Field and current distributions and ac losses in a bifilar stack of superconducting strips

In this paper, I first analytically calculate the magnetic-field and sheet-current distributions generated in an infinite stack of thin superconducting strips of thickness d, width $2a⪢d$, and arbitrary separation D when adjacent strips carry net current of magnitude I in opposite directions. Each strip is assumed to have uniform critical current density Jc, critical sheet-current density Kc=Jcd, and critical current Ic=2aKc, and the distribution of the current density within each strip is assumed to obey critical-state theory. I then derive expressions for the ac losses due to magnetic-flux penetration both from the strip edges and from the top and bottom of each strip and I express the results in terms of integrals involving the perpendicular and parallel components of the magnetic field. After numerically evaluating the ac losses for typical dimensions, I present analytic expressions from which the losses can be estimated.