K. Hoashi

K. Takahata

M. Tsuda

T. Hamajima

T. Satow

Y. Kakusho

National Institute for Fusion Science (NIFS-Repository)

§ 26. Study on Irregular AC Losses in LargeCIC ConductorHamajima, T., Tsuda, M., Kakusho, Y., Hoashi, K.(Yamaguchi Univ. Faculty of Eng.)Satow, T., Takahata, K.In recent years there has been a growing interest inirregular AC losses that cannot be measured from shortconductor sample tests. The irregular AC losses with longtime constants were typically observed in a JapaneseSMES model coil, and the similar long time constantswere estimated in a poloidal superconducting coilconductor of Large Helical Device in National Institutefor Fusion Science in Japan. Current loops, which must beirregularly formed in the cable, decay with the long timeconstants, and hence enhance the AC losses. The loopscan induce an imbalanced current distribution in aconductor, and lead to RRL (ramp rate limitation), whichwas observed in DPC coils at Japan Atomic EnergyResearch Institute.In this research, we propose a mechanism forming thelong loops. The CIC conductor is composed of severalstaged sub-cables. If one strand on the surface of asub-cable contacts with the other strand on the surface ofthe adjacent sub-cable, the two strands should encountereach other again at LCM (Least Common Multiplier)distance of all staged cable pitches, and thereby result informing a pair of a long loop. There are a number of suchlong loops in the CIC conductor. The time constants ofthese loops are fundamentally described as ratios of theirinductances to their contact resistances. The inductance ofthe loop is estimated to be about 5 Jill for 3.6m LCM inthe SMES CIC conductor, about 15 for 113m LCM inthe LHD-IV conductor.In order to estimate the time constants t = L/R of thelong LCM loops, we measured the cross contact resistancebetween the two strands of the SMES and LHD in LHe.The test results showed that the cross contactingresistances were about 50 J!O for both conductors. Theresults suggest that the time constants are around 0.1 s forSMES and 0.3 s for LHD, which are shorter than themeasured time constants of 5 to 100 s.It is important to investigate the contact conditions andthe contours of the loops. We orderly labeled all strands ina real CIC conductor, disassembling carefully the cableafter peeling the conduit. The orderly labeled strandpositions on a cross section for the SMES conductor are118shown in Fig. 1. It is found that the strands in a triplex arewidely displaced from their original positions as indicatedby bold circles.The fraction of the triplets with largely displacedstrands is computed for the SMES conductor and theLHD-IS and OV conductors, as listed in Fig. 2. It is foundthat around 5 % of all triplets have largely displacedstrands. However, it is confirmed from the data on bothsides of the conductor about 1 m in length, that almost ofall the triplets with displaced strands on one side becomeregular triplets without displaced strands on the other side.This suggests that the largely displaced strands havelonger line contact length than point contact one. The linecontact length must be more than 10 mm, while the pointcontact radius is estimated to be about 10 from Hertztheory. This leads to contacts more than 3 order long, andexplains the observed long time constants.Moreover, the loops with largely displaced strandshave irregular flux linked with the field. The irregularlylinked fluxes cause the additional irregular AC losses.Fig.l. Labeled strands in a CIC conductor';; 10....Q)"'5.8 -- -_. .--."Q)0III 6"'5. r---i5 r--->.1) 4 - f----III....I..... 2 - -0c00 au. SMES LHD-IS LHD-OVFig. 2. Number of the large displaced triplets in CICCs.Reference1) Hamajima, T., et aI., IEEE Trans. on Appi. Supercond.11, (2001) 18602) T Hamajima, et aI., IEEE Trans. on Appi. Supercond.12, (2002) 1616

Study on Irregular AC Losses in Large CIC Conductor

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