D. Arai

D. Miyagi

K. Takahata

M. Tsuda

T. "Morimura

T." Obana

National Institute for Fusion Science (NIFS-Repository)

I. IntroductionSuperconducting coils used for fusion reactors andSMES are formed from a Cable-in-Conduit-Conductor (CICC)1). The CICC is made of many twisted strands andhas superior characteristics such as large current capacity,high mechanical strength, and so on. However, it has been observed that the critical current of CICC was lower than the expected one. One of the reasons is unbalanced current distribution caused by inhomogeneous contact resistances between a copper sleeve and strands at a joint called “wrap joint”. We measured the contact resistance between the copper sleeve and each strand at a joint which simulated thewrap joint. The non-uniformity of contact resistancesbetween the copper sleeve and the strands was observed.II. Measurement of contact resistance between a copper sleeve and each strandTable 1 shows the Specifications of a CICC sample with wrap joint2). Fig. 1 shows the schematic view of the measurement sample and circuit. A thin indium sheet was wrapped around the cable that the conduit was removed.The wrap of indium sheet simulated the solder coated in a real wrap joint. Then, the copper sleeve was installed on the cable with the indium sheet. We measured the contact resistance between the copper sleeve and each strand using the four-terminal method at the liquid helium temperature (4.2 K). The current was set to 6.0 A.Fig.2 shows the contact resistance distribution between the copper sleeve and the strands. Zeroresistancein Fig.2 means that we could not measure the contact resistance due to too small voltage. The number of strands with the zeroresistance was 146 and the non-uniformity of the contact resistances was observed. This means that the strands with the zeroresistance were in contact with the copper sleeve directly and the other strands didn’t makecontact with the copper sleeve directly. The high resistance was caused by the contact resistance between the strands.In order to verify the validity of our measurement, weanalyzed the contact condition between the copper sleeveand the strands using the data of strand paths measured in the other CICC sample1). We judged the contact condition by comparing the coordinates between the copper sleeve and each strand at each cross-section of the CICC sample.We evaluated the number of strands making contact with the copper sleeve in the wrap joint of 75mm in length. In the analysis, 150 strands made contact with the copper sleeve.The number of the strands is almost the same with the measured one, 146 strands. Therefore, the resistance distribution between the copper sleeve and the strandsdepended on the contact condition between the copper sleeve and the strands at the joint. This result means thatsuch the non-uniformity of the contact resistances results in a non-uniform current distribution in a CICC.III. ConclusionWe evaluated a contact resistance distribution between a copper sleeve and strands at the joint which simulated a wrap joint. The contact resistance distributionwas inhomogeneous. This means that the current distribution in the wrap joint becomes unbalanced due to the non-uniform contact resistance distribution. Furthermore we analyzed the contact condition between the copper sleeve and the strands using the data of strands paths measured in the other CICC. The number of the strands which directly made contact with the copper sleeve in the analysis was nearly equal to that of experiment. The contact resistance distribution depended on the contact condition between the copper sleeve and the strands. Cable structure should be arranged to realize that all strands make contact with acopper sleeve to suppress inhomogeneous contact resistance distribution between a copper sleeve and strands.1) Morimura, T. et al.: The Papers of Technical Meeting on “Application of Superconductivity”, IEE Japan, (2013)ASC-13-018. (in Japanese)2) Morimura, T. et al.: The 2013 Annual Meeting of the Institute of Electrical Engineers of Japan, (2013) 5-158. (in Japanese)Table 1 Specifications of a CICC sample with wrap joint.CIC conductornumber of strands 486strand diameter [mm] 0.89cable length [mm] 210.0cable shape [mm] 20.5?24.8Copper sleevesleeve length [mm] 75.0sleeve shape [mm] 18.8?23.0Indium sheet thickness [m] 50.0Fig.1 Schematic view of a measurement system.0 0.5 1 1.5050100150Resitance between the strands and the copper sleeve[      ]Number of strandsCurrent = 6 AFig.2 Resistance distribution between a copper sleeve and strands in a CICC sample.291§4. Analysis of Joint-resistance between Copper Sleeve and Strands in  Cable-in-Conduit ConductorMorimura, T., Miyagi, D., Arai, D., Tsuda, M. (Tohoku Univ.),  Takahata, K., Obana, T.

Analysis of Joint-resistance between Copper Sleeve and Strands in Cable-in-Conduit Conductor

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