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Database on Characteristics of Nb3Sn Strands

#2707


Creation of a Database on Electromagnetic, Mechanical, and Thermal Parameters of Superconducting Nb3Sn Composite Strands and Components of Composite Strands

Tech Area / Field

  • MAT-ALL/High Performance Metals and Alloys/Materials
  • PHY-SSP/Solid State Physics/Physics

Status
3 Approved without Funding

Registration date
17.02.2003

Leading Institute
All-Russian Scientific Research Institute of Non-Organic Materials named after A. Bochvar, Russia, Moscow

Collaborators

  • CERN / European Laboratory for Particle Physics / AT Department, Switzerland, Geneva\nEuropean Fusion Department Agency, Germany, Munich\nMassachusetts Institute of Technology (MIT) / Plasma Science and Fusion Center, USA, MA, Cambridge

Project summary

The purpose of the Project is creation of a representative database on electromagnetic, mechanical, and thermal parameters of superconducting Nb3Sn composite strand and components of composite strand which are necessary for calculations of magnet system.

At present strands based on superconductor Nb3Sn are the basic material for high-field magnets manufacture. Successful experience of superconductors application in the small and large scale high-field magnetic systems has become the basis of the development of the very large-scale magnetic systems project, such as ITER (International Thermonuclear Experimental Reactor) project for thermonuclear fusion reactor with magnetic containment of plasma.

In large scale magnetic systems superconductors are used in cables containing hundred and thousands single superconducting wires (strands) incorporated in vacuum tight jackets made of composite materials having mechanical and thermal properties, significantly different from those properties of superconductors. The typical Nb3Sn strand has a diameter of ~1 mm and consists of several thousand Nb3Sn filaments (diameter about 3 mm) embedded in a matrix of copper-tin bronze. The filament/bronze region is surrounded by a diffusion barrier of Tantalum or Niobium.

On the outside of the strand there is a layer of high conductivity copper for stabilization of strand. The Nb3Sn compound is formed by a heat treatment at about 1000 K, when tin from tin-rich bronze is diffused into Niobium filaments. The strand components are therefore in strain equilibrium at 1000 K. With a temperature decrease to 4.2 K stresses build up in a strand due to different thermal contraction coefficients of its inpidual components.

The strain influence on the critical current (IС) is described in the frame of empirical scaling law, including fundamental parameters TС and BС2 as fitting ones. The scaling law allows an adequately accurate description of experimental data in a wide range of strains, temperatures and magnetic fields if stresses in Nb3Sn compound are known. At 4.2 K in an isolated Nb3Sn strand the contraction in a superconducting layer is assessed to be ~ 0.25%. Problems arise when describing a strand stressed state in a cable. Inaccurate assessments of the strand condition of a Nb3Sn strand in a cable that are explained by incomplete data on mechanical properties, thermal expansion coefficients of Nb3Sn strands and cable components as well as on the influence exerted by strands on superconducting characteristics result in overstated assessments of superconducting cable properties.

One of the objectives of the project is to measure and compile database on mechanical properties and thermal expansion coefficients of bronze having different composition and other components of a composite strand. In the same characteristics will be also acquired for internal tin Nb3Sn strands and the ones of variant designs produced by the method of solid phase diffusion and having different levels of stresses.

The second part of the project is aimed at studying the superconducting properties of Nb3Sn strands. Detailed studies will be given to critical current as a function of temperature, magnetic field and strain IC (T, B, e) as well as parameters describing transition characteristics (TC), in other words, the process of electrical field increase vs variations in current, temperature, magnetic field and strain.

The instability of superconductors is governed by both the specific features of a transition characteristic the shape of which promotes a positive feedback between a temperature of a wire and the extent of its heat emission output and by the availability of external thermal perturbations. Until recently the instability of a superconductor was investigated in the framework of electromagnetic based on the slightly upgraded concept of jerky transition of a superconductor to the normal state (models of critical state). The result acquired in this approximation led to some recommendations useful for the designing of current carrying elements and superconducting magnetic systems. The further investigations revealed that to accurately calculate it is necessary to account for the real form of the transition characteristics of a superconductor. Despite the many-year studies into Nb3Sn superconductors the issue relevant to the causes effecting the smearing of transition characteristics remains the least studied one. The physical model is not yet available that would adequately describe the ЕС behavior in a wide range of temperatures, magnetic fields and strains.

This project assumes implementing investigations of the volt-current and volt temperature characteristics of Nb3Sn strands. The results of these investigations shall form the bases of the ЕС parameter database. This information will be further used to develop a mode describing volt-current characteristics (vcc).

The finished project will result in compiling a representative database on superconducting parameters, mechanical properties and thermal expansion coefficients for magnetic system calculations addressing a wide range of specimens of superconducting composite Nb3Sn wires and components.

The compiled database is to be further used:


- to study physical processes governing the current carrying capacity and transition characteristics of Nb3Sn superconductors;
- to optimize properties of Nb3Sn base strands;
- to analyze the results generated via testing of ITER model coils.

The investigations shall be carried on by highly qualified experts having much experience in the area of applied superconductivity, materials science and solid state physics.

The project is based on the results acquired from Nb3Sn strand developments and studies implemented in the framework of the ITER project at the Institute. More than 30 papers on the Project topics were published in reference editions.

The project meets the objectives of integrating Russian scientists to International research programs of fundamental and applied research addressing developments of novel power sources; promotes the national goal of keeping and evolution of the scientific potentialities of the country.

The project is to cover 18 months. The main project sections are:


1. Collect and analyse data on project topic;
2. Choose and prepare specimen of Nb3Sn wire and strand components;
3. Examine fine structure of Nb3Sn filaments and strand components.
4. Conduct experiments on measuring critical current IC (T, B, e) of Nb3Sn strands in wide ranges of temperatures (T), magnetic fields (B) and strains (e);
5. Investigate transition characteristics;
6. Mathematical processing of experimentally acquired results;
7. Investigate mechanical properties and thermal expansion coefficients of Nb3Sn strands and strand components;
8. Compile database on superconducting composite wire parameters needed to design magnetic systems.

In the framework of co-operation with foreign collaborators it is assumed to conduct discussions and co-ordination of program of experimental studies; round-robin checks of results; working meetings to jointly discuss results; workshops and issuing papers.

The technical approach and methodology of the work consist in a systems theoretical and experimental research of electromagnetic, mechanical and thermal properties of a wide range of Nb3Sn strands and components of a composite strand that are needed to calculate design magnetic system. Various stress levels in Nb3Sn wire are realized by choosing produced by the method of solid phase reaction as well as internal tin specimens.


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