Controlling for outer-diameter of superconducting cable for PF Cable-In-Conduit Conductor of ITER

Poloidal Field (PF) coils, made of CICC (Cable-In-Conduit Conductors), are the most important part of ITER (International Thermonuclear Experimental Reactor) superconducting magnet system. The structure of superconducting magnet system in ITER and the CICC are introduced. The main factors influenced the cabling of the superconducting cable for CICC is discussed. Analyzed the outer diameter controlling technique and the relationship between ac loss and structure parameters of the superconducting cable. The technique route of cabling has been established. Especially, the technique of dies setting for shaping + multi-rolling compacting dies + half-dies for diameter keeping is explained in detail, which can efficiently control the diameter and the cabling quality of the superconducting cable, without injury to strands line. The technical parameters for cabling is fixed and one 765 m long dummy cable and one 115 m long superconducting cable of CICC for PF coil is manufactured successfully for ITER the last.     

CFD model of ITER CICC. Part VI: Heat and mass transfer between cable region and central channel

Dual-channel cable-in-conduit conductors (CICC) are used in the superconducting magnets for the International Thermonuclear Experimental Reactor (ITER). As the CICC axial/transverse size ratio is typically ∼1000, 1D axial models are customarily used for the CICC, but they require constitutive relations for the transverse fluxes. A novel approach, based on Computational Fluid Dynamics (CFD), was recently proposed by these authors to understand the complex transverse thermal-hydraulic processes in an ITER CICC from first principles. Multidimensional (2D, 3D) Reynolds-Averaged Navier-Stokes models implemented in the commercial CFD code FLUENT were validated against compact heat exchanger and ITER-relevant experimental data, and applied to compute the friction factor and the heat transfer coefficient in fully turbulent spiral rib-roughened pipes, mimicking the central channel of an ITER CICC. That analysis is extended here to the problem of heat and mass transfer through the perforated spiral separating the central channel from the cable bundle region, by combining the previously developed central channel model with a porous medium model for the cable region. The resulting 2D model is used to analyze several key features of the transport processes occurring between the two regions including the relation between transverse mass transfer and transverse pressure drop, the influence of transverse mass transfer on axial pressure drop, and the heat transfer coefficient between central channel and annular cable bundle region.     

Design,development and fabrication of indigenous 30 kA NbTi CICC for fusion relevant superconducting magnet

The fusion relevant superconducting magnet is under development in India using a cable-in-conduit-conductor (CICC) with operating current of 30 kA at 5.5 T and 4.5 K. The 30 kA NbTi based CICC is designed on the basis of desired critical design parameters as well as mechanical fabrication considerations. The 30 kA CICC has been designed having square cross-section (30 mm × 30 mm) consisting NbTi as superconducting cable, SS316LN as jacket material and SS304 foil as wrapping around the cabled strands. The design configuration of 30 kA NbTi CICC has been discussed in this paper. The NbTi base high current carrying strands have been fabricated indigenously using direct extrusion and cold drawing process. The 100 m long NbTi–Cu strands twisting, insertion of cabled strands into a circular conduit has been developed with pull through technology. The welding process qualification and effects of cold work on jacket material at room temperature have been elaborated in this paper. The manufacturing parameters and quality procedures for development of CICC have been successfully established and demonstrated with fabrication of 100 m NbTi based CICC without any technical difficulties.     

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 1.2 m 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 300–500 W m⁻² K⁻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.     

Modelization of the thermal coupling between the ITER TF coil conductor and the structure cooling circuit

The ITER Toroidal Field (TF) coils are required not to quench during the most demanding event: a plasma disruption followed by a fast discharge of the Central Solenoid (CS), the Poloidal Field (PF) coils and the Correction Coils (CC). This event creates large heat deposition in the ITER magnet stainless steel structures in addition to the conductor AC losses. In order to prevent quench occurring in the TF conductor, cooling channels, implemented in the TF coil structure (TFCS), have to remove a large fraction of the heat deposited. The first integrated TF and structure mock-up has been manufactured and then tested in the HELIOS cryogenic test facility (CEA Grenoble) to determine the thermal coupling between the TFCS and the TF conductor, both actively cooled by supercritical helium at 4.4 K and 5 bar. It consists in a stainless steel casing, a cooling pipe glued with resin in the casing groove, winding pack (WP) ground insulation, a radial plate and a copper dummy cable-in-conduit-conductor (CICC). Steady state as well as transient thermal characterizations have been completed in May 2015. Simulation results by thermal hydraulic codes (VENECIA/SuperMagnet) and some of the experimental data are presented and discussed. The thermal coupling between the helium in the cooling tube and the TF coil structure is then modelled as an equivalent heat transfer coefficient in order to simplify the thermal hydraulic (TH) models. Comparison between simplified coupling and detailed coupling is presented.     

Design and optimization of the 12.5 T EFDA dipole magnet

The aim of this paper is to provide an overview of a recent study carried out—within the framework of the European Fusion Program—to design a 12.5 T superconducting dipole. By focusing on the CICC based design option, the overall design procedure is presented. In particular, the 2D optimization of the dipole cross section is described including the magneto-static analysis of the winding and iron yoke, the mechanical analysis of the conductor jacket, insulation and outer cylinder, the conductor hot spot analysis, etc. As far as the thermo-hydraulic design is concerned, simulations of nominal as well as offset operating conditions (e.g., magnet quench) are presented with emphasis on their role played in the overall magnet design. For example, diagrams reporting the helium heat removal capabilities, pressure drop, mass flow, etc. are shown and their usefulness as guidance for the magnet designer described.     

Status of Nb3Sn strand development in Korea

Although there were many research activities for the development of superconducting Nb₃Sn strands, the major one started under KSTAR (Korea Superconducting Tokamak Advanced Research) project in 1996. After the success of a large scale production test of Nb₃Sn strand using the internal tin route, a new mass production facility is under operation since 2004.KAT (Kiswire Advanced Technology Ltd.), an affiliate of Kiswire Ltd., manufactured various types of Nb₃Sn strands using the internal tin process optimized for fusion magnets. For the Nb₃Sn strand of the KSTAR PF coil, each module has ～190 niobium filaments and 19 modules are restacked for the strand production. For the ITER TF strand, there are two types of basic design. One of them has 37and 19 modules with 169–219 niobium filaments in each module. The other has 19 modules with 164–190 niobium filaments in each module. Both the designs satisfy the requirements for ITER TF strand with enough margins. The characterization of the strands is performed by hysteresis loss measurement, RRR (Residual Resistivity Ratio), n-value, and critical current density measurement vs. temperature, magnetic field, and strain. The critical current density of the strands reached around 1100 A/mm² at 12 T and 4.2 K. A well defined quality assurance program helped to produce a high quality strand with a piece length of more than 15 km. KAT has been provided Nb₃Sn strand for KSTAR PF Coil and ready to produce the Nb₃Sn strand for ITER TF coil.In this paper, the design concept, the fabrication procedure and the result of the strand performance test are discussed.     

Thermo-hydraulic analyses associated with a CEA design proposal for a DEMO TF conductor

The future DEMO Toroidal Field (TF) magnets are likely to feature cable-in-conduit conductors (CICC) cooled by forced flow of supercritical helium. Design activities were carried out at CEA to provide a winding pack compatible with DEMO plant requirements. The CEA proposal comprises, for each of the 16 D-shaped windings, 10 double-pancakes (2 × 392 m long) wound in 10 turns. The conductor is a square-shaped Nb₃Sn double channel conductor with a central spiral, carrying a nominal current of 95.5 kA. We present a thermo-hydraulic analyses focused on the central, most critical pancake, where the maximum field is reached, aiming at evaluating the integrity of the proposed conductor design. Both normal and off-normal simulations were performed using detailed electromagnetic and neutron heating load maps as input, and evaluating operational quantities such as the temperature margin in burn conditions, and the hot spot temperature in quench conditions. We assessed the sensitivity of these quantities to some driving parameters, notably mass flow rate and the choice of friction factor correlation for the temperature margin, and quench initiation features for the hot spot temperature. Furthermore, the influence of the casing cooling on the temperature margin is analyzed. The study is carried out using two thermohydraulic models.     

Mechanical test on the ITER TF jacket

This paper focuses on mechanical tests on the ITER TF jacket 316LN stainless steel material. During manufacture the conductor will be compacted, spooled and aged at approximately 650 °C after cable insertion. Therefore, sample jackets were prepared under compaction, stretching and annealing in order to simulate the manufacturing process of TF coils. The mechanical properties of the given material were measured at 4.2 K and 300 K. Young’s modulus, yield strength (0.2% offset) and elongation are reported. SEM images showed that the TF jacket had more pronounced ductile behavior. The elongation at failure is more than 30% at cryogenic temperature, showing that the TF jacket has high mechanical performance. It is concluded that the results complies with ITER requirements.     

Design research on the conductor of 10 kA class HTS DC power cable

High temperature superconducting (HTS) DC power cable shows a wide application prospect in the field of power transmission for its nearly lossless and rather high capacity. A 360 m/10 kA HTS DC power cable system, which connects the rectifier output of a substation with the bus bar of an electrolytic aluminium cell, will be put into operation at Henan Zhongfu Industrial Co., Ltd. As one of the items in this project, a 5 m/10 kA HTS DC power cable was developed, which is used to investigate the conductor design, fabrication, current-carrying capacity and stability of the 360 m/10 kA HTS power cable.The HTS DC power cable core consists of five conductor layers wound with spliced Bi-2223 wires with the length of 600 m. The cable core has five layers and 23 conductors in each layer with the outer diameter of 45.42 mm. The superconducting power cable is fabricated and tested. The critical current is about 14.3 kA at 77 K. The superconducting power cable is charged to 10 kA with rate of 10 A/s and operates at steady-state for 30 min.In this paper, the 10 kA HTS DC power cable design, fabrication and test are presented. The experimental research of the performance of spliced superconducting wire and charging, steady-state operating performance of the cable was carried out.     

Experimental study on a Nb3Al insert coil under high magnetic field

Nb₃Al is one of the most promising superconductors to replace Nb₃Sn in large scale, high field superconducting magnet. Since the complicated conductor manufacturing process, long and stable Nb₃Al conductor is difficult to acquire in a commercial scale. Based on a 70 m length of Nb–Al precursor conductor, we designed and fabricated a Nb₃Al coil. The coil winding, low temperature diffusion heat treatment and epoxy impregnation are described in detail. The finished Nb₃Al coil is tested as an insert in a background magnet. The test is performed at the background field from 7 T to 15 T. The test results are analyzed and presented in this paper.     

Low temperature physical properties of a high Mn austenitic steel JK2LB

High manganese austenitic stainless steel JK2LB is developed by the Japan Atomic Energy Agency for applications as a conduit material for superconducting cable-in-conduit conductors for the magnets of international thermonuclear experimental reactor (ITER). The low temperature physical property data of this material are very important to ITER magnet design. Therefore in this paper, our measurements of the physical properties including room temperature Young’s modulus and thermal expansion, magnetization, thermal conductivity, specific heat and resistivity at temperatures from room temperature down to 2 K are reported. We found that JK2LB is antiferromagnetic at low temperatures with a Néel temperature of 240 K. This is consistent with a prediction based on the chemical composition of the austenite stainless steel. The antiferromagnetic phase transition is also evident in the resistivity vs. T curve. Nevertheless, no anomalies are observable in its specific heat and thermal conductivity from 2 K to 300 K. The thermal expansion of this steel between 10 K and 300 K is about 0.22%. Its Young’s modulus, specific heat and thermal conductivity are comparable to that of 316LN stainless steel.     

Change of interstrand contact resistance and coupling loss in various prototype ITER NbTi conductors with transverse loading in the Twente Cryogenic Cable Press up to 40,000 cycles

The multistrand NbTi conductors for the Poloidal Field (PF) Coils of the International Thermonuclear Experimental Reactor (ITER) are subjected to heavy transverse loading due to the Lorentz forces in the coils. The current in the multistage Cable-In-Conduit Conductors (CICC) exceeds 50 kA and the magnetic field reaches up to more than 6 T for a few tens of thousands of pulses. The large transverse forces, accumulating from strand to strand over the cable cross-section, cause a severe deformation of the cable bundle inside the conduit and this goes along with electromagnetic, mechanical, and thermohydraulic effects. In order to study the electromagnetic and mechanical behaviour in more detail, a Cryogenic Cable Press is build to simulate the effect of the Lorentz forces on a conductor comparable to the present design for ITER in magnet operating conditions. The magnetisation of the conductor (and from this the coupling loss expressed in nτ) and the interstrand resistance (R_c) between various strands and strand bundles inside the cable can be measured along the loading history, starting at virgin condition and accordingly subjected to various loads. The results, all obtained on eight full-size ITER type NbTi conductor samples with a variety of cable strand layout and coatings, are reported here.A consistent correlation is found between the experimental AC loss and interstrand contact resistance (R_c) results. It is also observed that there is a strong impact of cyclic loading on the AC loss and R_c which may change up to orders of magnitude. The variation of the AC loss due to transverse cyclic loading of CICC conductors in ITER coils can be accomplished by reducing the void fraction. The results point out that cyclic loading with a significant number of cycles, sufficient to reach a saturation after having passed the peak transverse resistance, should be included in next tests on large NbTi CICC's and PF Model Coils as the AC loss and ability of current sharing among strands will vary along the loading history.     

Nb3Sn material development in Russia

In the USSR and later in Russia, the main activities in technical superconductivity were concentrated in the institutes that belonged to the Ministry of Atomic Energy (Minatom). The development of new technologies shortly transferred to the large-scale industrial production of NbTi and Nb₃Sn superconductors in early 1970s. Two main technologies for multifilamentary Nb₃Sn strands were under investigation during that time – bronze-process and internal tin method. More than 25 ton of Nb₃Sn bronze-processed strands were produced for the fabrication of 90 ton of conductors for application in the magnet system of first in the world fusion facility (tokamak T-15) with magnet system based on the intermetallic compound. The characteristics of these strands and conductors have been briefly described. The requirements for the Nb₃Sn strands constantly increased and the main R&D on the enhancement of critical current density have been reviewed. For bronze-processed strands the increase of the tin content in large ingots was the crucial factor. The artificial doping of niobium filaments by niobium–titanium alloy was invented, which enabled to improve the workability of Nb₃Sn strands, with enhanced critical current density in high fields. For internal tin Nb₃Sn strands the main R&D were concentrated on the optimization of the layouts of the strand and on the multistage heat treatment because of the inevitable liquid phase formation which could result in severe distortion of the geometrical arrangement of the filaments and even in destruction of the whole strand. The main results of these investigations have been presented. The corresponding impact of these R&D on the design of bronze-processed and internal tin strands has been analyzed. The quantitative estimations of the grain size were made for bronze-processed and internal tin strands. It was shown that in bronze-processed and internal tin strands subjected to the standard ITER heat treatment characterized by two stages at 575 °C and 650 °C, the variation of Nb₃Sn grain size in the range of 30–300 nm could be observed. The correlations of microstructure and superconducting properties have been discussed. The ITER connected activities in Russia on the development of Nb₃Sn strands, which met the HP-II specification, have been outlined. The results of the ITER Model Coil Program have shown a degradation of the critical current of large cable-in-conduit conductors (CICC) built with Nb₃Sn strands. For this reason, the investigation on the strain dependence of critical current density in Nb₃Sn strands of different designs is of high interest and priority. The R&D on development of bronze-processed and internal tin Nb₃Sn strands with enhanced, by the nanostructured Cu–Nb material, mechanical strength have been reviewed.     

Electromagnetic analysis of a superconducting transformer for high current characterization of cable in conduit conductors in background magnetic field

A large cable-in-conduit-conductor (CICC) test facility has been designed and fabricated at the High Magnetic Field Laboratory of the Chinese Academy of Sciences (CHMFL) in order to meet the test requirement of the conductors which are applied to the future fusion reactor. The critical component of the test facility is an 80 kA superconducting transformer which consists of a multi-turn primary coil and a minor-turn secondary coil. As the current source of the conductor samples, the electromagnetic performance of the superconducting transformer determines the stability and safety of the test facility. In this paper, the key factors and parameters, which have much impact on the performance of the transformer, are analyzed in detail. The conceptual design and optimizing principles of the transformer are discussed. An Electromagnetic-Circuit coupled model built in ANSYS Multiphysics is successfully used to investigate the electromagnetic characterization of the transformer under the dynamic operation condition.     

Novel three-dimensional cocoon-like hydrogels for soft tissue regeneration

Large three-dimensional hydrogels (> 150 cm³) of bacterial cellulose (BC) were synthesized by using Gluconacetobacter hansenii ATCC 23769 under controlled agitated culture conditions. The macroscopic cocoon-like structures are gelatinous and translucent and may find applications in several areas, particularly in tissue and organ engineering. Internal microstructure was investigated by scanning electron microscopy (SEM) and field emission scanning electron microscopy (FESEM), which revealed that the cocoons are composed of cellulosic nanofibres randomly and three-dimensionally dispersed. The macroscopic bodies are delimited by a dense semi-permeable membrane with thickness between 0.2 and 2 mm, also composed of cellulosic nanofibres. Endothelial cells were seeded on the hydrogels and incubated for 7 days. HUVECs grew and migrated into the inner part of the structure. The three-dimensional BC hydrogel structures can be directly implanted in tissue deficient regions as scaffolds containing the appropriate cultured cells.     

The 4C code for the cryogenic circuit conductor and coil modeling in ITER

A new tool - the 4C code - has been developed, which allows the thermal-hydraulic simulation of the entire superconducting magnet system of the International Thermonuclear Experimental Reactor (ITER), with particular reference to: (1) the winding made of cable-in-conduit conductors (CICC), (2) the structures (the radial plates and the case of the toroidal field - TF - coils, for instance) and (3) the cooling circuits. In this paper the different components of the 4C code (1D 2-channel model of the CICC and of the structure cooling channels, 2D model of selected cross sections of the structures, 0D/1D model of the cryogenic circuit) are described in detail, together with the strategy adopted for the coupling between the different components and their integration in a single tool. The new tool is then applied to the modeling of two transients in an ITER TF coil: a simplified version of a cooldown of the coil and the response to a heat pulse applied in the winding.     

Volume and dislocation diffusion of iron,chromium and cobalt in CVD β-SiC

Impurity tracer diffusion of ⁵⁹Fe, ⁵¹Cr and ⁵⁷Co in CVD β-SiC has been studied in the temperature range between 973 and 1873 K. The temperature dependence of the volume diffusion coefficients of iron and chromium can be expressed by linear Arrhenius equations. The preexponential factor and the activation energy are estimated to be 8.7×10⁻¹⁵ m² s⁻¹ and 111 kJ mol⁻¹ for iron, respectively, and 9.5×10⁻¹⁵ m² s⁻¹ and 81 kJ mol⁻¹ for chromium, respectively. The diffusion coefficients of iron and chromium are much higher than those of the self-diffusion in β-SiC. Furthermore, the activation energies for the diffusion of iron and chromium are about one-tenth of those for carbon and silicon in β-SiC. Therefore, it seems that an interstitial mechanism is predominant for the diffusion of iron and chromium in β-SiC. On the other hand, the diffusion coefficient of cobalt above 1673 K is higher than that of iron, while at lower temperatures it is much lower than that of iron. The difference in the diffusion coefficients at 1173 K is more than three orders of magnitude. Thus, the temperature dependence of the diffusion coefficients of cobalt shows a strongly curved Arrhenius relation. This suggests that cobalt atoms diffuse by an interstitial mechanism at higher temperatures and by a substitutional mechanism at lower temperatures. From the deeper regions of the penetration profiles of iron, chromium and cobalt the dislocation diffusion coefficients of them have been estimated.     

State of the art powder-in-tube niobium–tin superconductors

Powder-in-tube (PIT) processed niobium–tin wires are commercially manufactured for nearly three decades and have demonstrated a combination of very high current density (presently up to 2500 A mm^?2 non-Cu at 12 T and 4.2 K) with fine (35 μm), well separated filaments. We review the developments that have led to the present state of the art PIT niobium–tin wires, discuss the wire manufacturing and A15 formation processes, and describe typical superconducting performance in relation to magnetic field and strain. We further highlight successful applications of PIT wires and conclude with an outlook on possibilities for further improvements in the performance of PIT niobium–tin wires.     

The toucan beak: Structure and mechanical response

The structure and mechanical response of a Toco toucan (Ramphastos toco) beak were established. The beak was found to be a sandwich composite with an exterior of keratin scales (50 μm diameter and 1 μm thickness) and a core composed of fibrous network of closed-cells made of collagen. The tensile strength of the external shell is about 50 MPa. Micro- and nanoindentation hardness measurements corroborate these values. The keratin shell exhibits a strain-rate sensitive response with a transition from slippage of the scales due to release of the organic glue, at a low strain rate (5 × 10^− 5 s^− 1) to fracture of the scales at a higher strain rate (1.5 × 10^− 3 s^− 1). The closed-cell foam consists of fibers having a Young's modulus (measured by nanoindentation) of 12.7 GPa. This is twice as high as the keratin shells, which have E = 6.7 GPa. This is attributed to their higher calcium content. The compressive collapse of the foam was modeled by the Gibson–Ashby constitutive equations.There is a synergistic effect between foam and shell evidenced by a finite-element analysis. The foam stabilizes the deformation of the keratin shell by providing an internal support which increases its buckling load under compressive loading.