Determination of Niobium Cavity Magnetic Field Screening via a Dispersively Hybridized Magnonic Sensor

TL;DR

This paper introduces a dispersively hybridized magnonic sensor method to measure the internal magnetic field in superconducting niobium cavities, revealing critical screening behavior and hysteresis effects relevant for quantum and dark matter applications.

Contribution

The study presents a novel dispersive measurement technique for internal magnetic fields in superconducting cavities, enabling precise detection of screening levels and vortex dynamics.

Findings

Measured the superheating critical field in Nb cavities.

Observed hysteresis due to trapped vortices.

Detected abnormal plateau behavior in cylindrical cavity.

Abstract

A method for determining the internal DC magnetic field inside a superconducting cavity is presented. The method relies on the relationship between magnetic field and frequency of the Kittel mode of a ferrimagnetic sphere, hybridised in the dispersive regime of the superconducting cavity. Results were used to experimentally determine the level of screening a superconducting Nb cavity provides as it changes from perfect diamagnetism to no screening. Two cavity geometries were tested, a cylinder and single post re-entrant cavity. Both demonstrated a consistent value of field that enters the cavity, expected to be the superheating critical field. Hysteresis in the screened field during ramp up and ramp down of the external magnetic field due to trapped vortices was also observed. Some abnormal behaviour was observed in the cylindrical cavity in the form of plateaus in the internal field above the first critical field, and we discuss the potential origin of this behaviour. The measurement approach would be a useful diagnostic for axion dark matter searches, which plan on using superconducting materials but need to know precisely the internal magnetic field.