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

Abstract

Abstract. 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 center 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.

Measurement of the magnetic field inside the holes of a drilled bulk high-Tcsuperconductor

“…Such characteristics are the consequence of strong pinning in the sample which leads to a hysteresis between the Measurement of the magnetic field inside the holes of a drilled bulk HTS 15 applied field and the local induction in the hole. That behaviour is qualitatively consistent with the Bean model [34].…”

“…The distribution of the magnetic field at both surfaces is different from what would be expected in a sample without defects [34]. The peculiar distribution observed at both surfaces most likely arises from cracks, defects or disoriented grains [27,35].…”

Modification of the trapped field in bulk high-temperature superconductors as a result of the drilling of a pattern of artificial columnar holes

“…The two holes on the second line can be arranged in two different ways: (I) either they are aligned with the holes of the first line and form the beginning of a rectangular lattice, or (II), the holes are shifted with respect to those of the first line by half a separation (0.8 mm) and start forming a centered rectangular lattice. In this latter case, the centres of the holes of the second line are in fact placed on the discontinuity lines associated with the holes of the first line [12]. A picture of the two samples obtained after the drilling process is shown in Figure 1.…”

“…This relation can be integrated numerically under the constraint that the penetration length ℓ be minimal at each point P (or in other words, the shortest penetration path be selected for each point P ) [12]. The magnetization can then be deduced from the resulting field distribution.…”

“…In previous works, we modelled the current stream lines in the presence of holes and calculated the trapped field in drilled samples of finite or infinite heights [12,13]. It was shown that, for a given critical current density, the maximum of the trapped magnetic field is always smaller in a drilled sample than in a non-drilled one.…”