Ac losses in field-cooled type I superconducting cavities

TL;DR

This paper develops a theoretical model for ac losses in field-cooled type I superconducting cavities, identifying surface absorption as the main dissipation mechanism and comparing predictions with experimental data.

Contribution

It introduces a new theory for ac losses in type I superconductors with trapped flux, focusing on surface absorption effects, which was previously underexplored.

Findings

Surface absorption dominates ac dissipation in trapped flux regions.

Theoretical estimates agree with measured quality factor degradation.

Normal regions' surface interactions are key to understanding losses.

Abstract

As superconductors are cooled below their critical temperature, stray magnetic flux can become trapped in regions that remain normal. The presence of trapped flux facilitates dissipation of ac current in a superconductor, leading to losses in superconducting elements of microwave devices. In type II superconductors, dissipation is well-understood in terms of the dynamics of vortices hosting a single flux quantum. In contrast, the ac response of type I superconductors with trapped flux has not received much attention. Building on Andreev's early work [Sov. Phys. JETP 24, 1019 (1967)], here we show theoretically that the dominant dissipation mechanism is the absorption of the ac field at the exposed surfaces of the normal regions, while the deformation of the superconducting/normal interfaces is unimportant. We use the developed theory to estimate the degradation of the quality factors in field-cooled cavities, and we satisfactorily compare these theoretical estimates to the measured field dependence of the quality factors of two aluminum cavities.