The fixed probe storage ring magnetometer for the Muon g-2 experiment at Fermi National Accelerator Laboratory

TL;DR

The paper describes the design, construction, and performance of a fixed probe NMR magnetometer used to precisely monitor the magnetic field in the Muon g-2 experiment at Fermilab, achieving stability over 8 years.

Contribution

It introduces a novel fixed probe NMR magnetometer with 378 probes and custom electronics, enabling high-precision magnetic field monitoring for the Muon g-2 experiment.

Findings

Median single shot resolution of 650 ppb after 8 years

Stable magnetic environment measurement over long periods

Effective use of modular RF components in magnetometer design

Abstract

The goal of the FNAL E989 experiment is to measure the muon magnetic anomaly to unprecedented accuracy and precision at the Fermi National Accelerator Laboratory. To meet this goal, the time and space averaged magnetic environment in the muon storage volume must be known to better than 70 ppb. A new pulsed proton nuclear magnetic resonance (NMR) magnetometer was designed and built at the University of Washington, Seattle to track the temporal stability of the 1.45T magnetic field in the muon storage ring at this precision. It consists of an array of 378 petroleum jelly based NMR probes that are embedded in the walls of muon storage ring vacuum chambers and custom electronics built with readily available modular radio frequency (RF) components. We give NMR probe construction details and describe the functions of the custom electronic subsystems. The excellent performance metrics of the magnetometer are discussed, where after 8 years of operation the median single shot resolution of the array of probes remains at 650 ppb.