Modelling of the behaviour of superconducting cables at different scales for the optimization of their electrical performances – COCASCOPE

1. Electrical and mechanical tests

Electrical tests aim to examine the influence of specific loadings, in particular bending and tensile loadings on the critical current.

Mechanical tests are designed to characterize the behaviour of strands under cyclical bending in order particularly to highlight possible ratcheting effects.

2. Identification of cables geometry using tomography

Identification of complex trajectories of strands within cables is needed as validation for the simulation. Statistical analysis tools will be used to characterize these complex geometries.

3. Simulation of the initial forming and of service life loadings

Mechanical simulation codes will be employed to simulate the initial forming of the cables in the scope of the study in order to predict their geometry and to evaluate the strains caused by this process.

1. New test of critical current under bending

A VAMAS test has been adapted by cutting two grooves in a circular mandrel so as to take into account bending effects.

2. Simulation of the forming of Rutherford cables

A dynamic explicit simulation code was employed to simulate the initial forming of Rutherford cables. A good correlation with geometries identified by tomography is obtained.

The characterization of strands behavior under cyclic bending should highlight possible mechanisms of degradation of conductivity properties.

Main results will be published.

As the electrical conductivity of superconducting cables depends on the local strains they undergo, an accurate modeling of their mechanical and electrical behaviours is required to be able to optimize their global electrical performances.

Superconducting cables display a multiscale internal structure. At a first level, the cable is made of an assembly of elementary strands arranged together according to more or less complex architectures, depending on their application. At a lower scale, strands appear as composite structures formed either by superconducting microfilaments embedded in a metallic matrix, or by a thin superconducting layer deposited onto a metallic substrate. Both reversible and irreversible conductivity losses are encountered for these superconducting components at microscopic scale, depending on the magnitude of local strains.

Relying on first obtained results which partially validated the proposed numerical simulation approach in the case of cable in conduit conductors, the project aims at developing the approach in order to address new issues related to the validation by a geometrical identification, the taking into account of effects induced by the initial forming process, as well as of the cyclic constitutive behaviour of strands.

Tomography techniques will be employed in order to experimentally identify geometrical configurations of conductors, and to attempt to observe the occurrence of damage at microscopic scale. Since the complexity of configurations prevents from comparing two configurations through a simple superposition, statistical tools will be specially developed in order to compare relevant geometrical measurements and to validate simulation results against these experimental identifications.

Concerning new applications, the project will focus on determining effects induced by forming processes on the electrical performances of conductors used for electric energy distribution. The new superconducting materials used there are indeed formed before the cabling process ; strains induced by the initial forming process play therefore a predominant role on their electrical characteristics.

Since the origins of conductivity losses under cyclic loading are not yet understood, the project will try to answer this question by carrying out experimental tests in order to identify the cyclic constitutive behaviour of strands subjected to combined bending and compression loadings. Meanwhile a modeling task will be carried out in order to formulate a homogenized cyclic constitutive law at the scale of a strand, taking into account its composite internal structure. These developments will be implemented in the simulation code to predict the macroscopic behaviour of the global conductor.

An experimental setup will be implemented in order to identify the electro-mechanicall behaviour of strands and small cables, at cryogenic temperature, and under magnetic field. These tests will allow characteristics of new kinds of strands to be identified, will provide input data for the electrical simulation code, and will validate the global approach by measuring the actual electrical performances of test cables.

The cluster formed by the various skills gathered in the project in relation with the different scales and features of the studied conductors, should be able to handle issues related to the different applications of superconducting cables, from their initial forming up to their in-service operation.