Magnetic field design in a cylindrical high-permeability shield: The combination of simple building blocks and a genetic algorithm

Magnetically sensitive experiments and newly developed quantum technologies with integrated high-permeability magnetic shields require increasing control of their magnetic field environment and reductions in size, weight, power, and cost. However, magnetic fields generated by active components are distorted by high-permeability magnetic shielding, particularly when they are close to the shield's surface. Here, we present an efficient design methodology for creating desired static magnetic field profiles by using discrete coils electromagnetically coupled to a cylindrical passive magnetic shield. We utilize a modified Green's function solution that accounts for the interior boundary conditions on a closed finite-length high-permeability cylindrical magnetic shield and determine simplified expressions when a cylindrical coil approaches the interior surface of the shield. We use an analytic formulation of simple discrete building blocks to provide a complete discrete coil basis to generate any physically attainable magnetic field inside the shield. We then use a genetic algorithm to find optimized discrete coil structures composed of this basis. We use our methodology to generate an improved linear axial gradient field, dB(z)/dz, and a transverse bias field, B-x. These optimized structures generate the desired fields with less than 1% error in volumes seven and three times greater in spatial extent than equivalent unoptimized standard configurations. This coil design method can be used to optimize activepassive magnetic field shaping systems that are compact and simple to manufacture, enabling accurate control of magnetic field changes in spatially confined experiments at low cost. (C) 2022 Author(s).