Finite element analysis of copper–Nb3Sn contact performance under transverse compression

It turns out from recent researches on Nb₃Sn conductor that contact strain is important for Nb₃Sn conductor, as it could cause the degradation of the conductor itself. Few experimental results describing the behavior of Nb₃Sn sub cables under transverse load are available. Numerical modeling is one method to investigate the contact strain inside the cable. In this paper, finite element method was used to analyze the two- and three strand contact strains under two pressing configurations: flat and concave plates. From the finite element analysis, it is found that the average von-Mises strain of bottom Nb₃Sn in Nb₃Sn–Nb₃Sn–Nb₃Sn contact model and copper–Nb₃Sn–Nb₃Sn contact model is the same under flat or concave plates; the average von-Mises strain of bottom Nb₃Sn in two strand contact model is also the same. It is also verified that the shape of the top copper press has no more influence on the strain distribution in Nb₃Sn strand under the transverse compression.     

The European Nb3Sn advanced strand development programme

Strands relevant for fusion with high critical current densities and moderate hysteresis losses were developed and already produced on industrial scale. Based on these achievements EFDA-CSU Garching has launched a Nb₃Sn strand development and procurement action inside Europe in order to assess the current status of the Nb₃Sn strand production capability. All six addressed companies have replied positively to the strand R&D programme which includes the three major Nb₃Sn production techniques namely the bronze, internal-tin and powder-in-tube (PIT) route. According to the strand requirements for the ITER TF conductor a critical current density of 800 A/mm² (at 12 T, 4.2 K and 10 μV/m) and overall strand hysteresis losses below 500 kJ/m³ have been specified as the minimum guaranteed strand performance.The second major objective of this programme is to motivate the strand manufacturers to develop and design high performance Nb₃Sn strands optimised for the ITER conductor. For this purpose, a target critical current density of 1100 A/mm² has been added to the specification. This paper describes the strategy behind the strand development programme, the actual status of the strand production as well as first preliminary results obtained from the strand suppliers.     

Coil winding pack FE-analysis for a HELIAS reactor

At the Max-Planck-Institut fuer Plasmaphysik (IPP) a reference design is being created of an upgraded five-periodic HELIAS type stellarator reactor which evolves from Wendelstein 7-X (W7-X) by scaling of the coil centre line geometries by a factor of four. This reactor type was extensively investigated at IPP with regard to physical characteristics and to some extent also to engineering issues. The upgrade concerns an increase of the induction at the plasma axis and correspondingly at the superconductor.The aim is to develop the reactor concept to a stage and such detail that major engineering problems are unveiled, and relevant comparisons with other concepts, including tokamaks, can be drawn in view of upcoming decisions concerning a DEMO reactor. Even though progress in plasma physics, and in particular future results of W7-X and other machines – particularly of ITER – will probably lead to somewhat different coil shapes, no principal changes of the reference design are expected.In this paper the option of a roll-formed square coil cable jacket is investigated. Detailed structural FE analysis of the coil winding pack demonstrates the feasibility of such a conductor which appears to be the most economical option. It also allows sufficient space for a cable current density very similar to that of the ITER TF coil with a similar overall winding pack cross section of ≈0.5 m². Already existing Nb₃Sn conductors could thus be safely applied in such a HELIAS reactor. Obvious progress of superconductor technology, particularly concerning Nb₃Al, will be beneficial concerning savings of conductor material, ease of manufacture, higher operation temperature, etc.     

Prospects of High Temperature Superconductors for fusion magnets and power applications

During the last few years, progress in the field of second-generation High Temperature Superconductors (HTS) was breathtaking. Industry has taken up production of long length coated REBCO conductors with reduced angular dependency on external magnetic field and excellent critical current density jc. Consequently these REBCO tapes are used more and more in power application.For fusion magnets, high current conductors in the kA range are needed to limit the voltage during fast discharge. Several designs for high current cables using High Temperature Superconductors have been proposed. With the REBCO tape performance at hand, the prospects of fusion magnets based on such high current cables are promising. An operation at 4.5 K offers a comfortable temperature margin, more mechanical stability and the possibility to reach even higher fields compared to existing solutions with Nb₃Sn which could be interesting with respect to DEMO.After a brief overview of HTS use in power application the paper will give an overview of possible use of HTS material for fusion application. Present high current HTS cable designs are reviewed and the potential using such concepts for future fusion magnets is discussed.     

The key role of twist pitch and mechanical pre-treatment in the transport properties of Nb3Sn wires subject to bending strain

In modelling the transport properties of multi-filamentary Nb₃Sn strands, the knowledge of the geometrical parameters of the superconducting filaments and the electrical and mechanical properties of the different materials composing the wire are required. In particular, the filament twist pitch and the transverse resistivity between filaments have a crucial role in the definition of the current transfer length and, consequently, in the simulation of the transport performances of superconducting wires subject to mechanical loads, as in cable-in-conduit conductors (CICC) for fusion magnets during operation. We have measured the critical current and the n-index of internal tin Nb₃Sn wires with different values of the filament twist pitch, having inserted the strand into a stainless steel jacket and under the application of pure bending strain. Results show that the degradation of the transport properties is affected by the twist pitch value and, in the limit case of non-twisted filaments, even an improvement is observed in presence of bending. Moreover, the reversibility of critical current after relaxing the mechanical load has also been checked and an improvement of the performances has been observed after pre-bending applications, presumably due to the strain relaxation. In addition, the differential analysis through the second derivative of the V–I curve evidenced a peaked critical current distribution for the UNTW-strands, while TW-strands under bending showed a higher degree of non-homogeneity, proven by broader distributions.     

System analysis study for Korean fusion DEMO reactor

A conceptual design study for a steady-state Korean fusion DEMO reactor (K-DEMO) has been initiated. Two peculiar features need to be noted. First, the major radius is designed to be just below 6.5 m, considering practical engineering feasibilities. But still, high magnetic field at the plasma center around 8 T is expected to be achieved by using current state-of-the-art high performance Nb₃Sn strand technology. Second, a two-stage development plan is being considered. In the first stage, K-DEMO will demonstrate a net electricity generation but will also act as a component test facility. Then, after a major upgrade, K-DEMO is expected to show a net electric generation on the order of 300 MWe and the competitiveness in cost of electricity (COE). Feasibility of such a practical, near-future demonstration reactor is studied in this paper, based on a zero dimensional system analysis code study. It was shown that a net electric generation on the order of 300 MWe can be achieved below the optimistic β_N limit of 5. The elongation of K-DEMO is around 1.8 with single null configuration. Detailed optimization process and the resultant various plasma parameters are described.     

Results of simulations of stability and quench behavior for pulsed tests of the ENEA 12T Nb3Sn CICC coil

In the ENEA Frascati Laboratory a facility is being assembled to test the ENEA Nb₃Sn CICC coil in pulsed regimes. The characteristics of the coil (dimensions, cable-in-conduit conductor, strand designed for use in variable field) are such to make these tests of primary importance to predict the behaviour of the ITER (International Thermo-nuclear Experimental Reactor) Central Solenoid Model Coil, which will be built in the next two years. In particular, the stability and quench behaviour of the coil will be tested and compared to the predictions of the thermo-hydraulic analysis code SARUMAN. Other important parameters will be the ramp rate limination, the limiting current and the conductor losses. Several testing scenarios (ramp up and discharge) are described and the present status of testing programme definition is given, together with the associated analyses.     

Development of Nb tube processed Nb3Al multifilamentary superconductor

The Nb tube process has recently been developed at NRIM (Japan) for fabricating Nb₃Al multifilamentary superconductors containing more than one million continuous ultrafine filaments of less than 0.1 m diameter. The adjustment of hardness of Al cores relative to Nb matrix by alloying Al cores with additives of Mg, Ag(-Ge), Cu(-Ge), Zn, etc. improved remarkably the cold workability of Nb/Al composites. Wires heat treated below 1000°C show superior properties to a commercial multifilamentary (Nb,Ti)₃Sn conductor: (1) higherJ _c (4.2 K) (1.5×10⁹ A/m² at 10 T), (2) smaller degradation inJ _c with mechanical strain, and (3) smaller ac loss (resulting from a smaller effective superconducting filament diameter: about 2 m). Furthermore, sensitivity to irradiation is nearly the same as that of (Nb,Ti)₃Sn. Other techniques to produce Nb₃Al are unsuitable for developing a conductor with multifilamentary structure, and thereby the Nb tube processed Nb₃Al multifilamentary conductors is the best available for fusion reactor magnets.     

Implementation and analysis of ITER strand test of CNDA for world-wide benchmarking

According to the International Thermonuclear Experimental Reactor (ITER) Procurement Arrangement (PA) of Cable-In-Conduit Conductor unit lengths for the magnet systems, at the start of process qualification, the Domestic Agency (DA) shall be required to conduct a benchmarking of the room and low temperature acceptance tests carried out at the strand suppliers and/or at its reference laboratories designated by the ITER Organization (IO). The first benchmarking was carried out successfully in 2009 and the second round in 2010. Bronze-Route (BR) Nb₃Sn strand and samples prepared by CERN were sent out to each participant in the first round. The second round was referred to the Internal-Tin(IT) Nb₃Sn and NbTi strand. The two rounds of benchmarking included tests for critical current, hysteresis loss, residual resistance ratio, strand diameter, Cu fraction, twist pitch, and plating thickness. As the referenced lab of Chinese DA (CNDA), the superconducting strand test lab from Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP) participated in the Benchmarking. The feedback from IO showed good results and high coherence. The test facility and test results for benchmarking of CNDA were presented in this paper.     

Qualification tests for ITER TF conductors in SULTAN

From February 2007 to May 2008, 18 short length conductor sections have been tested in SULTAN for design verification and manufacturer qualification of the ITER Toroidal Field (TF) conductor. The test program is focussed on the current sharing temperature, T_cs, at the nominal operating conditions, 68 kA current and 11.15 T effective field, which can be fully reproduced in the SULTAN test facility. A broad range of results was observed, with over 2 K difference among the T_cs of the conductors. In average, the results are poorer compared to the potential performance estimated from the strand scaling law. The key parameters to mitigate the degradation are not yet clearly identified. The experimental challenges to test conductors with performance degradation are highlighted, including enhanced instrumentation sets, the application of gas flow calorimetry to sense the current sharing power and the post-processing of voltage data to cancel the transverse potential across the cable. The updated schedule of the tests in SULTAN is presented with the short-term action plan for conductor test.     

Status of the ITER magnets

The first 2 years of the ITER IO has seen substantial progress towards the construction of the magnets, in three main areas. Firstly, the design has been developed under the conflicting constraints to minimise construction costs and to maximise plasma physics performance. Building construction momentum while updating the design to take account of new physics assessments of the coil requirements has been challenging. Secondly, with a stabilising design, it has been possible for the Domestic Agencies to launch the first industrial procurement contracts. And thirdly, critical R&D to confirm the performance of the Nb3Sn cable in conduit design is proceeding successfully.The design consolidation has been accompanied by design reviews involving the international community. The reviews conducted by magnet experts have enabled a consensus to be built on choosing between some of the design options in the original ITER basic design in 2001. The major design decisions were to maintain the circular Nb₃Sn conductor embedded in radial plates for the toroidal field (TF) coils and to maintain NbTi-based conductors for the PF coils. Cold testing, at low current, is also being introduced for quality control purposes for all coils.     

How should we test the ITER TF coils?

The international thermonuclear experimental reactor (ITER) toroidal field (TF) magnet system consists of 18 superconducting coils using a 68 kA Nb₃Sn conductor. In order to guarantee the performances of these coils prior to their installation, the test of at least one prototype coil at liquid helium temperature and full current is required. The test of all coils in the two-coil test configuration, with successive charging of each coil to nominal current is recommended. This requires a large test facility.     

Prospects for a high field ITER device

Scoping studies of a high-field ITER device have been performed. Possible advantages of high field operation include: reduced machine size and cost, decreased fusion power and tritium consumption, lower current, and higher density operation (which may increase the design window for the divertor targets). The use of high aspect ratio minimizes the need for increased field at the superconductor by increasing the ratio between the field at the plasma and the peak field at the coil. Higher aspect ratio also results in stresses in the magnet structure that are within present ITER design limits. The current densities in the conductors are consistent with the use of Nb₃Sn. The impact of high field on steady-state current drive is evaluated.Supported by USDOE Contract DE-AC02-78ET51013.This work performed under appointment to the Magnetic Fusion Energy Fellowship which is administered for the U. S. Department of Energy by Oak Ridge Associated Universities.Since this work was completed, similar conclusions have been reached by J. Perkins, LLNL.     

Neutron irradiation effects on superconducting wires and insulating materials

On the progress of the Deuterium–Deuterium (D–D) or Deuterium–Tritium (D–T) burning plasma devices, the importance of neutron irradiation on superconducting magnet materials increases and the data base is desired to design the next generation devices. To carry out the investigations on the effect of neutron irradiation, neutron irradiation fields are required together with post-irradiation test facilities. In these several years, a collaboration network of neutron irradiation effect on superconducting magnet materials has been constructed. 14 MeV neutron irradiation was carried out at Fusion Neutronics Sources (FNS) in Japan Atomic Energy Agency (JAEA) and fission neutron irradiation was performed at JRR-3 in JAEA. After the irradiation, the Nb₃Sn, NbTi and Nb₃Al samples were sent to High Field Laboratory for Superconducting Materials (HFLSM) in Tohoku University and the superconducting properties were evaluated with 28 T hybrid magnet. Also, the organic insulation materials are considered to be weaker than superconducting materials against neutron irradiation and cyanate ester resin composite was fabricated and tested at the fission reactor. One clear result on Nb₃Sn was the property change of Nb₃Sn by 14 MeV neutron irradiation over 13 T. The critical current was increased by 1.4 times around 13 T but the increment of the critical current became almost zero at higher magnetic fields and the critical magnetic field of the irradiated sample showed almost the same as non-irradiated one.     

The performance test and analysis of ITER Main and Correction Busbar conductor

The first ITER Main Busbar (MBCN1) and Correction Busbar (CBCN1) conductor samples were manufactured in ASIPP and tested in the SULTAN facility. This paper introduces the sample manufacture, including strand, cabling, jacketing and sample preparation, and discusses the performance of MBCN1 and CBCN1 conductors. The testing results show that both samples have high T_cs, and meet the ITER requirement.Due to the ITER acceptance standard T_cs of MB conductor was changed to 6.7 K at 45.5 kA/3.9 T. The performance of MBCN1 conductor after cyclic load fits the ITER requirement, but the sample was only tested at 57 kA/2.75 T before cycling test. Using some hypothesis and equation to extrapolate the T_cs performance of MBCN1 conductor before cycling test, the result also fits the ITER requirement.For CBCN1 conductor, the central line of the central cooling spiral shifted about 1.3 mm during the cabling. The deviation causes an increase of the max self-field by about 0.005 T, which could not influence the CBCN1 conductor real T_cs performance at peak field.     

Pulsed electron beam annealing of a15 tape superconductors

Thick films of Nb₃Ge and Nb₃Sn on niobium and Hastelloy B tape substrates were e-beam annealed using single 300 ns pulses and deposited energy densities, E_p, between 0.5 and 5 J/cm². In most cases the energy density thresholds causing melting, E_pt agreed reasonably with numerical simulation data: The melted and resolidified layers were microcrystalline A15's when and coarse-crystalline A15 at . Melting and resolidification resulted in film crazing/cracking due to the differential thermal contraction. The extent of film damage depended upon the film-substrate mismatch. The critical current density, j_c, was accordingly reduced. In the absence of crazing/cracking at an indication of j_c increase in Nb₃Sn was obtained. This increase was attributed to the formation of additional flux pinning centers such as grain boundaries and/or second-phase precipitates.     

Optimization of ITER Central Solenoid Insert design

The United States ITER Project Office (USIPO) is responsible for fabrication of the Central Solenoid (CS) for International Thermonuclear Experimental Reactor (ITER). The CS Insert (CSI) project should provide a verification of the conductor performance in relevant conditions of temperature, field, currents and mechanical strain. The US IPO will build the CSI that will be tested at the Central Solenoid Model Coil (CSMC) Test Facility at JAEA, Naka. One of the design goals of the CSI is to assure that the properties of the conductor near the median plane are measured accurately. Since Nb3Sn is strain sensitive and electromagnetic forces generate a significant strain that increases the current sharing temperature (T_cs), we need to design the Insert in such a way that the most strained conductor near the median plane would still have the lowest T_cs of all the rest of the conductor in the Insert. The difference between thermal contraction of the jacket and spacer material allows controlling axial distribution of the coil radial deformation. Numerical analysis of the CSI was performed using stainless steel, titanium and invar spacer material variants. Distribution of the T_cs was obtained from numerical results in the form similar to one proposed for ITER.     

Calorimetric method for current sharing temperature measurements in ITER conductor samples in SULTAN

Several Toroidal Field Conductor short samples with slight layout variations have been assembled and tested in the SULTAN facility at CRPP. The measurement campaigns started in 2007 and are still ongoing.The performance of every conductor is expressed in terms of current sharing temperature (T_cs), i.e. the temperature at which a defined electric field, 10 μV/m, is detected in the cable due to the incipient superconducting-to-normal state transition. The T_cs at specific operating conditions is the key design parameter for the ITER conductors and is the main object of the qualification tests. Typically, the average electric field is measured with voltage tap pairs attached on the jacket along the conductor. The inability however to explain observed premature voltage developments opened the discussion about possible alternative measuring methods.The He flow calorimetric method is based on the measurement of the resistive power generation in the conductor. It relies on the detection of very small temperature increases along the conductor in steady state operation. The accuracy and the reliability of the calorimetric method in SULTAN are critically discussed, with particular emphasis on the instrumentation requirements and test procedures. The application of the calorimetric method to the recent SULTAN test campaigns is described with its merits and limits. For future tests of ITER conductors in SULTAN, the calorimetric method for T_cs test is proposed as a routine procedure.     

Prospects for the use of high-Tc superconductors in fusion magnets and options for their test in SULTAN

In the last few years, the critical current densities of long commercially available REBa₂Cu₃O_7?x (RE-123, where RE represents Y or a rare earth element) coated conductors have reached values of 250 A/cm-width at 77 K and zero applied field. Even higher values of 600 A/cm-w (77 K, B = 0) have been demonstrated in shorter lengths. The attractive features of the use of these high-T_c superconductors (HTS) are operation temperatures above 20 K and/or magnetic fields higher than those envisaged for the ITER TF coils. Possible operation conditions for HTS fusion magnets have been studied taking into consideration the possible further improvements of RE-123 coated conductors. Investigations of stability and quench behavior indicate that stability is not a problem, whereas quench detection and protection need attention. Because of the high currents necessary for fusion magnets, many tapes need to be assembled into a transposed conductor. The qualification of HTS conductors for fusion magnets would require their test at magnetic fields of 11 T and currents well above 10 kA. The possibilities to test straight HTS conductor samples in SULTAN have been considered. For a test at 4.5 K, only the development of a low resistance joint between the HTS conductor under test and the NbTi transformer of SULTAN would be necessary. Tests up to 20 K would require that the HTS sample is connected with the NbTi transformer by a conduction-cooled HTS bus bar of large thermal resistance similar to the HTS module of a current lead. HTS conductor tests at temperatures around 50 K would be possible with modified cryogenics.