Review of Current Distribution Measurements and Reconstruction in Cable-in-Conduit Conductors for ITER

Abstract

Abstract-The cable-in-conduit conductors (CICC) for ITER magnets consist of more than thousand superconducting strands. Usually such cables exhibit a significant non-uniform current distribution between the strands due to inequality of strand impedances. This undesirable effect can complicate the measurement of the conductor critical transport properties, particularly for short samples. To improve the interpretation of measurement results, detailed knowledge of the current distribution is highly desired. We present a review of current unbalance measurements and current distribution reconstruction techniques in CICC as they were intensively performed during the last years. The experimental methods of current unbalance detection are described, starting with earlier attempts on sub-size CICCs and ending with more sophisticated methods on full-size conductors. The models of current identification developed by various research groups are briefly compared, emphasizing their similarities, differences and limitations. Finally, the results of current unbalance reconstructions performed by different groups on several full-size CICCs are discussed together with the implication of the results for ITER short sample conductor tests and coil performance.

Influence of the magnetic field profile on ITER conductor testing

“…It appears that the current distribution between the petals is within a maximum deviation of 35% from the average petal current (total transport current divided by six). The result is in line with the degree of current imbalance found in other experiments on full-size ITER type conductors [7,21]. An average deviation of about 30% (mostly between 25 and 50%) can be considered as a 'natural' current imbalance between the six petals due to nonuniform joints under normal manufacturing conditions.…”

Interpretation of conduit voltage measurements on the poloidal field insert sample using the CUDI–CICC numerical code

“…The result is in line with the degree of current unbalance found in Fitting parameters for scaling found by CEA [16] Fitting parameters other experiments on full size ITER type conductors. Table 3 summarizes the current unbalance identification results found by various research groups [18]. An average deviation of about 30% (mostly between 25% and 50%) can be considered as a ''natural'' current unbalance between the six petals due to non-uniform joints.…”

Current distribution monitoring enables quench and damage detection in superconducting fusion magnets

“… 30 outlines several techniques to improve the stability of the dense and ill-conditioned system. Previously, a similar technique based on Singular Value Decomposition (SVD) was employed to investigate current distributions in ITER cables 31 , however the high levels of current sharing did not permit the methodology employed here. It is possible to solve the least squares problem with a net transport current constraint (see Ref.…”