Field limit and nano-scale surface topography of superconducting radio-frequency cavity made of extreme type II superconductor

TL;DR

This paper investigates how nano-scale surface topography affects the maximum achievable field in superconducting RF cavities made of extreme type II superconductors, providing analytical formulas to estimate the suppression of the superheating field due to nano-defects.

Contribution

It introduces an analytical model for the suppression factor of the superheating field caused by nano-defects, applicable to bulk and multilayer superconductors, validated with Nb cavity data.

Findings

Nano-defects significantly reduce the superheating field.

The model's predictions align with experimental record fields.

Surface processing techniques influence the cavity's maximum field.

Abstract

The field limit of superconducting radio-frequency cavity made of type II superconductor with a large Ginzburg-Landau parameter is studied with taking effects of nano-scale surface topography into account. If the surface is ideally flat, the field limit is imposed by the superheating field. On the surface of cavity, however, nano-defects almost continuously distribute and suppress the superheating field everywhere. The field limit is imposed by an effective superheating field given by the product of the superheating field for ideal flat surface and a suppression factor that contains effects of nano-defects. A nano-defect is modeled by a triangular groove with a depth smaller than the penetration depth. An analytical formula for the suppression factor of bulk and multilayer superconductors are derived in the framework of the London theory. As an immediate application, the suppression factor of the dirty Nb processed by the electropolishing is evaluated by using results of surface topographic study. The estimated field limit is consistent with the present record field of nitrogen-doped Nb cavities. Suppression factors of surfaces of other bulk and multilayer superconductors, and those after various surface processing technologies can also be evaluated by using the formula.