Comparison of AC losses, magnetic field/current distributions and critical currents ofsuperconducting circular pancake coils and infinitely long stacks using coatedconductors

Abstract

A model is presented for calculating the AC losses, magnetic field/current densitydistribution and critical currents of a circular superconducting pancake coil. Theassumption is that the magnetic flux lines will lie parallel to the wide faces of tapes inthe unpenetrated area of the coil. Instead of using an infinitely long stack toapproximate the circular coil, this paper gives an exact circular coil model usingelliptic integrals. A new efficient numerical method is introduced to yield moreaccurate and fast computation. The computation results are in good agreementwith the assumptions. For a small value of the coil radius, there is an asymmetryalong the coil radius direction. As the coil radius increases, this asymmetry willgradually decrease, and the AC losses and penetration depth will increase, but thecritical current will decrease. We find that if the internal radius is equal to thewinding thickness, the infinitely long stack approximation overestimates the loss by10% and even if the internal radius is reduced to zero, the error is still only60%. The infinitely long stack approximation is therefore adequate for most practical purposes.In addition, the comparison result shows that the infinitely long stack approximation savescomputation time significantly.