Calculation of Magnetic Field Noise from High-Permeability Magnetic Shields and Conducting Objects with Simple Geometry

TL;DR

This paper presents analytical and numerical methods to evaluate magnetic field noise from high-permeability shields, aiding the design of low-noise environments for precision measurements.

Contribution

It introduces explicit formulas for magnetic noise in simple geometries using the fluctuation-dissipation theorem, and compares different noise sources and materials.

Findings

Analytical formulas match finite element analysis results.

Magnetic and non-magnetic conductors exhibit different noise characteristics.

A model predicts noise spectrum decay beyond the quasi-static regime.

Abstract

High-permeability magnetic shields generate magnetic field noise that can limit the sensitivity of modern precision measurements. We show that calculations based on the fluctuation-dissipation theorem allow quantitative evaluation of magnetic field noise, either from current or magnetization fluctuations, inside enclosures made of high-permeability materials. Explicit analytical formulas for the noise are derived for a few axially symmetric geometries, which are compared with results of numerical finite element analysis. Comparison is made between noises caused by current and magnetization fluctuations inside a high-permeability shield and also between current-fluctuation-induced noises inside magnetic and non-magnetic conducting shells. A simple model is suggested to predict power-law decay of noise spectra beyond quasi-static regime. Our results can be used to assess noise from existing shields and to guide design of new shields for precision measurements.