Contribution to mid-field Q slope in Niobium SRF cavities from linear RF loss mechanism due to topographic surface structures

TL;DR

This paper investigates how surface topography affects RF losses in niobium SRF cavities, using PSD analysis and a perturbation model to quantify additional losses from different surface treatments.

Contribution

It introduces a linear RF loss model based on surface PSD analysis that quantifies topographic effects on cavity performance.

Findings

PSD varies with surface treatment methods.

The perturbation model estimates additional RF losses due to roughness.

Loss ratios can be calculated for different surface conditions.

Abstract

Topographic structure on Superconductivity Radio Frequency (SRF) surfaces can contribute additional cavity RF losses describable in terms of surface RF reflectivity and absorption indices of wave scattering theory. At isotropic homogeneous extent, Power Spectrum Density (PSD) of roughness is introduced and quantifies the random surface topographic structure. PSD obtained from different surface treatments of niobium, such Buffered Chemical Polishing (BCP), Electropolishing (EP), Nano-Mechanical Polishing (NMP) and Barrel Centrifugal Polishing (CBP) are compared. A perturbation model is utilized to calculate the additional rough surface RF losses based on PSD statistical analysis. This model will not consider that superconductor becomes normal conducting at fields higher than transition field. One can calculate the RF power dissipation ratio between rough surface and ideal smooth surface within this field range from linear loss mechanisms.