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 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 3 10 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.

Effect of Periodic Cyclic Deformation on the Voltage Current Transition of<tex>$hboxNb_3hboxSn$</tex>Strands Tested in the Novel ‘TARSIS’ Setup

“…Qualitatively the results are in agreement with [10]. As far as it concerns a first order quantitative analysis, the results presented here are not obtained on a CSMC strand but can be interpreted with the estimated average bending amplitude in ITER Nb Sn CICC's.…”

<i>I</i>_c Analysis of Nb₃Sn Strand Cable-In-Conduit Conductor under the Electromagnetic Force by the Structural Mechanics

“…4 The histogram of beam length distribution. between experiment and calculation pointed out the bad effect of simple assumption [8]. We think it is necessary to introduce the precise information about beams.…”

“…Figure 7 describes the J c profile for each strands. According to the [8], 0.8% curvature means 50% degradation of I c . Then, we assume the curvature dependence of J c should be the following function,…”

Critical current measurement with spatial periodic bending imposed by electromagnetic force on a standard test barrel with slots

“…A drawback of this method is that the exact zero strain condition is defined to coincide with the first observation in I c decrease. 13 This approach may be argued since the initial bending of the wire goes with very low force and considering the results from other bending experiments, summarized above, we cannot exclude that the I c remains practically constant at low bending strain or may even show some increase. Adopting the initial change of the critical current as an indicator for the zero strain level may be misleading in the possible presence of strain relaxation counterbalancing a decrease in I c due to bending.…”

“…For applied transverse strain, we can distinguish between homogeneously distributed loads on a wire, being, for example, sandwiched between flat surfaces, 9,10 spatial periodically applied stress by using crossing strands, 11,12 and finally bending strain. For bending strain we can differentiate between spatial periodical loading with a wavelike bending pattern 13,14 and bending along a bend surface or cylinder with constant radius. [15][16][17][18][19] As bending strain in the Nb 3 Sn filaments is largely a distribution of axial strain over the wire cross section, knowledge of the I c versus axial strain dependency is required for analysis of the bending strain influence.…”