# High Frequency Nonlinear Response of Superconducting Cavity-Grade Nb surfaces

## TL;DR

This study investigates the microscopic nonlinear responses of superconducting Nb surfaces using a novel microwave microscope, revealing intrinsic and extrinsic effects that could explain SRF cavity breakdown at lower-than-expected gradients.

## Contribution

It introduces a new measurement technique and models to analyze the nonlinear microwave response of Nb surfaces, providing insights into SRF cavity failure mechanisms.

## Key findings

- Periodic and non-periodic nonlinear responses observed
- RSJ model explains periodic response well
- Vortex semi-loops account for non-periodic response

## Abstract

Nb Superconducting Radio-Frequency (SRF) cavities are observed to break down and lose their high-Q superconducting properties at accelerating gradients below the limits imposed by theory. The microscopic origins of SRF cavity breakdown are still a matter of some debate. To investigate these microscopic issues temperature and power dependent local third harmonic response was measured on bulk Nb and Nb thin film samples using a novel near-field magnetic microwave microscope between 2.9K-10K and 2GHz-6GHz. Both periodic and non-periodic response as a function of applied RF field amplitude are observed. We attribute these features to extrinsic and intrinsic nonlinear responses of the sample. The RF-current-biased Resistively Shunted Junction (RSJ) model can account for the periodic response and fits very well to the data using reasonable parameters. The non-periodic response is consistent with vortex semi-loops penetrating into the bulk of the sample once sufficiently high RF magnetic field is applied, and the data can be fit to a Time-Dependent Ginzburg-Landau (TDGL) model of this process. The fact that these responses are measured on a wide variety of Nb samples suggests that we have captured the generic nonlinear response of air-exposed Nb surfaces.