Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn filaments induce complex strand trajectories and a highly inhomogeneous strain state. This “cable strain map” also evolves with operating loads (Lorentz force/hoop stress). The SAMAN experiment, conducted in the FBI facility at Karlsruhe Institute of Technology, aimed to stretch subsize, ITER-like conductors, in order to observe the evolution of the critical current associated with these loadings. The application of the Multifil finite element code, developed at Ecole Centrale de Paris, has helped quantifying the local strains along every individual strand, and their evolutions during cooldown (from heat treatment), energizing and stretching phenomena. Using Multifil output mechanical data as input in the CEA electrical code CARMEN has allowed computing the critical current in every strand, thus leading to an understanding of the critical current degradation of such subsize conductors. This paper shows, for two SAMAN samples, what is the impact of bending strain concentration on a CICC current transport capability.

Development of&amp;lt;tex&amp;gt;$hboxNb_3hboxSn$&amp;lt;/tex&amp;gt;Cable-in-Conduit Conductors With Stainless Steel Jackets for Central Solenoid of JT-60SC

Development of Nb/sub 3/Sn cable-in-conduit (CIC) conductors with stainless steel (SS) jackets for the central solenoid (CS) and divertor coil of the superconducting tokamak called JT-60SC, is presented. Full-size and subsize conductors were manufactured using two types of high-copper-ratio and high-J/sub c/ strands made by different processes (bronze and internal-tin). Use of a SS jacket causes decrease of critical current of the conductor because compression strain due to difference of the thermal contraction between the SS jacket and Nb/sub 3/Sn cable is generated. The strains are estimated from the results of critical current measurements using short full-size samples which were pool-cooled. In order to investigate J/sub c/-B-T characteristics of a CIC conductor, a novel apparatus, which can control the temperature and magnetic field, has been developed. This apparatus makes it possible to measure directly the critical current and temperature of a subsize conductor, which is composed of two superconducting strands, one copper wire and a SS jacket. Strains of the subsize conductors are estimated from the results of the J/sub c/-B-T characteristic measurements. By comparison of the estimated strains between the full-size and subsize samples, we concluded that the characteristics of the full-size conductor can be predicted by measuring of J/sub c/-B-T characteristics of subsize conductors. Finally, the strain of the actual conductor under the operational condition is estimated. The result confirms that the developed conductors are applicable to CS and divertor coil of JT-60SC.

The CEA JOSEFA Test Facility for Subsize Conductors and Joints

The JOSEFA (joint subsize experiment facility) experimental test facility, installed at CEA/Cadarache is devoted to perform tests at cryogenic temperature on subsize superconducting conductor and joint samples under parallel or transverse magnetic field. This facility was built in 1993 to investigate the performances of joints of cable-in-conduit conductors at subsize level and further upgraded in the framework of European tasks. The samples of hairpin type using subsize ITER conductors are cooled by a circulation of supercritical helium in a temperature range from 5 to 15 K and tested at a maximum current up to 10 kA. Two different helium bath cooled magnets allow to apply DC or AC transverse magnetic field up to 3.5 T or longitudinal magnetic field up to 7.5 T. A sliding system with a 240 mm stroke on the sample cryostat allows to test separately in the same sample either the conductor or the joint performances. The paper reports on how, through the conductor and joint development tasks, the facility performances were successfully increased and tested. The ITER TFMC joints using Nb/sub 3/Sn conductors were first developed on this facility. The last developments, performed on the ITER PF NbTi conductors and joints proved this facility to be a versatile and useful tool for superconducting magnet developments and showed the interest of possible upgrading to finalize conductor design.

Recent results from R&D on superconductors at CRPP

Prototype conductors for the ITER magnets have been tested extensively in the SULTAN facility at CRPP in the last decade. The in-depth characterization of the high current carrying conductors provided a valuable feedback in the design, with performance optimization and cost reduction. From the transient stability results on full size, short length conductor samples, the temperature margin required to withstand the plasma disruption has been found to be much smaller than originally assumed. A comparison between two specially designed sub-size conductors (the SeCRETS experiment) showed that the copper fraction in the superconducting Nb3Sn composite can be reduced without affecting the stability, leading to a substantial reduction of the overall amount of superconducting composite to be procured for the ITER magnets. The ac loss measurements carried out over a broad range of ac field frequency, brought evidence of two regimes of losses, complementing the test results of the ITER model coils and indicating that the correct eddy currents loss to be retained in the design for plasma disruption and initiation is much smaller than the value extrapolated from the slow charge of the model coils. The impact of the coolant speed on the stability behaviour of NbTi and Nb3Sn cable-in-conduit has been observed both in transient field and dc experiments, leading to the conclusion that the assumption of a constant value for the heat exchange coefficient is inadequate in the design criteria.

DC behaviour of NbTi cable-in-conduit conductors for ITER-FEAT

Using the SULTAN test facility the d.c. behaviour of three subsize NbTi cable-in-conduit (CIC) conductors has been studied. The d.c. performance of the cables is compared with the critical currents and the current sharing temperatures expected from the strand data. As a basis for this comparison the critical current of the NbTi strands has been measured in the temperature range of 4.2–6.5 K in magnetic fields of up to 7.5 T. The d.c. performance of the NbTi subsize conductors has been found to be not significantly affected by cyclic loading. Taking into consideration the self field of the conductor the cable critical current and the current sharing temperature are in agreement with the strand data.

A high resolution magnetization sensor for AC loss measurement on cable-in-conduit conductors

A special sensor is proposed for magnetization measurement of superconductors with round cross section under pulsed transversal field. The sensor consists of a saddle shaped pick-up coil attached to the cylindrical superconducting sample, with a controlled geometrical distribution of the turns. The pick up coil carries out a two-dimensional flux integration and the software calculates the loop area in Joules. The sensor has been used during a campaign of ITER subsize conductor tests, showing an excellent agreement (better than 1%) with the results of boil-off calorimetric measurements. The detailed design, the algorithm and the test results are discussed in the paper. Other applications of this test method are proposed.

Experimental investigation of AC losses and effective time constant in the high current cable

The report is dedicated to the results of AC-loss measurements in the ITER (International Thermonuclear Experimental Reactor) Nb 3Sn strand and ITER design subsize cable samples. The test facility described below includes the set of superconducting magnet systems furnished with boil-off calorimetric inserts and calorimetric schemes including gas-flow meters. The calorimetric method and the magnetization loop recording method were used simultaneously for the AC-loss measurements. The coincidence between the results of both measuring techniques was confirmed. The original calorimetric scheme makes it possible to measure the released energy during one pulse of the magnet field. The results of AC loss measurements in ITER subsize conductor samples manufactured in Russia are given as an example.

Coupling currents losses bench mark test of ITER subsize conductor

To solve the discrepancies frequently observed between coupling currents losses results measured on similar samples at different labs, a bench merit test action has been carried out among five laboratories of the four partners of the ITER project. The specimen for the bench mark action is a short sections of Nb/sub 3/Sn cable-in-conduit conductor with the same layout as the last but one stage of the ITER conductor. The five labs measure the AC losses by boil-off calorimetry. The test programme, selected according to the capabilities of the test equipment, allows a direct comparison of the test results over a broad range of field pulses.

Effect of non-uniform current distribution on the stability in Nb-Ti cable-in-conduit superconducting conductor

30kA-NbTi Demo Poloidal Coil (DPC-U) exhibited instability such as the conductor quenches at 40% of the rated current which is still expected conductor critical current, instability was caused by the non-uniform current distribution in the DPC-U conductor whose strands were insulated from each other. To investigate the instability of the DPC-U conductor, a stability experiment of a subsize conductor consisting of 27 strands was performed and the effect of the current imbalance on the stability was investigated. The current imbalance was forcibly established in the conductor using two current sources in this experiment. The experimental results indicate that the stability margin decreases as the current imbalance becomes larger and that the stability margin of the conductor is governed by the stability of the strand with the highest current in the conductor. Also, it is confirmed from the experimental results that the instability of DPC-U has to be attributed to the current imbalance in the conductor.

A review of conductor development for large fusion magnets

This paper is a survey of the development progress towards high-field, high-current superconducting conductors suitable for the next experimental fusion machines. The reference is the International Thermonuclear Experimental Reactor, ITER a world-wide project presently in the Engineering Design Activity Phase. The main conclusions are that the available Nb/sub 3/Sn strand is not far from the ITER specifications, the feasibility of the cabling procedure has been demonstrated, suitable jacket materials have been identified, and the pull-through draw-down jacketing technique has been successfully applied to significant lengths of subsize conductor. R&D-activity is required to upgrade the background knowledge to the level necessary for the fabrication of the long-length, full-size ITER conductors. Qualification of welding procedures for Incoloy 908 is required. Further investigation has to be made on the effect of chromium plating on the RRR of the strands and on the transverse resistivity of the cable vs void fraction and/or transverse loads. >

Performance of a 'react and wind' 12 T KfK-NET-TF subsize conductor under static and cyclic axial strain

A forced flow Nb/sub 3/Sn subsize conductor for the 12-T Next European Torus (NET) toroidal field coils fabricated by the react-and-wind process was developed. It consists of a flat core housing the Nb/sub 3/Sn cable soldered between two stabilizing copper units which are surrounded by a stainless steel jacket. The jacket was tightly drawn onto the already reacted cable. Critical current, I/sub c/, vs. both magnetic field, B, and axial strain, in /sub a/, measurements show that this delicate procedure has not adversely affected the I/sub c/ vs. (B, in /sub a/) characteristic. At B=12 T the I/sub c/ vs. in /sub a/ curve exhibits a maximum of I/sub c/=5.25 kA at a prestrain of in /sub a/=0.36% which degrades to I/sub c/=3.93 kA at in /sub a/=0. This behavior is not influenced by 10/sup 3/ strain cycles between 0.1 and 0.36% strain. The results have demonstrated both the mechanical integrity and the predicted properties of react-and-wind conductors.

Conductor Design for NET Toroidal Field Coils

The paper describes the results of studies for a NET toroidal field coil conductor carried out at KfK-Karlsruhe. The conductor concept is based on the same design principles as used in the Euratom-LCT coil, well proven in all conductor tests and the domestic tests of the coil. These principles are applied to the peculiarities of Nb/sub 3/Sn for a rated current of 20 kA at 12 T, taking into account ac losses and nuclear heating. A flat Nb/sub 3/Sn cable is soldered to a surrounding CuNi tape after reaction. Around this rectangular conductor core, Cu profiles are cabled on distance by the Roebel-process and subsequently soldered onto the CuNi tape. The whole system is surrounded by a steel conduit. The conductor data result from electric, thermohydraulic and stability calculations as well as mechanical evaluations. Expected fabrication processes are discussed, and measurements on a first simplified subsize conductor model are presented.