Designing Gradient Coils for the LANL nEDM Experiment

The LANL nEDM experiment seeks to measure the neutron electric dipole moment to a sensitivity of δdn~3x10-27e-cm using ultra-cold neutrons (UCN). The experiment measures the precession of spin-polarized UCN in a precisely applied vertical magnetic field. To achieve this, the experiment will use mu-metal shielding, along with secondary coils that can cancel out any ambient field. To design the secondary coils, we use bfieldtools, a Python library that uses stream functions to compute an ideal set of coils to generate the target magnetic field. The target magnetic fields include gradients decomposed into spherical harmonic functions, which provide a complete orthogonal basis of functions, a linear combination of which can cancel out any arbitrary magnetostatic potential that may occur. We generate two sets of coils for each spherical harmonic gradient- one that ignores any external shielding coupling, and another that assumes perfect shielding(infinite permeability)- which will be used together in a ratio dependent on the coupling to the mu-metal shielding. The generated coils are then exported using a MATLAB script to COMSOL, where their target field is calculated and any deviation can be quantified. I will be presenting details of the design workflow.