Sensitivity of Niobium Superconducting Cavities to Trapped Magnetic Flux Dissipation

TL;DR

This paper investigates how different niobium cavity preparations affect their sensitivity to trapped magnetic flux, revealing that surface treatment and mean free path influence RF losses in superconducting cavities.

Contribution

It demonstrates the dependence of trapped flux sensitivity on cavity surface treatment and mean free path, aligning experimental results with theoretical models.

Findings

Standard electropolished cavities have residual resistance sensitivity of ~0.6 nΩ/mG.

Nitrogen-doped cavities show higher sensitivity of 1 to 5 nΩ/mG.

Shorter mean free paths reduce sensitivity in the dirty limit, longer mean free paths reduce it in the clean limit.

Abstract

Future particle accelerators such as the the SLAC ``Linac Coherent Light Source-II'' (LCLS-II) and the proposed Cornell Energy Recovery Linac (ERL) require hundreds of superconducting radio-frequency (SRF) cavities operating in continuous wave (CW) mode. In order to achieve economic feasibility of projects such as these, the cavities must achieve a very high intrinsic quality factor ($Q_0$) to keep cryogenic losses within feasible limits. To reach these high $Q_0$'s in the case of LCLS-II, nitrogen-doping has been proposed as a cavity preparation technique. When dealing with $Q_0$'s greater than 1\e{10}, the effects of ambient magnetic field on $Q_0$ become significant. Here we show that the sensitivity to RF losses from trapped magnetic field in a cavity's walls is strongly dependent on the cavity preparation. Specifically, standard electropolished and 120$^\circ$C baked cavities show a residual resistance sensitivity to trapped magnetic flux of $\sim0.6$ and $\sim0.8$ n$\Omega$/mG trapped, respectively, while nitrogen-doped cavities show a sensitivity of $\sim$ 1 to 5 n$\Omega$/mG trapped. We show that this difference in sensitivities is directly related to the mean free path of the RF surface layer of the niobium: shorter mean free paths lead to less residual resistance sensitivity to trapped magnetic flux in the dirty limit ($\ell<<\xi_0$) while longer mean free paths lead to lower sensitivity in the clean limit ($\ell>>\xi_0$). These experimental results are also shown to have good agreement with recent theoretical predictions for pinned vortex lines oscillating in RF fields.