Nanoscale Nonlinear Radio Frequency Properties of Bulk Nb : Origins of Extrinsic Nonlinear Effects

TL;DR

This paper develops a cryogenic microwave microscope with a magnetic write head to map local nonlinear RF properties of bulk niobium, revealing nano-scale weak links affecting superconducting RF cavity performance.

Contribution

It introduces a novel localized microwave measurement technique using a magnetic write head to identify nano-scale defects in bulk niobium at cryogenic temperatures.

Findings

Localized strong RF magnetic field on bulk Nb surface (>102 mT)

Detection of nonlinearity from nano-scale weak link Josephson junctions

Mapping of electrodynamic response at multi-GHz frequencies

Abstract

The performance of Niobium-based Superconducting Radio Frequency (SRF) particle accelerator cavities can be sensitive to localized defects that give rise to quenches at high accelerating gradients. In order to identify these material defects on bulk Nb surfaces at their operating frequency and temperature, a wide bandwidth microwave microscope with localized and strong RF magnetic fields is developed by integrating a magnetic write head into the near-field microwave microscope to enable mapping of the local electrodynamic response in the multi-GHz frequency regime at cryogenic temperatures. This magnetic writer demonstrates a localized and strong RF magnetic field on bulk Nb surface with $B_{surface}$ > 102 mT and sub-micron resolution. By measuring the nonlinear response of the superconductor, nonlinearity coming from the nano-scale weak link Josephson junctions due to the contaminated surface in the cavity fabrication process is demonstrated.