Noise Suppression and Balancing Experiments Using a Superconducting Magnetic Gradiometer

摘要

We present a noise suppression technique which allows detection of tiny fluctuating magnetic gradients in the presence of a very large dc gradient field. We describe our superconducting gradiometer, its response to rotations in an arbitrary magnetic field, and various external sources of noise in the gradiometer output. As ambient dc gradient field is shown to be identical to the gradients produced by some 'equivalent dipole' and gradient noise in the gradiometer from a nearby magnetic object is treated as arising from small perturbations in the position and orientation of the equivalent dipole. We describe an empirical alignment technique for aligning the gradiometer to suppress noise introduced by such gradient fluctuations and use it to demonstrate suppression of total gradiometer noise to essentially the level of inherent instrument noise at frequencies down to one milliHertz. A computer calculation which models the gradiometer noise as a function of gradiometer orientation reproduces essentially all features of experimental gradiometer noise measurements, and provides evidence that our model for the instrument's response to gradient fields is correct.