New method of current distribution studies for ramp rate stability of multistrand superconducting cables

Abstract

The Ramp Rate Limitation phenomena were studied using local field sensors to observe the intrinsic processes within the cable. Sensitive miniature Hall sensors and small pick-up coils placed around Cable-in-Conduit Conductor were used to measure local magnetic fields and field derivatives associated with currents in the cable. Using this method both fast jumps and slow changes in local magnetic fields at different conditions were observed. Fast jumps occured during ramping background magnetic field and may indicate a fast current redistribution processes. Slow changing of local fields may be associated with current loops closed through the current lead joints. Such current loops may also indicate the non-uniformity of current distribution in the cable strands. The new method is a promising tool for future investigations of stability of multistrand cables.

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

“…The unknown currents are the solution to the inverse identification problem. Assuming linear material properties with magnetic field, the governing system of linear equations can be written in a matrix form as follows: (1) where is the vector of known experimental data (in general case, magnitudes of the self magnetic field and external field), is the unknown vector of current amplitudes and is the coefficient matrix relating the currents to the measurement data. System (1) is typically under-determined (ill posed) and has no unique solution, as the number of probes is limited (6 12) compared to a number of strands in the CICC (typically more than 1000).…”

“…INCE the pioneering work [1] in 1995, Hall sensors (HS) and pick-up coils (PUC) were used in various experiments to detect the current redistribution process in superconducting cables, for instance [2], [3]. In the year 2000, the first measurements of the self-field profile were performed around sub-size cable-in-conduit conductors (CICC) [4] aimed for the ITER conductor development program.…”

Experimental study of the current redistribution in pulsed operation inside the Nb/sub 3/Sn CICC of an ITER relevant magnet

“…By means of numerous local miniature field sensors (Hall probes and pickup coils), located in a few positions along the conductor, an attempt was made to determine the current distribution inside the cable. Slow local field changes (SLFC) with time constants above 100-1000 s were observed [1], [2]. They could be associated with induced current loops closed through the cable joints.…”

“…Particularly, this was experimentally proven for small model cables [1]- [3]. By means of numerous local miniature field sensors (Hall probes and pickup coils), located in a few positions along the conductor, an attempt was made to determine the current distribution inside the cable.…”

Review on Boundary-Induced Coupling Currents

“…Ramp rate limitation phenomena, studied on cable-in-conduit conductors, were also explained by the existence of current loops or BICCs with time constants larger than 100 s passing through the low-resistance joints with the current leads 28,29 . It is concluded that the surplus current caused by the BICC quenches a strand or subcable leading to a quench of the entire cable if the cooling is not sufficient and the adjacent strands are not able to take the current of the quenching strand.…”