Multi-mode Analysis of Surface Losses in a Superconducting Microwave Resonator in High Magnetic Fields

TL;DR

This study introduces a method to analyze surface losses in superconducting microwave resonators under high magnetic fields, revealing that end caps dominate quality factor degradation, which benefits high-field applications like dark matter detection.

Contribution

A novel multi-mode measurement technique was developed to decompose surface resistance contributions in superconducting cavities under high magnetic fields.

Findings

End caps are the primary source of surface resistance in high magnetic fields.

Parallel surface resistances of cavity walls remain relatively unaffected.

Results support the feasibility of high-Q superconducting cavities in strong magnetic environments.

Abstract

This paper reports on a surface impedance measurement of a niobium titanium superconducting radio frequency (SRF) cavity in a magnetic field (up to $10\,{\rm T}$). A novel method is employed to decompose the surface resistance contributions of the cylindrical cavity end caps and walls using measurements from multiple $TM$ cavity modes. The results confirm that quality factor degradation of a NbTi SRF cavity in a high magnetic field is primarily from surfaces perpendicular to the field (the cavity end caps), while parallel surface resistances (the walls) remain relatively constant. This result is encouraging for applications needing high Q cavities in strong magnetic fields, such as the Axion Dark Matter eXperiment (ADMX), because it opens the possibility of hybrid SRF cavity construction to replace conventional copper cavities.