M. Eisterer

Neutron irradiation of MgB2 bulk superconductors

Sintered samples of MgB2 were irradiated in a fission reactor. Defects in the bulk microstructure are produced during this process mainly by the 10B(n,α)7Li reaction while collisions of fast neutrons with the lattice atoms induce much less damage. Self-shielding effects turn out to be very important and lead to a highly inhomogeneous defect distribution in the irradiated samples. The resulting disorder enhances the normal state resistivity and the upper critical field. The irreversibility line shifts to higher fields at low temperatures and the measured critical current densities increase following irradiation.

Unusual effects of anisotropy on the specific heat of ceramic and single crystal MgB2

Abstract The two-gap structure in the superconducting state of MgB 2 gives rise to unusual thermodynamic properties which depart markedly from the isotropic single-band BCS model, both in their temperature- and field dependence. We report and discuss measurements of the specific heat up to 16 T on ceramic, and up to 14 T on single crystal samples, which demonstrate these effects in the bulk. The behavior in zero field is described in terms of two characteristic temperatures, a crossover temperature T c,π ≅13 K, and a critical temperature T c = T c,σ ≅38 K, whereas the mixed-state specific heat requires three characteristic fields, an isotropic crossover field μ 0 H c2,π ≅0.35 T, and an anisotropic upper critical field with extreme values μ 0 H c2,σ, c ≅3.5 T and μ 0 H c2,σ, ab ≅19 T, where the indexes π and σ refer to the three-dimensional and two-dimensional sheets of the Fermi surface. Irradiation-induced interband scattering tends to move the gaps toward a common value, and increases the upper critical field up to ∼28 T when T c ≅30 K.

'Magnetoscan': a modified Hall probe scanning technique for the detection of inhomogeneities in bulk high temperature superconductors

We present a novel technique for the investigation of local variations of the critical current density in large bulk superconductors. In contrast to the usual Hall probe scanning technique, the sample is not magnetized as a whole before the scan, but locally by a small permanent magnet, which is fixed near the Hall probe, during the scanning process. The resulting signal can be interpreted as a qualitative measure of the local shielding currents flowing at the surface.

Influence of the upper critical-field anisotropy on the transport properties of polycrystalline MgB2

The intrinsic properties of MgB2 form the basis for all applications of this superconductor. We wish to emphasize that the application range of polycrystalline MgB2 is limited by the upper critical field Hc2 and its anisotropy. In wires or tapes, the MgB2 grains are randomly oriented or only slightly textured and the anisotropy of the upper critical field leads to different transport properties in different grains, if a magnetic field is applied and the current transport becomes percolative. The irreversibility line is caused by the disappearance of a continuous superconducting current path and not by depinning as in high-temperature superconductors. Based on a percolation model, we demonstrate how the changes of the upper critical field and its anisotropy and how the changes of flux pinning will influence the critical currents of a wire or a tape. These predictions are compared to results of neutron irradiation experiments, where these parameters were changed systematically.

Specific heat of MgB2 after irradiation

We studied the effect of disorder on the superconducting properties of polycrystalline MgB2 by specific-heat measurements. In the pristine state, these measurements give a bulk confirmation of the presence of two superconducting gaps with 2Δ0/kBTc = 1.3 and 3.9 with nearly equal weights. The scattering introduced by irradiation suppresses Tc and tends to average the two gaps although less than predicted by theory. We also found that by a suitable irradiation process by fast neutrons, a substantial bulk increase of dHc2/dT at Tc can be obtained without sacrificing more than a few degrees in Tc. The upper critical field of the sample after irradiation exceeds 28 T at T → 0.

Enhanced transport currents in Cu-sheathed MgB2 wires

Copper-sheathed MgB2 wires, prepared by an in-situ process, were exposed to neutron radiation in order to introduce defects into the superconductor. The high level of disorder (4.6 × 10−2 dpa) leads to a decrease of the transition temperature by more than 4 K, but to an increase of the slope of the irreversibility line, thus resulting in higher irreversibility fields at low temperatures. The transport currents are significantly enhanced at 4.2 K for fields above 2 T.

Specific heat of MgB_2 after irradiation

We studied the effect of disorder on the superconducting properties of polycrystalline MgB2 by specific-heat measurements. In the pristine state, these measurements give a bulk confirmation of the presence of two superconducting gaps with 2Δ0/kBTc = 1.3 and 3.9 with nearly equal weights. The scattering introduced by irradiation suppresses Tc and tends to average the two gaps although less than predicted by theory. We also found that by a suitable irradiation process by fast neutrons, a substantial bulk increase of dHc2/dT at Tc can be obtained without sacrificing more than a few degrees in Tc. The upper critical field of the sample after irradiation exceeds 28 T at T → 0.

Overview of Progress on the EU DEMO Reactor Magnet System Design

The DEMO reactor is expected to be the first application of fusion for electricity generation in the near future. To this aim, conceptual design activities are progressing in Europe (EU) under the lead of the EUROfusion Consortium in order to drive on the development of the major tokamak systems. In 2014, the activities carried out by the magnet system project team were focused on the toroidal field (TF) magnet system design and demonstrated major achievements in terms of concept proposals and of consolidated evaluations against design criteria. Several magnet system R&D activities were conducted in parallel, together with broad investigations on high temperature superconductor (HTS) technologies. In this paper, we present the outcomes of the work conducted in two areas in the 2014 magnet work program: 1) the EU inductive reactor (called DEMO1) 2014 configuration (power plant operating under inductive regime) was the basis of conceptual design activities, including further optimizations; and 2) the HTS R&D activities building upon the consolidated knowledge acquired over the past years.

Assessment of the local supercurrent densities in long superconducting coated conductors

The authors report on measurements of the local supercurrent density in long Y1Ba2Cu3O7−δ based coated conductors by the magnetoscan technique. Significant inhomogeneities were found, which are well resolved by the resulting magnetic field map. A single central line scan along the length of the conductor reflects the inhomogeneities over the entire width of the sample, thus offering the possibility of very fast characterization. Modifying the applied field leads to different results highlighting either the overall critical current or details of the defect structure. In addition, numerical simulations of the current dynamics were carried out for a qualitative and quantitative interpretation of the results.

Thickness dependence of the critical current density in superconducting films: A geometrical approach

We analyze the influence of the magnetic field generated by the supercurrents (self-field) on the current density distribution by numerical simulations. The thickness of the superconducting film determines the self-field and consequently the critical current density at zero applied field. We find an equation, which derives the thickness dependence of the critical current density from its dependence on the magnetic induction. Solutions of the equation reproduce numerical simulations to great accuracy, thus enabling a quantification of the dependence of the self-field critical current density with increasing film thickness. This result is technologically relevant for the development of coated conductors with thicker superconducting layers.