[en] As the present NET specifications call for a toroidal magnetic field of about 5 T on the plasma axis the geometry of the TF coils system will require a maximum magnetic field on the conductor of about 11 T. A superconducting magnet system fulfilling these conditions can be designed on the basis of the present technology choosing one of the two following solutions: 1) a Nb-Ti conductor cooled by means of pressurized superfluid helium at 1.8 K; 2) an A-15 based conductor cooled by liquid Helium at temperatures around 4.5 K

[en] The design of the poloidal field (PF) coils of the International Thermonuclear Experimental Reactor (ITER) relies on the use of 45 kA NbTi cable-in-conduit conductors. An R and D programme is carried out jointly between CEA and ENEA to acquire knowledge on the behaviour of such conductors. In addition to the manufacture and tests of two subsize joint samples at CEA Cadarache, the design of a poloidal field full-size-joint sample (PF-FSJS) has been carried out. This sample is composed of two conductor legs made of stainless steel-jacketed NbTi full size cables, of a lower joint built according to the 'twin box' concept developed at CEA, and of two upper terminals to be connected to the facility current leads. A description of the sample design, the techniques used for the different manufacturing steps of the conductor legs, the terminations and the assembly is related. This prototype sample will be tested in the Sultan test facility at Villigen (Switzerland) and will be a model for the manufacture of joints for the PF coils

[en] The Central Solenoid (C S) of ITER (International Thermonuclear Experimental Reactor) is designed to operate at 13 T, 0.5 - 1 T s, 5.8 K. There is an experimental evidence that reduced performances in variable field, not explained by A C losses, could occur in CIC (cable-in-conduit) conductors. Such ramp rate limitations have to be well understood in order to be avoided in the ITER conductors. In the ENEA (National Agency for New Technology, Energy and Environment) Frascati Laboratory a facility has been assembled to test in pulsed regimes (up to 8 T, 4 T / s, 12 K) the ENEA (National Agency for New Technology, Energy and Environment) Nb₃ Sn CICC coil. The characteristics of the coil (dimensions, CIC conductor, strand designed for use in variable field). Main objectives of the program are experimental data on ramp rate limitations, pulsed-current distribution in the cable, limiting currents and global A C losses. This paper reports the results of the measurements carried out up now, and those of the simulations performed by means of the thermo-hydraulic analysis code SARUMAN

[en] We review progress in the design of high field superconducting cable-in-conduit conductors (CICCs) for fusion applications, with special attention to the results of recent key experiments, leading to the state-of-the-art CICC technology: the ITER Toroidal Field and Central Solenoid programs, the EFDA Dipole conductor development program, the NHFML Hybrid Magnet project, the EU-TF Alt conductor demonstration, and the CRPP React and Wind flat cable test. For these projects, the main CICC design driver was the mitigation of Nb_3Sn conductor performance degradation with electro-magnetic loading cycles. This was achieved by proper choice of cable layout and of conductor geometry, depending on the specific operating conditions and project requirements. In all cases, the necessity to limit cable movements inside the conductor jacket was identified to be of crucial importance. The main aspects of CICC manufacture are also discussed here, at least for what is the experience gained by the authors in both CICC jacketing and cabling processes. Finally, the state of the art of high-temperature superconducting (HTS) cables is discussed: at present, this technology is still in its infancy, but it is highly likely that major technological improvements could eventually lead to a widespread use of HTS CICCs. (topical review)

[en] We performed magnetic characterizations, using a vibrating sample magnetometer, of an internal tin-type Nb₃Sn technological strand. In order to compare the properties of this sample under different strain conditions, the same strand was tested under compression, obtained by swaging and compacting into thin stainless steel tubes before the heat treatment, as well as after a complete relaxation of the filament strain components, by chemical etching of the Cu and bronze matrix components. In particular, we analysed the Kramer extrapolated upper critical field B_c2K, and the zero-field critical temperature T_c(0) and its distribution, for all the samples. In order to clarify the effect of the strain due to both the stainless steel reinforcement and the copper matrix thermal pre-compression, we compared our experimental results with the predictions from well established scaling laws based on uniaxial models. Good agreement has been found with measured critical temperature T_c and extrapolated upper critical field B_c2K values, with proper choice of fit parameters. On the other hand, we observed a broadening of the superconducting T_c transition with increasing strain. This has to be ascribed to the effect of radial strain components, which cannot be neglected in the present work.

[en] The performance of ITER toroidal field (TF) conductors still have a significant margin for improvement because the effective strain between −0.62% and −0.95% limits the strands’ critical current between 15% and 45% of the maximum achievable. Prototype Nb₃Sn cable-in-conduit conductors have been designed, manufactured and tested in the frame of the EUROfusion DEMO activities. In these conductors the effective strain has shown a clear improvement with respect to the ITER conductors, reaching values between −0.55% and −0.28%, resulting in a strand critical current which is two to three times higher than in ITER conductors. In terms of the amount of Nb₃Sn strand required for the construction of the DEMO TF magnet system, such improvement may lead to a reduction of at least a factor of two with respect to a similar magnet built with ITER type conductors; a further saving of Nb₃Sn is possible if graded conductors/windings are employed. In the best case the DEMO TF magnet could require fewer Nb₃Sn strands than the ITER one, despite the larger size of DEMO. Moreover high performance conductors could be operated at higher fields than ITER TF conductors, enabling the construction of low cost, compact, high field tokamaks. (paper)

[en] In this paper, the general equilibrium equations for a geometrically nonlinear version of the Timoshenko beam are derived from the energy functional. The particular case in which the shear and extensional stiffnesses are infinite, which correspond to the inextensible Euler beam model, is studied under a uniformly distributed load. All the global and local minimizers of the variational problem are characterized, and the relative monotonicity and regularity properties are established.

[en] 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.

[en] In the framework of the preparation for the realization of the international thermonuclear experimental reactor (ITER), the construction and test of relevant models of seven different parts of the reactor was decided. Two of them are related to the superconducting coils: the toroidal field model coil (TFMC) and the central solenoid model coil (CSMC). For these superconducting coils, due to the expected high values of the current (≥60 kA) and voltage (≥5 kV with respect to the ground) the adopted technology was that of cable in conduit conductor (CICC). Until recently, little experience of this technology existed. Therefore, an extensive research and development programme has been carried out, in the last 10 years, by the ITER partners and particularly in Europe, to design, industrialize and test these large conductors and their joints. The EURATOM associations CEA and ENEA played a leading part in this phase. The CICC concept is described and the results of the developments are presented. About 7 km of conductors were manufactured in the industry and for that more than 10 tonnes of Nb₃Sn strands were produced in Europe. In this large programme, Europe is particularly in charge of the TFMC, which will be tested this summer at Forschung Zentrum Karlsruhe (Germany). In the framework of this programme, three full size conductors and joint samples were tested at the European Sultan test facility (Centre de Recherches de Physique des Plasmas, Villigen, Switzerland), to validate the technological choices and check that the ITER specifications were met. The results of these tests are presented in detail. Starting from the strand critical properties, the conductors made of about 1000 strands did reach their expected performance. The joints of these large conductors are very special and delicate components. Their behaviour was quite successful and the joint resistance of these samples (of the order of 1 nΩ) was well within the specifications. (author)

[en] A computational tool is being developed for the analysis of superconductive magnets, combining detailed descriptions of termination joints and cables to thermo-hydraulic (TH) models. In parallel, an experiment (Stability Experiment Upgrade--SexUp) has been designed with the target to study the current distribution in cable in conduit conductors (CICC). Finally, the establishment of a reliable method for the measurement of the current distribution profile on the cable cross section is being implemented on the ITER toroidal field model coil (TFMC)