Helium magnetometers

Abstract

Optically pumped helium (4He,3He) magnetometers have provided magnetic field data for military, space exploration and geophysical laboratory applications for over five decades. More recently they are increasingly being used for experiments in basic research. The characteristics of He magnetometers that have made them instruments of choice for these varied applications include high sensitivity, high accuracy, simplicity of the resonance line, small heading errors due to light shifts, temperature independence of resonance cells, linear relationship between the magnetic field and the resonance frequency, excellent stability for gradiometer operation and robustness for field and space use. All He magnetometers manufactured from 1960 to 1990 utilized an RF electrodeless discharge He-4 lamp as an optical pumping source of 1083 nm resonance radiation. With the invent of optical fiber lasers at 1083 nm from the 1990s on, laser-pumped He magnetometers are characterized by sensitivities up to two orders of magnitude better than lamp-pumped He magnetometers and are more accurate, smaller, and very stable for use in magnetic gradiometers. A quantum step forward in terms of precision was achieved by utilizing the benefits of free spin precession. For polarized helium-3 the coherent spin precession time T*2 can reach up to 100 h at low magnetic fields and even at high magnetic fields (> 0.1 T) nuclear spin precession times of ~5 min have been reported. This opens a new chapter of ultra-high precision magnetometry where the signal readout is accomplished by using SQUIDs, optical pumped alkalimagnetometers or NMR techniques. The following article provides a comprehensive overview on helium magnetometry starting from some historical remarks to the latest developments including future perspectives.