Experimental Investigation on the Effect of Transverse Electromagnetic Force on the V-T Curve of the CIC Conductor

Abstract

The voltage-temperature characteristic curve (V-T curve) observed in the large-current Nb/sub 3/Sn CIC conductor, which was used in the ITER CS Insert, showed more gradual take-off toward normal state than that of an individual strand composing the conductor. More gradual take-off corresponds to a reduction in so-called n-value, and measured n-values of the strand and conductor of the CS Insert were 30 and 7, respectively. This reduction cannot be explained by a tensile strain of the conductor caused by a hoop deformation which is uniform along the conductor length. Investigation is therefore required to clarify the strain states of each strand, especially those caused by a transverse electromagnetic force acting on each strand. In a CIC conductor, since strands are twisted to form a cable, each strand is mechanically supported by nearby strands at an interval (typically 5 mm) set by the twist pitch. Between two supporting points, the strand is free to move under the transverse force and a cyclic deformation may occur along the strand length. This deformation will produce nonuniform bending strain along the strand. In order to verify the above consideration and to quantitatively evaluate the effect of this deformation, we prepared an apparatus to simulate this cyclic deformation by artificially applying a transverse load on the strand and its V-T characteristic was measured. When the strand received the transverse force of 10/sup 3/ /spl sim/10/sup 4/ N/m (which is expected value for a strand of the CS Insert operated at 13 T, 46 kA), n-value reduced to less than 15 from the original value of 30, which agreed to the phenomena observed in the CS Insert. This indicates that the transverse force acting on each strand causes the reduction in n-value of the CIC conductor.