AC losses of HTS tapes and bundles with de-coupling barriers

Plastronic Inc. is working on a project for the Ballistic Missile Defence Organization to improve the strength and AC loss characteristics of Bi-based high temperature superconductors (HTS) for AC applications like motors, generators, and transmission cables. High strength, high amperage conductor is made by encapsulating bundles of HTS tapes with a nonsilver sheath. With this technique, Cu-alloy spacers can be placed between Bi-2223/Ag HTS tapes to de-couple the tapes under AC currents. AC losses were measured on conductor bundles with and without Cu-alloy spacers at AC currents up to 70 A and frequencies from 50 to 400 Hz. At frequencies up to about 100 Hz, it was found that the bundle with resistive barriers had AC loss equal to the sum of individual tapes as expected from the Norris ellipse model. At higher frequencies, the Cu-alloy barrier was not as effective in reducing the AC loss. For single tapes, Zr-oxide was used to coat the HTS filaments before stacking to form a multifilament tape. AC loss measurements of 6-filament tapes with and without the oxide coating showed little effect of the coating when the loss was normalized to the square of the critical current as the theory suggests.