The influence of temperature on levitation properties of CC-tape stacks

Abstract

This article presents the study of the temperature effect on the levitation characteristics of stacks of CC-tapes from various manufacturers SuperOx, Theva and Sunam. Tapes differ in the values of the critical current density, architecture, and parameters of the metal substrate. Stacks of tapes were assembled from 12 by 12 mm square pieces of CC-tape. The number of elements in the stacks varied from 5 to 70. The dependences of the levitation force F_z (the vertical component of the force in the system permanent magnet—HTSC stack) on temperature were obtained both in zero field cooling (ZFC) and in field cooling (FC) modes. The measurements were carried out in the temperature range of 32–100 K. We have established that, down to the minimum operating temperatures, a decrease in temperature leads to a continuous increase in levitation force in ZFC mode. In FC mode, the F_z value tends to saturate after the initial increase in force. Cooling below the boiling point of liquid nitrogen up to 65 K gives a levitation force gain of 5%–10%. At the same time, cooling to 35 K gives an increase in levitation force by 7%–23%. It was found that an increase in the number of elements in the stack from 5 to 70 leads to an increase in the levitation force with a tendency to saturation for all temperatures studied. Excellent agreement between the experimental and calculated dependences of the levitation force on temperature and the number of elements in the stacks is demonstrated. Calculations show that for Sunam tapes, 90% of the levitation force is achieved with a stack thickness of 100 tapes at the boiling point of liquid nitrogen, and with 70 tapes at lower temperatures. The distributions of the magnetic field during the approach/removal of magnets, as well as the distribution of the trapped magnetic flux at different temperatures and numbers of elements in the stacks are modelled.