Nb3Al Strands Research Articles

Advanced fusion technologies developed for JT-60 superconducting tokamak

Modification of JT-60 as a full superconducting tokamak (JT-60SC) is planned. The objectives of the JT-60SC programme are to establish scientific and technological bases for steady-state operation of high performance plasmas and utilization of reduced-activation materials in an economically and environmentally attractive DEMO reactor. Advanced fusion technologies relevant to the DEMO reactor have been developed for the superconducting magnet technology and plasma facing components of the JT-60SC design. To achieve a high current density in a superconducting strand, Nb3Al strands with a high copper ratio of 4 have been newly developed for the toroidal field coils (TFCs) of JT-60SC. The R&D to demonstrate the applicability of the Nb3Al conductor to TFCs by a react-and-wind technique has been carried out using a full-size Nb3Al conductor. A full-size NbTi conductor with low ac loss using Ni-coated strands has been successfully developed. A forced cooling divertor component with high heat transfer using screw tubes has been developed for the first time. The heat removal performance of the carbon fibre composite target was successfully demonstrated on an electron beam irradiation stand.

Evaluation of strain applied to strands in a 13 T–46 kA Nb3Al cable-in-conduit conductor

A 13 T–46 kA Nb3Al coil, a Nb3Al insert, has been developed to demonstrate the applicability of the react-and-wind technique to toroidal field (TF) coils of a fusion reactor, such as ITER. It is estimated that a conductor is subjected to ≈0.4% bending strain after heat treatment when the react-and-wind method is applied to the TF coil fabrication. Therefore, 0.4% bending strain was artificially applied to the Nb3Al insert conductor after the heat treatment. Since a stainless steel conduit was used in this conductor, the strands are subjected to the compressive strains due to thermal stress. The effective strains applied to the strands are estimated by comparing the critical current test results of the Nb3Al insert at 10–13 T and calculation. Those due to thermal stress and conductor bending were evaluated to be ≈−0.4% and ≈0%, respectively. The effective strain by the conductor bending is sufficiently small so as not to affect the critical current performance of the Nb3Al conductor. In addition, the evaluated strain of the Nb3Al conductor is compared with those of same scale Nb3Sn conductors, in which there is an unexpected strain, which is proportional to the electromagnetic force. Such strain was not observed in the Nb3Al conductor. One of the explanations is higher rigidity of the Nb3Al strand than the Nb3Sn one. This shows that a Nb3Al conductor is suitable for application to large magnets, such as the TF coil of the fusion machine, which experiences large electromagnetic force in the conductor.

Ｎｂ３Ａｌインサート・コイルの臨界電流性能評価

Nb3Al superconductor has the potential capability of producing a higher magnetic field than that by Nb3Sn and is less sensitive against strain, which will simplify the fabrication of a large coil. From this point of view, development work for a Nb3Al conductor (cable-in-conduit conductor with stainless steel conduit) was started at the Japan Atomic Energy Research Institute and the Nb3Al Insert was successfully developed and tested in 2002. In the tests, a nominal current of 46 kA was achieved in a 13 T magnetic field and critical currents were measured in terms of magnetic field and temperature in order to clarify the strain state of the cable, which determines the overall performance of the conductor. From the results, the thermal strain of the Nb3Al conductor was estimated to be around −0.4%, corresponding to a 10% reduction in the critical current. This is much smaller than that observed in the same type of Nb3Sn conductor (−0.7% strain and 50% reduction). In addition, there was no effect of the electromagnetic forces on critical current observed in the Nb3Al conductor during coil charge while an evident reduction has been reported for Nb3Sn conductors. The higher rigidity of the Nb3Al strands and smaller sensitivity of the critical current against strain can account for these characteristics. Furthermore, although the insert was fabricated using a react-and-wind method, this did not produce any observable effect on the strain of the cable. These results demonstrated the excellent performance of the Nb3Al conductor to be used for a high-field, large coils that will experience large electromagnetic forces.

核融合炉応用を目指したＮｂ３Ａｌ導体の開発と大型超電導コイルへの適用

The Japan Atomic Energy Research Institute has been involved in developing Nb3Al conductors since the middle of 1980s based on the consideration that Nb3Al conductors are capable of producing a higher magnetic field than Nb3Sn conductors owing to the extremely high critical current density in high magnetic fields. At the beginning of the development work, a fabrication technique for Nb3Al strands using a Jelly-roll process was established. This process requires heat treatment at 750°C for 50 h instead of a temperature of more than 1,800°C as required by the conventional method. Using this technique, about 1 ton of strands was produced and a 150-m Nb3Al cable-in-conduit conductor was fabricated. For the next step, to demonstrate the applicability of the Nb3Al conductor to a large coil, a coil 1.5-m in diameter, called the Nb3Al Insert, was manufactured. A react-and-wind method was tried for the production process as it simplifies the fabrication of large coils such as an ITER-TF coil. Performance tests of the Nb3Al Insert were conducted in 2002. The Nb3Al Insert could be charged to the designed point of 13 T and 46 kA without showing any instability. Thus, the world's first large superconducting coil using a Nb3Al conductor was successfully developed, thus indicating the possibility of producing fusion magnets that can operate in higher magnetic fields than those used with Nb3Sn conductors.

Ｎｂ３Ａｌ導体の電磁力下での圧力損失特性―圧力損失特性と撚線剛性の関係―

The Japan Atomic Energy Research Institute (JAERI) has demonstrated the superconducting performance of a large coil using a newly developed cable-in-conduit Nb3Al conductor comprised of 1,152 superconducting strands bundled on a central cooling channel. The conductor was developed as one of the R&D activities of the International Thermonuclear Experimental Reactor (ITER). The hydraulic performance of the Nb3Al conductor under electromagnetic force was studied to clarify the relation between pressure drop under electromagnetic force and cable stiffness. Under an electromagnetic force of 500 kN/m, the pressure drop of the Nb3Al conductor decreased by 5%, which is smaller than the 13% decrease measured for a Nb3Sn conductor. This characteristic was well simulated by taking into account the high stiffness of the Nb3Al cable as related to the high strength and stiffness of Nb3Al strand. Furthermore, the pressure drop of several Nb3Sn conductors tested in previous R&D studies was investigated by means of the same evaluation method to study the effect of cable types. It was concluded that the pressure drop under electromagnetic force can be explained by taking the cable stiffness of any type of CIC conductor into account. This paper describes the measurement results, detailed evaluation method, and considerations that led to this conclusion.

The strain and temperature scaling law for the critical current density of a jelly-roll Nb3Al strand in high magnetic fields

The engineering critical current density (JE) and the index of transition, N (where E = αJN), of a Nb3Al multifilamentary strand, mass-produced as a part of the Fusion programme, have been characterized as a function of field (B), temperature (T) and strain (ε) in the ranges B ≤ 15 T, 4.2 K ≤ T ≤ 16 K and −1.79% ≤ ε ≤ +0.67%. Complementary resistivity measurements were taken to determine the upper critical field (BC2(T, ε)) and the critical temperature (TC(ε)) directly. The upper critical field defined at 5%ρN, 50%ρN or 95%ρN, is described by the empirical relation BC2ρN(T, ε) = BC2ρN(0, ε)[1 −(T/TCρN(ε))ν]. The upper critical field at zero Kelvin and the critical temperature are linearly related where BC2ρN (0, ε) ≈ 3.6TCρN (ε) − 29.9, although strictly BC2ρN (0, ε) is a double-valued function of TCρN (ε). JE was confirmed to be reversible at least in the range −0.23% < ε < 0.67%. The JE data have been parameterized using the volume pinning force (FP) where FP = JE × B = A(ε)BC2n (T, ε)bp (1 − b)q and b = B/BC2(T, ε). A(ε) is taken to be a function of strain otherwise the maximum value of FP (found by varying the field) was a double-valued function of BC2 when the temperature was fixed and the strain varied. To achieve a very high accuracy for the parameterization required by magnet engineers (∼1 A), the data were divided into three temperature–strain ranges, BC2(T, ε) described by the empirical relation and the constants p, q, n and ν and the strain-dependent variables A(ε), BC2(0, ε) and TC(ε) treated as free-parameters and determined in each range. A single scaling law that describes most of the JE data has also been found by constraining BC2(T, ε) using the resistivity data at 5%ρN where ν = 1.25, n = 2.18, p = 0.39 and q = 2.16. When BC2(T, ε) is constrained at 50%ρN or 95%ρN, the scaling law breaks down such that p and q are strong functions of temperature and q is also a strong function of strain. Good scaling provides support for identifying BC25%ρN (T, ε) as the characteristic (or average) upper critical field of the bulk material. The JE data are also consistent with a scaling law that incorporates fundamental constants alone, of the Kramer-like formwhere the Ginzburg–Landau (GL) parameter κ is given by the relationγ is the Sommerfeld constant and t = T/TC(ε). At an applied field equal to the upper critical field found from fitting the Kramer dependence (i.e. at BC2(T, ε)), the critical current is non-zero and we suggest that the current flow is percolative. The functional form of FP implies that in high fields the grain boundary pinning does not limit JE, this is consistent with JE-microstructure correlations in other superconducting materials.

Evaluation of critical current performance of 13 T–46 kA steel-jacketed Nb 3Al conductor

A 13 T–46 kA Nb 3Al conductor with steel jacket has been developed in the framework of ITER-EDA. The thermal strain will appear on the Nb 3Al strands after heat treatment, when the steel jacket is used because of difference in their thermal expansion. The conductor critical current performance is degraded by this process. The critical current performance of this conductor was then studied to verify no degradation during the fabrication and small deterioration in the critical current performance even when the steel jacket is used. The thermal strain is evaluated to be <0.4% for the developed Nb 3Al conductor. The critical current deterioration for this case is <10%. Also, the critical current of the conductor without the thermal strain shows good agreement with that calculated by the critical current of a strand multiplied by its number, 1152. Consequently, no degradation during the conductor fabrication process can be verified. Moreover, the critical current is expected to be more than 90 kA for the thermal strain of 0.4%, resulting in a large margin against the nominal current, 46 kA.

ステンレス鋼製コンジットを使用したニオブアルミ導体の臨界電流性能評価

An Nb3Al conductor with stainless steel (SS) conduit has been developed in the framework of ITER Engineering Design Activity. The critical current performance of this conductor was experimentally investigated by using a short conductor sample. The results show that the critical current achieves the expected, resulting in demonstrating the validity of the fabrication method. The thermal strain is generally applied to the strands if SS conduit is used because of the different thermal expansion between the strands and SS conduit. However, it was not applied in our sample, since the conductor length, a few meters, is too short to keep it by friction force between the cable and conduit. The thermal strain applied to the strands in a long conductor is therefore estimated by using the solid model, in which the strain on the strands and conduit are calculated from the equilibrium of their thermal forces, ignoring the effect of cabling on relaxation of the strain on the strands. The results indicate that the Nb3Al strands are subjected to a compressive thermal strain of less than 0.4%. The critical current degradation by this is less than 10%, even though the SS conduit is used. Consequently, its critical current is estimated to be 90kA, which is sufficiently high compared with the nominal, 46kA, at 13T and 4.5K. Furthermore, it should be noted that the large thermal force, 80kN, appears after the heat treatment at room temperature.

AC loss and contact resistance in copper-stabilized Nb/sub 3/Al Rutherford cables with and without a stainless steel core

Calorimetric measurements of AC loss and hence interstrand contact resistance (ICR), were measured on three samples of Rutherford cable wound with Cu-stabilized jelly-roll type unplated Nb3Al strand. One of the cable types was furnished with a thin core of AISI 316L stainless steel and the other two were both uncored but insulated in different ways. The cables were subjected to a room-temperature-applied uniaxial pressure of 12 MPa that was maintained during the reaction heat treatment (RHT), then vacuum impregnated with CTD 101 epoxy, and repressurized to 100 MPa during AC-loss measurement. The measurements were performed at 4.2 K in a sinusoidal field of amplitude 400 mT at frequencies of 1 to 90 mHz (no DC-bias field) that was applied both perpendicular and parallel to the face of the cable (the face-on, FO, and edge-on, EO, directions, respectively). For the cored cable the FO-measured effective ICR (FO-ICR), was 5.27 &#61549;&#61527;. Those for the uncored cables were less than 0.08 &#61549;&#61527;. As shown previously for NbTi- and Nb3Sn-based Rutherford cables, the FO-ICR can be significantly increased by the insertion of a core, although in this case it is still below the range recommended for accelerator-magnet use. Post-measurement dissection of one of the cables showed that the impregnating resin had permeated between the strands and coated the core with a thin, insulating layer excepting for some sintered points of contact. In the uncored cables the strands were coated with resin except for the points of interstrand contact. It is suggested that in the latter case this tendency for partial coating leads to a processing-sensitive FO-ICR.

Development of Nb 3Al superconductors for International Thermonuclear Experimental Reactor (ITER)

A fabrication technique of a Nb 3Al strand using mass-production billets was developed for the framework of International Thermonuclear Experimental Reactor (ITER) Engineering Design Activity (EDA). The weight of a multifilament billet is about 70 kg, which corresponds to the total length of the wire of about 16 000 m. The critical current density is over 600 A mm −2 at 4.2 K, 12 T, and the hysteresis loss for the field parallel to the wire axis is lower than 600 mJ cm −3 at ±3 T. The residual resistivity of the chrome-plated wire is drastically improved to be lower than 1.6×10 −10 Ωm with setting the large copper area at the centre, which reduces the chrome diffused into the copper. The yield of the superconducting strand has been improved, resulting in a yield of 70% for wire longer than 1500 m. The total volume of fabricated Nb 3Al superconducting strand for ITER Nb 3Al insert coil, which is fabricated in the ITER-Central Solenoid (CS) model coil program, is about 210 000 m, i.e. 1000 kg. This is the first experience of mass-production for Nb 3Al superconductors in the world.

Development of a 13-T and 40-kA Nb 3Al conductor for toroidal coils of fusion reactors

Since Nb 3Al is less sensitive against strain than Nb 3Sn, we can avoid degradation of the critical current performance due to strain, or any difficulties in the fabrication of the toroidal field (TF) coil, which originate as a result of sensitivity of the critical current of Nb 3Sn on the strain. From 1986, therefore, the authors started the development of an Nb 3Al strand aiming at the application in TF coils of fusion reactors. The high critical current performance, 600–700 A mm −2 at 12 T and 4.2 K, could then be achieved, which was as good as Nb 3Sn. In addition, a large-current Nb 3Al cable-in-conduit conductor will be tested, the critical current of this conductor was then evaluated to be 120 kA at 12 T and 4.2 K. These data demonstrate the applicability of the Nb 3Al conductor to the ITER-TF coil. In addition, the 13-T and 40-kA class coil, Nb 3Al insert coil, was tested to demonstrate the applicability of the Nb 3Al conductor, with a react-and-wind technique, to the ITER-TF coil under the framework of the ITER-EDA. The fabrication of the Nb 3Al strand for this coil is underway.

ジェリロールＮｂ３Ａｌ導体の開発とその特性

Recently, the development of Nb3Al conductors has progressed rapidly with the aim apply them to fusion magnets. In general, the critical current density of Nb3Al is insensitive to strain (or stress) in comparison with that of Nb3Sn. This property is useful in the construction of high-field and large magnets; particularly, fusion magnets. The Nb3Al strands are copper-stabilized multifilamentary composites produced using a jelly-roll process. In this paper, the status of Nb3Al conductor development and the characteristics of the Nb3Al conductor are reported.

The applicability of the react-and-wind technique to the ITER-TF coil with a Nb 3Al conductor

The ITER-TF coil whose size is a 17 m height and a 11.5 m wide, has been designed with maximum field of 13 T in winding. At present the TF coil is considered to be fabricated with a wind-react-transfer technique using Nb 3Sn conductors. However, recently the development of Nb 3Al conductors is advanced with rapidity. The critical current density of more than 700 A/mm 2 within non-copper has been achieved. Furthermore, the degradation of the critical current density at 0.4% strain and at 12 T was only 5% for a Nb 3Al strand in comparison with 20% for a Nb 3Sn strand. Therefore, we have tried to investigate if the ITER-TF coil could be fabricated with the react-and-wind technique using Nb 3Al conductor, by which high reliability of the coil fabrication and reduction of the fabrication cost are realized. As the result, it was found that the fabrication of the ITER-TF coil is possible within 0.4% strain in Nb 3Al strand with no changes of the present coil and conductor dimensions.