Field-dependent surface resistance of a superconducting RF cavity caused by surface impurity

TL;DR

This paper investigates the field-dependent surface resistance in superconducting RF cavities, linking Q-slope phenomena to surface impurities and proposing a model to evaluate surface treatments' effects on cavity performance.

Contribution

It introduces a surface-impurity model explaining Q-slope in SRF cavities and analyzes experimental data from various surface treatments to validate the model.

Findings

The model successfully fits experimental data for different Q-slope types.

Surface treatments significantly influence the field-dependent surface resistance.

The model can evaluate and optimize surface treatment effectiveness.

Abstract

Q-slope issue, which is caused by the field dependent surface resistance, puzzled people for a long time in SRF fields. In this paper, we related the Q-slope with surface treatments; and proposed a surface-impurity model to explain the field-dependent of surface resistance of SRF cavities. Eighteen cavity-test results have been analyzed to examine the model. These cavities were treated by different recipes: Nitrogen-doping; BCP and HF-rinsing; EP with 120{\deg}C baking; and EP without 120{\deg}C baking. The performance of these cavities, which is normally represented by cavity quality factor versus accelerating gradient or surface magnetic field curves (Q0 vs. Eacc or Q0 vs. B), has included all types of Q-slope, such as Low-field Q-slope, Medium-field Q-slope, and Anti-Q-slope. The data fittings are quite successful; the fitting results will be shown. The model can be used to evaluate the effectiveness of the surface treatments. At last, the paper discussed the way to build a high-Q high-gradient SRF cavity.