Coupling loss with long time constants due to large displacement of strands in a large CIC conductor

ac losses consist of both regular losses that are proportional to squared cable twisting pitch and irregular losses that could not be estimated from short conductor sample test results. It was explained from our previous works that irregular loops in the conductor, which are caused by strand displacement as a result of low void fraction of the CIC conductor, produce the losses with long time constants up to several hundred seconds. The observed long time constant indicates that the typical loop length should be about LCM (least common multiplier) of all sub-staged cable pitches, and that contact conditions between the two strands forming the loop should be line contact. In order to investigate the contact conditions in detail, we traced 81(= 3 × 3 × 3 × 3) strands at intervals of 11 mm along of CIC sample conductor with 1 m in length whose strands are NbTi/Cu without any surface coating. The measured traces of 81 strands show that asymmetric strand positions, in other words, small and large displacements of strands from their original positions due to compressing the conductor provide many line contacts. It is found that the averaged line contact length reaches about 10 mm that is three orders of magnitude larger than the point contact length, i.e. 10^-2 mm. The time constant of coupling loss is also shown in this paper by estimating the line contact resistance and inductance of current loop.