Sensitivity profile of the Storey–Purcell array

The Storey–Purcell array is an arrangement of four coaxial coils, composed of a Helmholtz pair nested inside a larger opposing Helmholtz pair whose respective area–turns products are sufficiently closely balanced to cancel the dipole moment of the combination when the magnitude of series current is the same in each coil pair. Ideally, the external field will exhibit 25‐pole character and falloff with distance as r −7. The array has desirable properties as a flux sensor because it is closely coupled to internal oscillating flux yet strongly decoupled to fluctuating external fields arising in distant sources. We have built an array and measured the external axial field component up to ten radii away. Our measurement technique relies on the principle of reciprocity for mutual inductance. Our analysis indicates area–turns unbalance of less than 500 ppm is required for the measured sensitivity profile to approach the calculated shape characteristic of a 25 pole. Comparison of measured field values to those calculated from the magnetic multipole expansion show our imperfect construction results in an area–turns unbalance of the order of 2.5% or 25 000 ppm.