Cable-in-conduit Conductor Research Articles

Thermohydraulic analysis of Tore Supra/WEST TF coil quench: Associated smooth quench occurrence in tokamak

The Toroidal Field (TF) system of the Tore Supra/WEST tokamak comprises 18 NbTi superconducting coils, cooled by a static superfluid helium bath at 1.8 K and carrying a nominal current of 1255 A. The 19th December 2017, at the end of plasma run #52205, a quench of TFC-09 was detected first on a secondary thermohydraulical signal (helium liquid level) and triggered the current Fast Safety Discharge (FSD). A numerical model of this quench has been developed with SuperMagnet (CryoSoft). The whole TFC-09 circular coil is modelled by THEA as a single large Cable-In-Conduit Conductor (CICC) with 2028 large rectangular strands (monolithic conductors of length equal to coil average perimeter). The external quench helium relief circuit (cold and warm safety valves, rupture disk and magnetic valve with corresponding pressure set) is modelled by FLOWER.The helium pressure in the coil, upstream of the cold safety valve (maximum value of 9 bar), and the expulsed helium temperature have been used as comparison between measurements and calculations which depend on the Minimum Quench Energy (MQE) used in THEA with small initial heat deposition length (few tens of centimeters) at low field region (external leg). This energy, in the order of few kJ, is compared to the real shape and energy of neutron and gamma flux caused by highly energetic runaway electrons colliding the outboard plasma facing components and which induced the quench. The expulsed helium mass flow rate of nearly 6 kg/s during 5 s has also been calculated. This event confirms on one hand the criticality and the possible occurrence of a so called “smooth quench” caused by small initial heat deposition length and at low field region and on the other hand the important interest of secondary quench detection which can be useful for other tokamak magnets safe operation and protection.

A Pressure Drop Model for Helium Flow in CIC Conductors Based on Porous Media Analogy

Pressure drop of cable-in-conduit conductors (CICCs) cooled by a flow of liquid or supercritical helium is one of the key parameters for the design of large superconducting magnet systems, which determines the heat removal capability and the thermal stability. In this article, an analytical model for predicting the pressure drop in CICCs is developed based on an analogy to porous media with the equivalent geometry configuration. This new prediction model indicates that the pressure drop in CICCs is affected by the geometry parameters of cable and the physical properties of the liquid helium, such as void fraction, tortuosity, pore-throat ratio, hydraulic diameter, density, and viscosity of the liquid helium. To verify the validity of the pressure drop model proposed in this article, the predictions are compared with the experimental data, the results show that the predictions are in good agreement with the experimental data. Then, an analytical expression for predicting the friction factor is derived, the obtained expression is in good agreement with experimental data and is superior to the Katheder correlation at large Reynolds number. In addition, the effects of the major geometric parameters on pressure drop are analyzed, the results explain why the Katheder correlation can be used to predict the pressure drop of CICCs.

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.

The Effect of Specific Manufacturing Characteristics on PF ITER Full-Size Joint Performance

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

Analytical Modelling of CICCs Coupling Losses: Broad Investigation of Two-Stage Model

In spite of their complex geometry, cable in conduit conductors (CICCs) have to be modelled with a rather simple description for coupling losses when operating in transient regime. Difficulties to predict ac losses in superconducting cable have already been shown in previous models such as the new analytical one developed at CEA named COLISEUM (after Coupling Losses analytical Staged cables Unified Model) and CEA heuristic one multi partial shielding (MPAS). In this paper, we present a parametric analysis for coupling losses in superconducting strands and cables subjected to time-varying transverse magnetic field by using the recently developed COLISEUM. This analysis aims at understanding trends of the model in a broad domain of investigation and their associated limits of application. We show that for a wide range of parameters, it is possible to reduce this model of four time constants to a smaller subset. This reduction brings simplifications to the current COLISEUM and enables it to be consistent with the number of time constants considered in the MPAS model from CEA.

Test Facility for Critical Current Measurement of Superconducting Conductors

High-current superconducting conductors are widely used in fusion magnets, high-energy accelerators, and superconducting power transmission. In order to carry out experimental research on new high-current conductors such as Bi 2 Sr 2 CaCu 2 O x (Bi-2212) Cable-in-Conduit conductor (CICC), YBa 2 Cu 3 O 7-x (YBCO)-twisted stacked-tape cable (TSTC) conductor, and Rutherford cable, we have developed a new test facility for their critical current measurement. The key component of the facility is a 6-T dipole magnet with a useful bore of 80 mm, which is connected through a pair of 6-kA binary-current leads to the external dc power supply. The homogeneous high field length is 400 mm with uniformity better than 98%. Instead of the traditional expensive dc power supply and complicated current leads, a superconducting transformer is used to supply the test sample with current up to 30 kA. Quench detection and protection system are also used for both the primary coil of superconducting transformer and the 6-T dipole magnet. An NbTi Rutherford cable with ten strands has been tested to verify the test facility. The status of our test facility and the test results are described in this paper.

Cable-in-Conduit Dipoles for the Ion Ring of JLEIC

The electron-ion collider JLEIC requires 3-T arc dipoles with large aperture and homogeneous field suitable for long beam life for all field levels from injection to maximum collision energy. A superferric dipole has been designed using a novel NbTi cable-in-conduit (CIC) conductor and a flux plate to suppress persistent-current multipoles. Fabrication and tests of the 13-kA CIC conductor are presented. A 1.2-m model dipole is under construction, with completion expected in early 2019. Details of the coil technology and magnet fabrication will be presented. A closely similar 6-T dipole has been designed, capable of doubling the ion energy for JLEIC. It utilizes a 2-layer 23 kA CIC cable using the same wire as the single-layer CIC. Fabrication and cable technology for the 2-layer CIC are presented.

Quench Simulation of REBCO Cable-in-Conduit Conductor With Twisted Stacked-Tape Cable

Quench protection is a critical issue for magnet using high-temperature superconducting (HTS) materials such as REBCO. For cable-in-conduit conductor (CICC) with twisted REBCO tapes, which is considered for high-current HTS conductor for fusion magnet, the anisotropic critical current density (Jc) of tape and complex conductor layout will further complicate the quench behavior. The CryoSoft code THEA is used to simulate the quench behavior of various subscale CICCs in view of a quench experiment in SULTAN facility. The CICCs are composed of twisted tape stack assembled into copper shell (strand). It has been found that neglecting the anisotropy of Jc in quench simulation will significantly underestimate the hot spot temperature by about 40 K. Therefore, it could leave potential risk to damage the magnet. It is also shown that the thermal contact resistance between strand and the steel jacket (the conduit) plays a very important role on quench behavior. The hot spot temperature can be reduced by more than 100 K if the thermal resistance is reduced by two order of magnitude. On the other hand, variations of interstrand electrical and/or thermal resistance have only very little influence (few kelvin).

Design and analysis of a high power supply for the research system of superconducting magnets

To test superconductors with high performance such as Nb3Sn Cable-in-Conduit Conductor (CICC) and superconducting magnets from small scale to large scale, the background magnetic field with high homogeneity is necessary. As a major national science and technology infrastructure project in china, the peak magnetic field of the Research System of Superconducting Magnets (RSSMs) can be up to 15 T and the maximum change rate of magnetic field is 1.5 T/s. To supply electrical power for the RSSMs including the background field superconducting magnet and the tested objects in the meanwhile, a direct current (DC) high power supply rated at 360 MW was designed, the rated current, ripple current and rated voltage are 240 kA, 1 kA and 1.5 kV respectively. Additionally, the maximum output pulse current, the ramp up speed and the pulse width in pulse mode of the DC high power supply are 1000 kA, 5000 kA/s and 100 ms respectively. As a result, the alternating current (AC) power supply system was estimated from the view point of available power. To reduce the ripple of the DC high power supply, the topology of eight power blocks' parallel connection with the interleaved control method is put forward. The simulating results of circulating current and pulse operation indicate the design proposal is an alternative option.

Calculation method for pulsed magnetic field energy supplied to Nb3Sn ITER CS conductors during SULTAN stability tests

Cable-In-Conduit Conductors (CICCs) for the ITER Central Solenoid (CS) magnets are designed to operate in the presence of fast changing current and magnetic field during the plasma-operating scenario. For ITER, the AC loss of several types of Nb3Sn CICCs was experimentally tested, but only very limited experimental data is available for quantitative analysis of the minimum quench energy (MQE). In the SULTAN testing facility (Swiss Plasma Centre) few CS conductors were tested on MQE, but the magnetic field amplitude and ramp rate settings are far from the actual ITER operating conditions. Nevertheless, such tests are needed as a basis to calibrate and benchmark the codes that describe the quench behavior. Moreover, during the stability tests in Sultan, the temperature measurements show severe fluctuations, which can introduce a large error for the energy calculation. An interpretation is given for the temperature fluctuation and a procedure is proposed to significantly reduce the error in the pulsed energy calculation.

Electromagnetic and thermal stability of the ITER Central Solenoid during a 15 MA plasma scenario

The combination of the electromagnetic conductor model JackPot ACDC with the thermo-hydraulic model THEA is able to reproduce and predict the behavior of cable-in-conduit conductors (CICCs) under any varying current and magnetic field. The combined JackPot-THEA model is used to simulate the turns of the CSU2 quadra-pacake of the ITER Central Solenoid (CS) that are subjected to the most demanding conditions of the 15 MA plasma scenario. The considered section is a pancake with a 150 m length of Nb3Sn CICC from helium inlet to helium outlet, from the inner towards outer turns. The simulation results in terms of temperature and local strand electric field levels are compared to those obtained from earlier minimum quench energy analysis in order to evaluate possible risk of quench. The results confirm sufficient stability of the ITER CS coil, both from electrical and thermal point of view. The temperature evolution and flow of a helium heat slug along the conductor during the 15 MA plasma scenario is also analyzed. It appeared that sequential multiple burn cycles can be carried out without accumulation of temperature in the helium heat slug, essential for continuous energy production for a fusion power plant.

Mechanical Modeling and First Case Study on ITER TF CICC Loading Cases With Upgraded Finite Element Code Simulations

The strands of the Cable-In-Conduit Conductors (CICCs) for the ITER magnet system TF coils are made of Nb3Sn, which is a brittle and strain-sensitive superconducting material. The strain distribution in the filaments is driven by the cool-down process and the electromagnetic forces acting on the strands in operation. The impact of electromagnetic and thermal warm-up-cool-down cycles on the conductor performance degradation has been investigated experimentally both on short samples in the SULTAN facility and in the solenoidal configuration of the TF Insert tests in the CSMC facility. A deeper understanding on how cyclic loading of the conductor affects its electrical performance could allow one to identify the best working conditions so as to limit the degradation. In this paper, a mechanical model of one last sub-cable (petal) of the TF CICC is presented. The strain distribution in the strands is simulated via a new version of the MULTIFIL finite element code. The effects of the alternate application of thermal and electromagnetics cyclic loading are tentatively analyzed, similarly to experiments performed in the SULTAN facility in the tests of the TFIO1 conductor sample. An upgrade of the model is implemented, concerning the thermal cycle simulation.

Thermohydraulic behaviour of the cryogenic system during protected quenching of the superconducting magnet of a hybrid magnet

The superconducting outsert of the hybrid magnet at CHMFL is cooled by 4.5 K supercritical helium contained in CICC (cable-in-conduit conductors), which must be capable of extracting any heat load (external and internal) during ramping and high field operating. In the event that the heat loads exceed the cooling capacity of the supercritical helium, superconducting transitions on CICC may happen and a quench protection will be triggered by the control system. The process will cause high heat input in a very short time to the supercritical helium, and consequently, large temperature and pressure fluctuations will occur in the related cryogenic system. In order to test the performance of the cryogenic system during the process of superconducting magnet quenching, quenching experiments with design operating currents of the superconducting magnet were carried out. The testing results shows that the helium cryogenic system can keep a stable operation in the worst protected quenching of the superconducting magnet, and also return to the normal situation automatically.

Design and test of the 4.5 K supercritical Helium coiled subcooler

Helium forced flow cooling was demonstrated as a reliable way for superconducting (SC) magnets. For the large Cable-in-Conduit Conductors (CICC) in high magnetic field, supercritical helium forced flow cooling is utilized. A coiled sub-cooler for 4.5 K supercritical helium, which is the vital component of the system, has been designed and manufactured to supply required helium for the SC magnet. The coiled sub-cooler installed in the valve box of the SC magnet has been tested during the experiment process of SC magnet. In this paper, the design details and the heat transfer performance of the coiled sub-cooler are presented. The analysis of test results reflects the design principles for the coiled sub-coolers.

Study on the effective Young’s moduli of CICC strand with multi-stage structures

A bottom-up geometrical model is established to investigate the mechanical properties of the cable-in-conduit conductors (CICC) superconducting cables for ITER (International Thermonuclear Experimental Reactor). A compact method is formulated to develop the uncompact problems during the construction of the third-stage and fourth-stage strands. Based on the homogenization theory and Costello’s wire rope theory, an analytical model is provided to predict the effective Young’s moduli of the hierarchical superconducting cable. The finite element calculations are performed to analyze the same situation. The numerical results agree well with the results from the analytical model. It is found that the effective Young’s moduli are a function of winding laps for various stages of the strands. This work helps to understand the mechanical properties of the hierarchical superconducting cable and also provides optimal design for higher stage strand.

Electromagnetic Design and Challenges for 14 T High-Tc Toroidal Field Superconducting Magnet for CFETR

The design work of the China Fusion Engineering Test Reactor (CFETR) was started in 2012. Related concept design work for its superconducting magnet system was carried out in the past years. In order to get higher plasma operation parameters, the system was optimized with the major radius (R) of 7.2 m, the minor radius (r) of 2.2 m, and the plasma center magnetic field ($B_{t}$) of 6.5 T. With these higher parameters, the fusion power may exceed 1 GW and the maximum magnetic field of the toroidal field superconducting magnet system will reach to 14.3 T. Due to the fact that the performance of the Nb3Sn strand used for ITER project is very difficult to be applied for such a high field of CFETR, new cable-in-conduit conductors (CICC) structure and conductor design work become the key point of CFETR. This paper illustrates the design and electronic magnetic analysis work of the toroidal field (TF) coils of CFETR. In addition, such high operation current and magnetic field bring to huge electromagnetic force, which will cause lots of challenges for the design work of the TF coil such as the stability margin, mechanical safety, and so on. It is not easy to solve all the issues for such a big superconducting coil for the fusion device in a short time, but we hope the analysis results and database can be good references for the next step R&D work.

Development of cable-in-conduit conductor for ITER CS in Japan

The National Institutes for Quantum and Radiological Science and Technology has the responsibility to develop a cable-in-conduit conductor (CICC) for the ITER central solenoid (CS). Qualification tests of CICCs fabricated in the initial development stage were carried out at the SULTAN test facility; the superconducting performance (Tcs = current-sharing temperature) was found to be degraded by the repeated cyclic loading that simulates realistic ITER operating conditions. From destructive examination and neutron diffraction tests, this degradation appears to result from bending strain on the strands generated by electromagnetic forces. In response, the cabling of the CICC was optimized by shortening the twist pitch to make it stiffer against electromagnetic forces. No Tcs degradation of the optimized CICC was seen in the subsequent SULTAN test; further, a CS insert (CSI) test was performed at the CS model coil test facility, which included hoop strain for a more realistic simulation of ITER conditions. Good performance was also achieved in the CSI test.

Multi-scale 3D modelling of a DEMO prototype cable from strand to full-size conductor based on X-ray tomography and image analysis

The design of a superconducting magnet system of a fusion reactor candidate is usually based on the Cable in Conduit Conductor (CICC) concept. CICC consist of complex structures with several hundreds of highly packed, multistage twisted superconductor and copper strands, cooling structures – all wrapped in thin steel foils and jacketed in relatively thick stainless-steel pipes. As recently demonstrated, such complex morphology can be captured by fully 3D X-ray tomography. One of the three alternative winding pack (WP) options for the toroidal field coils of the European DEMO superconducting magnet system as proposed by the Italian ENEA, features a layer-wound WP design adopting a wind-and-react conductor with high aspect ratio rectangular cross section. A multi-physics modelling requiring high accuracy from the strand to the full-size conductor scale was tested on two X-ray tomography systems. For the microstructural integrity analysis of the internal tin one mm diameter Nb3Sn strand, manufactured by Western Superconducting Technologies, a microtomograph with ˜1 μm feature recognition was used. For the full-size conductor (66 x 25 mm2 internal dimensions) a high energy (> 300 kVp) microtomograph with voxel resolution of ˜20 μm was employed. Voids and/or geometrical imperfections were detected, accurately pin-pointed and locally investigated at high resolution before and after heat treatment. Dimensional measurements on the 3D internal structure of the strand showed the small dimensional changes of the strand after the heat treatment. A significant result was the determination of the twist-pitch factor in a purely non-invasive way. The 3D reconstructions of the CICC cables were used to verify the cabling and compaction processes as well as the structural integrity of the strands, wrapping foils and spiral cooling structures. Also, 3D reconstruction of strand trajectories provided a strong input for a multiphysical modelling/interpretation of mechanical–electrical properties of CICC in cryogenic–electromagnetic environments.

Post-mortem analysis of ITER CS helium inlets fatigue tested at cryogenic temperature

In the ITER Magnet System, ten thousand tonnes of superconducting Cable In Conduit Conductor (CICC) are cooled down by a forced flow of supercritical helium, which is supplied from helium inlets. For the ITER Central Solenoid (CS), consisting of six independent pancake wound modules, the He inlets consist of three overlapping holes covered by an oblong shaped boss, welded to the CS jacket through full penetration, multi-pass Tungsten Inert Gas (TIG) welding. Since they are located in a region of high cyclic tensile stresses, i.e. first turn at the inner diameter of the pancake, the CS inlets are one of the most critical structural components. Qualification of the design is done by analysis and a comprehensive design optimization has been performed by finite element (FE) simulations. In order to guarantee the required fatigue life at cryogenic temperature of these component, a post – welding process consisting in ultrasonic shot – peening is required. Based on a qualified weld procedure, six mock – ups including each two He – inlets on the opposite surfaces have been produced to run a mechanical fatigue testing program at cryogenic temperature to validate the findings of the FE simulations. Five were peened and one not peened. The paper describes the results of a comprehensive post – mortem failure analysis which includes non – destructive (penetrant testing, leak testing, X-ray computed tomography) as well as destructive examinations (microoptical and hardness tests, scanning electron microscopy). The initiation site, propagation of the crack as well as the tensile overload region have been identified and studied. An estimation of the cycles from initiation to failure based on the width of the fatigue striations was performed. The paper also includes a full assessment of the welds according to the most stringent acceptance levels of the standards in force.

The Effect of Strain on the Transport Properties of Superconducting Strand and Cable in a Conduit Conductor

A proper understanding of the degradation of the transport properties of cable-in-conduit conductors (CICCs) due to a changing strain distribution or to crack formation in the filaments is essential to determine the operational limits of the conductors and to optimize their design. Based on the electrical and strain properties of the superconducting strand, the performance of short samples of the CICC can be analyzed, such as the ones tested in the SULTAN facility, or the full-size CICC used in real magnets. Mathematical fitting expressions are proposed to implement strand properties into the cable model. In combination with the strain maps generated by the mechanical model MULTIFIL, these expressions are introduced in the electromagnetic code JackPot to predict the current sharing temperature of the CICC of the international thermonuclear experimental reactor (ITER) Central Solenoid. A comparison is made with SULTAN short samples tests as well.