Technical Papers and Presentations

Disciplines such as neutron polarimetry and medical imaging depend on highly uniform magnetic fields to preserve target polarization, and have turned to "magic boxes" to reliably generate these fields. Searches for the permanent Electric Dipole Moment of the neutron (nEDM) are sensitive to systematic errors introduced by magnetic field gradients, and are similarly interested in "magic box" magnets: magnetized shells of a high-permeability alloy (ASTM A753 Alloy 4). Magic boxes earned their name because their performance so outclassed what was predicted by their builders' early finite element simulations. It is possible to generate a more accurate model with the COMSOL Multiphysics^® simulation software's AC/DC Module and LiveLink™ for MATLAB^®.

The search for nEDM at Los Alamos National Laboratory is developing a multi-layered 3.5[m] cube-shaped shielded room, with a 2.4[m] interior. nEDM measurements are conducted at 1[uT], with a maximum allowable relative magnetic gradient of 1e-4[1/m] across the entire experimental volume. One proposed source of this B₀ holding field is the innermost shielding layer in a sealed magic box configuration. Generating a sufficiently uniform field with the necessary access penetrations requires several current sheets of different widths and strength.

A magic box model with multiple current-carrying regions presents a huge parameter space, and finding a globally optimal configuration with a single optimizer is very difficult. Instead, by using LiveLink™ for MATLAB^® and a COMSOL Multiphysics^® Floating Network License, one can optimize that geometry with a genetic algorithm, where each generation's configurations are solved in parallel, and the number of simultaneous solvers is RAM-limited. On a modern workstation, a reasonably complex model with 9 free parameters may be optimized in a day or two.

Combining these tools allows us to determine which parameters have the strongest influence on the resulting magnetic field's quality, and is helping guide the design and construction of next-generation magnetically shielded rooms.