A built-in coil system attached to the inside walls of a magnetically shielded room for generating an ultra-high magnetic field homogeneity

Tianhao Liu; Allard Schnabel; Jens Voigt; Wolfgang Kilian; Zhiyin Sun; Liyi Li; Lutz Trahms

doi:10.1063/5.0027848

A built-in coil system attached to the inside walls of a magnetically shielded room for generating an ultra-high magnetic field homogeneity

Rev Sci Instrum. 2021 Feb 1;92(2):024709. doi: 10.1063/5.0027848.

Authors

Tianhao Liu¹, Allard Schnabel¹, Jens Voigt¹, Wolfgang Kilian¹, Zhiyin Sun², Liyi Li³, Lutz Trahms¹

Affiliations

¹ Physikalisch-Technische Bundesanstalt Berlin, 10587 Berlin, Germany.
² Laboratory for Space Environment and Physical Sciences, Harbin Institute of Technology, 150001 Harbin, China.
³ Department of Electrical Engineering and Automation, Harbin Institute of Technology, 150001 Harbin, China.

PMID: 33648084
DOI: 10.1063/5.0027848

Abstract

The homogeneity of the magnetic field generated by a coil inside a magnetic shield is essential for many applications, such as ultra-low field nuclear magnetic resonance or spin precession experiments. In the course of upgrading the Berlin Magnetically Shielded Room (BMSR-2) with a new inserted Permalloy layer of side length 2.87 m, we designed a built-in coil consisting of four identical square windings attached to its inside walls. The spacings of the four windings were optimized using a recently developed semi-analytic model and finite element analysis. The result reveals a strong dependence of the field homogeneity on the asymmetric placement of the inner two windings and on the chosen material permeability value μ_s. However, our model calculations also show that these experimental variations can be counterbalanced by an adjustment of the inner winding positions in the millimeter range. Superconducting quantum interference device-based measurements yield for our implementation after fine adjustments of a single winding position a maximum field change of less than 10 pT for a total field of B₀ = 2.3 µT within a 10 cm region along the coil axis, which is already better than the residual field of the upgraded BMSR-2.1 after degaussing. Measurements of free spin precession decay signals of polarized Xe129 nuclei show that the transverse relaxation time for the used cell is not limited by the inhomogeneity of the new built-in coil system.