Suitable method for increasing current carrying capacity of HTS toroidal coils for SMES and DC reactor

Non-uniform current distribution in a conductor composed of multiple HTS tapes is one of the important problems for increasing current carrying capacity of a large-scale coil system such as Superconducting Magnetic Energy Storage (SMES) and DC reactor. We focused on a manufacturing error in the HTS tape thickness and analytically investigated the relationship between the manufacturing error and the non-uniform current distribution. Even in a transposed conductor, the non-uniform current distribution is easily occurred by a small error in the tape thickness due to high coupling constant among the tapes. The non-uniformity of the current distribution reduces the current carrying capacity. However, the non-uniform current distribution can be improved by adopting a pancake coil with a large radius and small number of turns. An HTS tape with large thickness is also effective for reducing the non-uniformity of current distribution. Adopting parallel double pancake coils composed of single HTS tape is one of the promising candidates for increasing current carrying capacity effectively without any problems of the non-uniform current distribution. The suitable connecting method among the parallel double pancake coils for uniform current distribution, however, should be developed.