The objective of the EuCARD2 WP10 (Future Magnets) research activity is to demonstrate HTS magnet technology for accelerator applications, by building a short demonstrator dipole with an aperture of 40 mm, operating field of 5 T, and understood field quality. One of the magnet requirements is of small inductance, for use in long magnet strings, hence the superconducting cable must have large current carrying capacity, in the range of 10 kA at operating conditions of 4.2 K and 5 T. An initial down-selection of the cable material and geometry resulted in the choice, as baseline layout, of REBCO tapes assembled in Roebel cable geometry. In this paper we describe the requirements derived from magnet design, the selection process that led to the choice of material and geometry, the reference design of the cable, and its options. Activities have started to address fundamental issues, such as tape performance and tape processing through the cable construction, to key performance parameters such as cable critical current under stress or magnetization. We report here the main highlights from this work, which is described in details in a number of companion papers. Finally, we give an outline of the conductor prototyping and manufacturing plan for the construction of the demonstrator magnet.

EuCARD-2 is co-funded by the partners and the European Commission under Capacities 7th Framework Programme, Grant Agreement 312453.

Roebel assembled coated conductor (RACC) cables and Conductor on round core (CORC) cables are competing concepts for application in ramped high field magnets and AC operated electrical machinery (motors, generators, transformers). Since the cable geometry and the cable concepts are completely different, application relevant technical features as engineering current density, mechanical stability, applied striations, AC losses and effect of background fields are closely related to the cable design and are more or less favored by the two concepts. It is the aim of this contribution to compare both concepts with respect to the different aspects. We present the most recent data for the KIT RACC- cable performance, the achieved current capacity and the potential for further increase and the measured current anisotropy with the sensible influence of the used coated conductors. Also we show the bending ability for magnet fabrication (solenoid and racetrack) and the efforts to apply filaments in the strands by laser grooving for AC loss reduction. The current carrying capacity of RACC cables is correlated with the applied packing density and the transposition length, but also with the specific properties of used tape material, especially the current anisotropy. In contrast CORC cables allow adding tape layers with unchanged transposition length for higher currents. The engineering current densities for both cable concepts are discussed and prospects for a further increase are evaluated. Cable bending is necessary for magnet fabrication and is experimentally investigated on the KIT continuous bending strain rig. RACC cables show excellent results, at easy bending direction -smaller bending radii are achievable compared to the CORC system. Finally, the so far known AC loss data on cables with filaments will be reviewed and discussed addressing the influence of the cable geometry.

By combining resistive inserts, made of Bitter and polyhelix coils, with a large bore superconducting "outsert", the hybrid magnet being built at LNCMI-Grenoble aims at producing in a first step, an overall continuous magnetic field of 43 T in a 34 mm warm bore aperture. The superconducting coil relies on the novel development of a Nb-Ti/Cu Rutherford Cable On Conduit Conductor (RCOCC) cooled down to 1.8 K by a bath of superfluid helium at atmospheric pressure to deliver a nominal magnetic field of 8.5 T in a 1.1 m cold bore diameter. After reminding the specificity of hybrid magnets, namely the strong electromagnetic couplings between resistive and superconducting coils, the main requirements of the RCOCC for efficient and reliable magnet operations, are given. The test and validation phases prior to the industrial production of the RCOCC are reported, including the trial production of the ½ hard-drawn (H02 cold-worked) Cu-Ag0.05% hollow stabilizer in continuous lengths of 325 m long. Studies, developments and the prototyping of the production line for the assembly and the soft-soldering of the Rutherford cable on the Cu-Ag hollow stabilizer are also presented as well as the integration phase within LNCMI-Grenoble.

This project is supported by the CNRS, the French Ministry of Higher Education and Research in the framework of the “Investissements pour l’avenir” LaSUP, the European Funds for Regional Development (FEDER) and the Rhône-Alpes region.

From 2011 the European Fusion Energy Reference Laboratory in Durham has conducted seven types of verification test (critical current, hysteresis loss, residual resistivity ratio, twist pitch, diameter, chromium plating thickness and copper-to-non-copper ratio) on Nb3Sn strands already tested by the manufacturers for inclusion in the ITER toroidal field coils [1]. The scale of this work (> 10,000 measurements) provided Durham with an opportunity to develop and enhance the techniques required to conduct these measurements quickly and reliably. As well as additional specialist measurements on NbTi strands for MRI magnets and on NbTi Rutherford cables for accelerator magnets, Durham has started verification measurements on the NbTi strands that are to be used in one of the Poloidal Field magnets for ITER (PF6). We will describe the work in Durham that has maintained high quality, high quantity measurement throughput and will provide an update on the most recent developments in the laboratory.
[1] T. Boutboul, P. Readman, E. Viladiu, M. Losasso, J. Caballero, J. Abou-Yehia, and R. Batista, "Status of the Procurement of the European Superconductors for the ITER Magnets," IEEE Transactions on Applied Superconductivity, vol. 24, pp. 4, 2014

Steve Lishman and his team in the Physics Department Mechanical Workshop

Bronze processed and internal Sn processed Nb₃Sn wires with a small Ti additive are being widely used for ITER, NMR and other applications. However, after the heat treatment for Nb₃Sn synthesis, the Sn concentration in the matrix reduces to a few %. Then the matrix could not be expected as a reinforcement member, which makes the handling of the wires quite delicate. As an alternative method for the improvement of mechanical performance, we chose gold brass (Cu-15wt%Zn) as the matrix1). In the present study, we fabricated multifilamentary Nb₃Sn wires through internal Sn diffusion process using brass matrix, and investigated the diffusion behavior of each element.
The sub-bundle wires consist of gold brass matrix with a Sn-Ti central core and 8 Nb cores. Then 19 pieces of the sub-bundle wires were restacked and cold-drawn into wires eventually having 8 x 19 Nb cores and 19 Sn-Ti cores. Intermediate annealing was not required for the drawing. As a reference, a similar wire using Cu matrix was fabricated. The heat treatment of the wires was performed under 1 atm. argon atmosphere. In the 8 x 19 core wires, thick Nb₃Sn layers were formed at 700^oC. Zn does not penetrate into Nb₃Sn layer, and stays homogeneously in the matrix. Zn accelerates the diffusion of Sn in the matrix without void formation. A preliminary measurement indicated promising high-field performance in the wires. Moreover, cost reduction of wires may be expected, since brass is a common cheap material. Brass wires show appreciably better mechanical performance than Cu wire does, then the improvement of mechanical performance in Nb3Sn wires may be hopeful.
1) K.Tachikawa et al. : to be published in Physics Procedia (June, 2015).