Bulk high-Tc superconductors with drilled holes: how to arrange the holes to maximize the trapped magnetic flux?

Drilling holes in a bulk high-Tc superconductor enhances the oxygen annealing and the heat
exchange with the cooling liquid. However, drilling holes also reduces the amount of magnetic
flux that can be trapped in the sample. In this paper, we use the Bean model to study the
magnetization and the current line distribution in drilled samples, as a function of the hole
positions. A single hole perturbs the critical current flow over an extended region that is
bounded by a discontinuity line, where the direction of the current density changes abruptly. We
demonstrate that the trapped magnetic flux is maximized if the centre of each hole is positioned
on one of the discontinuity lines produced by the neighbouring holes. For a cylindrical sample,
we construct a polar triangular hole pattern that exploits this principle; in such a lattice, the
trapped field is ∼20% higher than in a squared lattice, for which the holes do not lie on
discontinuity lines. This result indicates that one can simultaneously enhance the oxygen
annealing, the heat transfer and maximize the trapped field.