Construction, Measurement, Shimming, and Performance of the NIST-4 Magnet System

TL;DR

This paper details the construction, measurement, and optimization of the NIST-4 magnet system, a key component of a watt balance, including field profiling, shimming techniques, and systematic effect estimates.

Contribution

It presents the detailed construction process, field measurement results, and shimming methods used to achieve a highly uniform magnetic field in the NIST-4 watt balance magnet system.

Findings

Radial magnetic flux density variation less than 10^-4 over 5 cm

Effective shimming techniques for flat field profile

Order of magnitude estimates for systematic effects

Abstract

The magnet system is one of the key elements of a watt balance. For the new watt balance currently under construction at the National Institute of Standards and Technology, a permanent magnet system was chosen. We describe the detailed construction of the magnet system, first measurements of the field profile, and shimming techniques that were used to achieve a flat field profile. The relative change of the radial magnetic flux density is less than $10^{-4}$ over a range of 5 cm. We further characterize the most important aspects of the magnet and give order of magnitude estimates for several systematic effects that originate from the magnet system.