Rotation-Based Hall Probe Achieves 1 mrad Magnetic Field Direction Accuracy at Jefferson Lab

Researchers at Jefferson Lab constructed a rotation-based Hall probe device in which a spinning sensor generates an AC signal proportional to the transverse magnetic field component. Nulling this signal by aligning the rotation axis with the field vector directly indicates direction. The axis orientation was transferred to lab coordinates via a laser reflected from a mirror on the rotor. In the 3He target test the method met the milliradian requirement for polarized target experiments.

The approach eliminates the usual calibration steps required by static Hall probes and is inherently insensitive to gain drift. A second implementation replaced the rotating probe with a spinning field concentrator and stationary sensor, reaching 18 μG transverse resolution after 10 s averaging and 70 μrad directional precision in a 0.25 G field. Both versions were validated under realistic experimental conditions rather than in idealized bench tests.

Existing fluxgate and vector magnetometer arrays still dominate precision field mapping, yet require periodic recalibration and temperature compensation. The rotation method offers a low-cost, drift-free alternative that could be integrated into neutron electric dipole moment searches and precision beta-decay experiments now planning sub-mrad field alignment. Adoption will depend on mechanical stability of the rotor assembly over multi-day runs.

Future work should quantify long-term axis wander and test performance in fringe fields exceeding 100 G, conditions common in spectrometer magnets.