Abstract
The magnetic flux trapped inside high-purity niobium samples after field cooling was investigated by indirect magneto-optical imaging using an indicator film. The detected magnetic field was compared with the field present during the phase transition, and the distribution of trapped flux was investigated. The measurements were performed on an untreated sample cut from an ingot and a sample that was heated at 1400 °C for 4 h. For untreated niobium, the trapped flux was found to be homogeneously distributed and almost all applied field during phase transition was trapped. In contrast, the heat treated niobium showed significantly reduced trapping. Neither did the grain boundaries play a major role as pinning centers nor did the crystal orientation influence the amount of trapped flux significantly, which is contrary to recent assumptions. However, niobium hydrides, which formed during the cool-down-stage to cryogenic temperatures, were found to enhance trapping considerably.
Highlights
Superconducting materials have numerous applications in research
Using MO imaging in combination with electron backscatter diffraction (EBSD) mapping to identify the crystal orientation, we were able to distinguish the absolute amount of trapped magnetic flux for grains with different orientations and we evaluated the trapped flux at grain boundaries
A magnetic field of 10 mT was applied perpendicular to the sample
Summary
Superconducting materials have numerous applications in research. Superconducting wires enabled the high current densities needed for the bending and focusing magnets, e.g., in the Large Hadron Collider, and superconducting radio-frequency (SRF) cavities are keycomponents of numerous new synchrotron light sources such as the European X-Ray Free-Electron Laser and the Linac Coherent Light Source-II. Many challenges of operating superconductors are common between direct current (DC) and RF application, such as the supply of adequate cooling power. Superconducting wires, which carry DC currents, are operated in the Shubnikov phase where the vortices have to be pinned strongly in order to reach high critical current densities.. All magnetic flux should be expelled from the material during the phase transition into this state. Pinning centers inside the material provide energetically favorable locations that resist the free redistribution of vortices. The vortices can get trapped and dissipate power once exposed to the RF field.
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