Giordano Tomassetti

Design of an Industrially Feasible Twisted-Stack HTS Cable-in-Conduit Conductor for Fusion Application

Taking advantage of the large experience of the ENEA Superconducting Laboratory in the manufacture and characterization of large scale superconducting systems, a project was launched, aimed at using High Temperature Superconductor (HTS) 2G wires for the manufacture of a cable-in-conduit conductor (CICC). In particular, the main aim was the definition of a conductor design fully compatible with existing cabling technologies, to be promptly transferred to an industrial scale production. The considered layout is based on 150 HTS tapes, arranged as five layered structures of twisted tapes wound on a helically slotted core with external round jacket. All manufacturing steps (slotted core production, tape stacking and insertion into the ducts, external wrapping and jacketing) are fulfilled by using equipments and technologies available at TRATOS Cavi S.p.A. These CICCs are intended for operation using forced flow of Helium. A 2D local thermal model has been developed for the optimization of the cooling configuration. This conductor is designed to target 20 kA at 4.2 K and 15 T (or, alternatively, higher temperature, in self-field and LN2 cooling) corresponding to a Je of about 55 A/mm2. The production of a short dummy sample revealed that the exploited industrial production process is very promising for the development of HTS CICC.

Electrical Characterization of ENEA High Temperature Superconducting Cable

ENEA is currently involved in the design and manufacture of a fully high temperature superconductor (HTS) cable in the cable-in-conduit conductor (CICC) configuration exploiting commercial second generation ReBaCuO (Re: Rare Earth and Y) coated-conductors. The final cable will be composed of five slots obtained in a helically twisted aluminum central core and filled with 2G tape stacks. This conductor is designed to operate above 10 kA in 12 T background field at 4.2 K or at about 10 kA in self-field at 77 K. A first sample of about 1-m length with one fully superconductive slot has been manufactured using 15 tapes provided by Superpower, Inc. and 12 tapes from the SuNAM Company grouped in two sub-stacks divided by a Kapton foil. Each tape of the stack has been characterized individually by measuring critical current values Ic at 77 K (liquid N2 bath) in self-field and n-index. Results revealed that the tapes showed no degradation of critical current values when compared with suppliers specifications confirming that the proposed manufacturing process is fully compatible with commercial coated-conductors. Inter-tape resistance(Rinter) has also been measured and the observed dependence of Rinter on the tape position in the stack has been put in correlation with transverse stress distribution calculated by finite element models. A second sample with a full superconducting slot has been manufactured using 18 SuNAM tapes. Preliminary results on the stack transport measurements performed at 77 K in self-field will be presented and discussed. All the samples were manufactured by using already existing industrial equipments at Tratos Cavi SpA.

Electrothermal Analysis of a Twisted Stacked YBCO Cable-in-Conduit Conductor

A YBCO cable-in-conduit conductor (CICC) for high field magnet applications as well as for energy transmission is under development at the ENEA Frascati Superconductivity Laboratory. The cable is designed to carry up to 20 kA at 5 K and 12 T and is composed of five stacks of 30 YBCO tapes each, arranged in a twisted stacked configuration inside an aluminum stabilizer and an external jacket. The cooling is provided by forced convection of supercritical helium that flows in a central channel and in cavities manufactured between the superconducting stacks and the aluminum core. A first dummy prototype of the cable has been produced by TRATOS Cavi S.p.A., Italy, in order to assess the feasibility of the manufacturing process. This paper presents a 3-D coupled electric and thermal model of the cable, developed at the University of Bologna in a COMSOL Multiphysics environment. The model adopts a homogenization procedure to describe the tape stacks in a finite-element method approach with considerable saving on the number of degrees of freedom. The model is applied to the computation of the cable current sharing temperature and to the investigation of quench due to a distributed disturbance.

Characterization of the Critical Current Capabilities of Commercial REBCO Coated Conductors for an HTS Cable-in-Conduit Conductor

In 2013, the Superconductivity Laboratory at ENEA (the Italian National Agency for New Technologies, Energy, and Economica Developments) started a project aimed at using second generation (2G) High-Temperature Superconductors (HTS) tapes for the manufacture of a cable-in-conduit conductor. The considered layouts are based on a number of HTS tapes, which are arranged as five layered structures of twisted tapes wound on a helically slotted core with an external round jacket, and the conductor is designed to target 20 kA at operating conditions (T = 5 K, B = 15 T; or T = 77 K, self-field). With the commercialization of 2G HTS tapes, several options are now available on the market that can meet the ENEA cable specifications. Thus, for a sound design of the ENEA cable, a measurement campaign was launched aiming to determine the critical current, i.e., Ic, of commercial tapes as a function of an external applied field (up to 12 T) at different temperatures (in the 5-77 K range). Transport measurements were performed at 77 K in the high-field region, whereas the low-field and low-temperature critical currents have been determined from magnetization loops measured with a vibrating sample magnetometer system. Within a COMSOL Multiphysics environment, a 2-D finite-element method (FEM) magnetostatic model has been developed, in which a uniform Ic distribution is assumed in the HTS stacks. Both the experimental and FEM results are discussed in terms of cable performance, showing that current commercial tapes can already meet the ENEA cable specifications at T = 5 K, B = 15 T.

Bending Tests of HTS Cable-In-Conduit Conductors for High-Field Magnet Applications

A high-temperature superconducting (HTS) cable-in-conduit conductor (CICC) suitable for high-field magnet applications and comprised of twisted-stacked coated-conductor tapes arranged around a helically slotted core has been recently proposed and tested, demonstrating full compatibility with existing cabling technologies. To form the desired shape of any coils for high-field magnet applications, any suitable CICC option needs to be bent. For a magnet design, it is then very important to characterize the bending behavior of the CICC and, in particular, to find the smallest bending radius achieved without performance degradation. To this aim, bending tests were carried out on 1-m-long dummy samples of five helical slots in an extruded aluminum core, in which four REBCO tapes and dummy stainless-steel tapes were mounted in each slot. A controlled bending moment has been applied to the HTS CICC samples at room temperature, and each individual superconducting tape has been electrically characterized as a function of bending radius by measuring the critical current and n-index values at 77 K in a self-field condition. Results are analyzed and explained with the help of a three-dimensional cable model implemented in ANSYS and analytical calculations. The experimental and numerical results presented in this paper will demonstrate that the twisted-stack slotted-core technology can meet the bending requirements of high-field magnet designs.

Investigation of Re BCO Conductor Tape Joints for Superconducting Applications

Electric joints are an essential part of every current carrying device. In the field of high temperature superconducting applications, quality of a contact between superconducting tapes and electric terminations is crucial for the device performance. When connecting tapes to each other, soldered contacts are commonly used. Recent reports have demonstrated that the resistance of this contact depends on the pressure applied during a soldering process, and the soldering temperature. When superconducting tapes are used in cables or in other applications, they have to be connected to copper terminations, e.g., to a current lead. Thus, tape-to-copper contacts are important. In superconducting cables, tapes are stacked together having a resistive electrical contact from tape to tape. The resistance of this contact depends on the pressure applied on the stack due to, for example, jacketing of the cable or during cool down. In this paper, the dependence of resistance between tapes on the applied pressure is reported. In this contribution two kinds of contacts, soldered ones and mechanical pressed, are presented and discussed. They are realized, respectively, between two superconducting ReBCO tapes from SuperPower, Inc. (SPI) and SuNAM, which were used for manufacturing HTS cable, and between superconducting tapes and copper.

Pareto-Optimization of HTS CICC for High-Current Applications in Self-Field

The ENEA superconductivity laboratory developed a novel design for Cable-in-Conduit Conductors (CICCs) comprised of stacks of 2nd-generation REBCO coated conductors. In its original version, the cable was made up of 150 HTS tapes distributed in five slots, twisted along an aluminum core. In this work, taking advantage of a 2D finite element model, able to estimate the cable’s current distribution in the cross-section, a multiobjective optimization procedure was implemented. The aim of optimization was to simultaneously maximize both engineering current density and total current flowing inside the tapes when operating in self-field, by varying the cross-section layout. Since the optimization process involved both integer and real geometrical variables, the choice of an evolutionary search algorithm was strictly necessary. The use of an evolutionary algorithm in the frame of a multiple objective optimization made it an obliged choice to numerically approach the problem using a nonstandard fast-converging optimization algorithm. By means of this algorithm, the Pareto frontiers for the different configurations were calculated, providing a powerful tool for the designer to achieve the desired preliminary operating conditions in terms of engineering current density and/or total current, depending on the specific application field, that is, power transmission cable and bus bar systems.

Surrogate Modelling to Minimize Contact Resistance of HTS ReBCO Terminations

Recent advances in HTS ReBCO tapes processing and performances have opened attractive perspectives for applications. For the development of HTS-based technology there are several technological challenges to be addressed such as cable terminations. In this contribution, the effect of load, temperature and joint process time on solder joint process of SuperPower SCS4050 tapes with the Pb-Sn solder paste, was investigated using a surrogate model. The prediction of contact resistance RSj = Rj × S (S is the joint contact area) as a function of manufacturing parameters is a complex and time-consuming multi-physical problem involving a coupling of mechanical, thermal and electrical equations. Therefore, a surrogate model was implemented to the aim of predicting the effects of parameters on the joint resistance, without developing a computationally intensive multi-physical model. Taking full advantage of the reduced number of tests performed, the surrogate model demonstrated to be useful to supply significant information for tests, allowing to focus directly on the most promising sets of parameters, avoiding an expensive trial-and-error experimental campaign.