Self-field effects upon the critical current density of flat superconducting strips

TL;DR

This paper presents a comprehensive theory for understanding how self-field effects influence the critical current density in flat superconducting strips, emphasizing the dominance of bulk pinning over edge effects in practical materials.

Contribution

The authors develop a self-consistent model for the critical current density considering field-dependent pinning, applying the Kim model to relate local flux density to transport current.

Findings

Jc(Ba) closely matches JpK(Ba) at high fields Ba > Ba*

Self-field effects significantly suppress Jc at low fields Ba < Ba*

Bulk pinning dominates over edge effects in typical superconducting films

Abstract

We develop a general theory to account self-consistently for self-field effects upon the average transport critical current density Jc of a flat type-II superconducting strip in the mixed state when the bulk pinning is characterized by a field-dependent depinning critical current density Jp(B), where B is the local magnetic flux density. We first consider the possibility of both bulk and edge-pinning contributions but conclude that bulk pinning dominates over geometrical edge-barrier effects in state-of-the-art YBCO films and prototype second-generation coated conductors. We apply our theory using the Kim model, JpK(B) = JpK(0)/(1+|B|/B0), as an example. We calculate Jc(Ba) as a function of a perpendicular applied magnetic induction Ba and show how Jc(Ba) is related to JpK(B). We find that Jc(Ba) is very nearly equal to JpK(Ba) when Ba > Ba*, where Ba* is the value of Ba that makes the net flux density zero at the strip's edge. However, Jc(Ba) is suppressed relative to JpK(Ba) at low fields when Ba < Ba*, with the largest suppression occurring when Ba*/B0 is of order unity or larger.