Nadezda Bagrets

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.

Cryogenic Test Facility CryoMaK

Within the growing field of cryogenic applications, there is an increasing demand on cryogenic material characterization. The Cryogenic Material Test Facility Karlsruhe CryoMaK is able to perform a variety of tests. The facility has a long standing tradition in cryogenic testing going back to 20 years in the Institute of Technical Physics at the Karlsruhe Institute of Technology (KIT). Mainly focused on mechanical testing also thermal conductivity and expansion measurements are performed. Additional test rigs are used to examine superconducting performance under applied tensile load in magnetic field.

Low Temperature Thermal and Thermo-Mechanical Properties of Soft Solders for Superconducting Applications

High temperature superconducting (HTS) systems require electrical connection of the superconductors with each other or with a parallel shunt metal. Soft soldering is commonly used for the electrical interconnection of superconducting strands. However, soldering of these electrical interconnections should be carried out at possible low temperatures to prevent degradation of the superconducting material and avoid reduction of oxygen within superconductor. Therefore, solders with low melting temperatures are of special interest for superconducting applications. For the design of these electrical joints, along with electrical properties, it is important to know the thermal and thermo-mechanical properties at low temperatures of the solder to be used. For example, a large mismatch between thermal expansion of the superconductors and the solder cause stresses during cool down. This may lead to a degradation of the superconducting properties. Thermal conductivity, thermal expansion and mechanical properties of the solder at low temperatures influence the quench propagation and life time of joints, and, therefore, the performance of the superconducting device. In this report we present low temperature thermal and thermo-mechanical properties of several soft solders, which are commonly used for cryogenic applications.

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.

Mechanical and Thermal Properties of Central Former Material for High-Current Superconducting Cables

Ongoing projects to realize high-current superconducting cables for transport or magnet applications need to incorporate a high number of coated conductor tapes. Several design layouts published, such as conductor-on-round-core cable, stacks, or Roebel-Rutherford, can achieve this. Design layouts proposed by the research institutes ENEA and KIT use a central former having grooves along the length where stacks of tapes or Roebel strands can be embedded. The former material is a substantial fraction of the cable cross section influencing the overall performance of the cable. In this work, materials used as the central former are investigated after severe plastic deformation, using equal channel angular pressing (ECAP) and equal channel angular rolling (ECAR) processes. Applying these methods allows producing continuously long-length profiles. Materials under investigation are aluminum alloy EN AW 6063 and oxygen-free high-conductivity copper. The influences of substructural characteristic obtained by ECAP and ECAR technology on mechanical properties, as well as thermal and electrical conductivity, at operational cryogenic temperatures, at 77 K and 4.2 K, are observed.

Thermal Properties of ReBCO Copper Stabilized Superconducting Tapes

In recent years, high temperature superconductors (HTSs) have been developed intensively for different applications such as high field magnets, fault current limiters, and current leads. Two-dimensional HTS-based conductors, the coated conductors, used for these applications are multilayered structures on substrate tapes, which can be laminated in addition by a metal, e.g., by copper or stainless steel. Depending on the constituents and the conductor architecture, the thermal conductivity of the whole conductor can vary over several orders of magnitude. Thermal conductivity data are required for calculation of the temperature distribution in a superconducting tape or cable, and for prediction of quench propagation in the conductor itself, and finally in a superconducting device (e.g., coil arrangement). Thermal conductivity data should be measured first of all for the HTS tape, which is used for production of the superconducting appliance. In this paper, we present results of thermal conductivity measurements of the yttrium-barium-copper-oxide copper stabilized superconducting tape in the direction along the tape. Moreover, measurements of the thermal conductivity of the tape in the normal direction are presented.

Data Acquisition of a Tensile Test Stand for Cryogenic Environment

Superconducting magnets and components are exposed to mechanical forces during cool down or current operation. The mechanical strength of used materials has to fulfill the specified requirements. A tensile test in cryogenic environment is one option in material testing to assess usability of materials. The PHOENIX facility at Karlsruhe Institute of Technology-Institute for Technical Physics is designated to analyse specimen on tensile load under cryogenic conditions. PHOENIX was adapted for economic operation. A total number of ten samples can be tested one after the other during one cool down cycle. PHOENIX is subject to a quality management system and it is planned to be accredited according to ISO 17025 standard in the near future. Focus of work will be the qualification of steel samples for quality assurance of cryogenic magnet components of the poloidal field and toroidal field coils in the framework of the ITER project. Recent instrumentation and software provide a standard degree of automation for the measurement tasks to be performed during a tensile test. The paper describes instrumentation equipment and implemented software features of the measurement and control system.

Determination of mechanical and thermal properties of electrical insulation material at 4.2 K

For the electrical insulation of superconducting magnets or magnet components, different insulation materials are used that ensure mechanical stability and good electrical insulation properties at low temperatures. Especially glass-fiber-reinforced or sand-filled epoxies are typical material composites that can be used at temperatures down to 4.2 K. For current lead construction within fusion technology, an optimized filled epoxy with respect to mechanical shear strength had to be identified. Therefore different steel/epoxy compositions where examined using the standardized three point bending method. The test specimen were formed putting five layers of glass-fiber lubricated with epoxy between two 1.4429 (316LN) steel frames. The epoxy facing surface of the steel was blasted with glass or corundum. A systematic investigation was performed using different epoxies to get an optimized mechanical performance. The results will be discussed together with data from a sand-filled epoxy compound. For both compoun...

Investigation of Soldered REBCO Tape–Copper Joints for Superconducting Applications

In the growing field of high-temperature superconductor (HTS) applications, soldered tape-copper joints are of significant importance. This is because the resistance of the joint often plays a decisive role in the design of a superconducting device, such as current leads or current terminations of cables. A common requirement to the joint is as-low-as-possible resistance and reliable reproducibility, which is crucial for achieving a homogeneous current distribution being independent within each HTS tape placed in a superconducting device. From previous studies, it is known that resistance of HTS tape-copper joint can vary for different tapes and solders. This resistance can be considered as a sum of several contributions: REBCO-layer-copper-laminating-layer interface resistance in HTS tape, copper-laminating-layer-solder and solder-copper interface resistances, and solder and copper resistances. Here, we present current status of investigations on tape-copper joints and trials to estimate different contributions in their resistance value.