AC loss in superconducting composites: Continuous and discrete models for round and rectangular cross sections, and comparisons to experiments

In this work, AC loss in superconducting composites was described using both an anisotropic continuum model and a discrete model. The efforts were concentrated in three main areas. First, the eddy current coupling loss of composites with rectangular cross section was calculated using an anisotropic continuum description based on a block model with different effective resistivities in each block. In this case, a numerical approach was used. This treatment, like the more typical lumped component network model, was able to describe many factors influencing the eddy current loss in the rectangular composites, such as twist pitch, aspect ratio, and core resistivity. However, the influence of core thickness and the presence of an outer sheath were also described with this model. Certain simplifying assumptions were used here to minimize computation time, while allowing the essential information to be extracted.; In the second area, the eddy current loss of round composites were calculated from a discrete (network) point of view, and analytic expressions were developed which allow comparison to analytic expressions which were derived from effective medium theory. We need to measure only the contact resistance between the strands. The eddy current coupling loss of seven-strand MgB2 cables were then calculated by this model. With this model, it was possible to use a measured contact resistance between the strands to both predict the loss and compare to effective medium based resistivities. The results from the block model and from the analytical model give results in reasonable agreement.; In the third part of the work, we attempt to compare the developed expressions to experiment. In some cases, data extant in the literature were used; in other cases, direct measurements were performed. For the rectangular geometry composites, existing data were sufficient. In the case of round composites, direct experiments were performed. The specific working medium chosen was round, seven-strand MgB2 cables.; In comparing measurements to theory, it was necessary to add the hysteretic component of loss. In this case, it was also necessary to include the effect of magnetic shielding. It was shown that there are two kinds of hysteretic losses in these ferromagnetic/SC composites; the real loss and the apparent loss. Reduction of the real loss is always smaller than reduction of the apparent loss due to the shielding effect of sample signal with respect to the pick-up coil of VSM (Vibrating Sample Magnetometer).; In turn, the eddy current component for round composites was controlled by the contact resistance. The contact resistance was in some cases about two orders higher than the control samples when certain insulating surface treatments were performed on the surface of the individual strands of the cable. Loss predictions form contact resistant measurements were very low. However, actual loss measurements gave losses even lower, due to a low strand permeability.