On the Position of the Apparent Current Center Inside CICC during External Magnetic Field Ramp

Abstract

We believe that ramp rate limitation phenomena (RRL) are strongly influenced by nonuniform current distribution between strands. To check this assumption, local field sensors such as Hall probes were used to study current redistribution inside of an ITER-type cable-in-conduit (CICC) subcable during fast field ramps.1 It is possible to determine the position of the apparent magnetic center of all currents in the cable by combining signals from at least two Hall sensors located in one cross-section around the CICC The apparent current center is a point where all current may be placed to create the same local field intensity measured at the sensors. We often found that the current center does not match the geometrical center of the cable. This definitely shows that the current inside the conduit is distributed non-uniformly Moreover, during magnetic field ramp the apparent current center demonstrates a sophisticated movement inside the CICC. Similar displacement of the current center was found in the sample whose conduit was tilled by oil and frozen at 4.2 K to exclude possible mechanical motion of the strands. Detailed experimental procedures and data about the position of the apparent current center are presented. A two-stream model is proposed to estimate the current distribution inside the conduit.