Design and measurement of a magnetic field for precision spectroscopy of helium

Systematic error caused by stray magnetic fields is often one of the major sources of the uncertainties in many precision measurements. It is necessary to accurately control the magnetic fields in these measurements. The spectrum of the fine structure of helium can be used to determine the fine structure constant, and also to test the quantum electrodynamic theory of multi-electron systems. In this work, we present a magnetic shield that can eliminate stray magnetic fields and a cosine coil that generates a uniform magnetic field. The central magnetic strength produced by the coil is measured through the spectrum of a He-4 metastable atomic beam. The magnetic shield reduces the strength of the external magnetic field down to less than 0.8 mGs. The variation of the magnetic strength in the spectrum region is less than 10 mGs at a magnetic strength up to 20 Gs. In this case, the uncertainty introduced by the magnetic field will be less than in the spectrum of helium fine structure.