Trapped magnetic flux in superconducting niobium samples

Trapped magnetic flux is known to be one cause of residual losses in bulk niobium superconducting radio frequency cavities. In the Meissner state an ambient magnetic field should be expelled from the material. Disturbances such as lattice defects or impurities have the ability to inhibit the expulsion of an external field during the superconducting transition so that the field is trapped. We have investigated the effect the treatment history of bulk niobium has on the trapped flux and which treatment leads to minimal flux trapping. For that purpose, we measured the fraction of trapped magnetic flux in niobium samples representing cavities with different typical treatment histories. The differences between single crystal and polycrystalline material as well as the influence of spatial temperature gradients and different cooling rates were investigated. In addition, the progression of the release of a trapped field during warm-up was studied. We found that heat treatment reduces trapped flux considerably and that single crystal samples trap less flux than polycrystalline niobium. As a consequence, the single crystal sample with $1200°\mathrm{C}$ baking trapped the smallest amount of field which is about 42%. Moreover, the release of the trapped field during warm-up was observed to progress over a broad temperature range for the baked single crystal samples.

Figure 1

Comparison between the perfect Meissner effect and the suppression of the flux expulsion due to flux pinning.

Figure 2

Setup to scan trapped flux in Nb samples.

Figure 3

Position of the field probe above the sample.

Figure 4

Calculated magnetic field of a homogeneously magnetized disk at the probe position compared to the measured trapped field as a function of the radius.

Figure 5

Dependency of the trapped field on the applied field for all measured samples: None of the samples exhibited a dependency on the applied field. The data for samples 1 and 2 overlap—they both trap 100% of the applied field.

Figure 6

Dependence of the trapped field on the cooling rate for a single crystal sample with $800°\mathrm{C}$ tempering.

Figure 7

Release of the trapped field during warm-up: The tempered single crystal samples released the trapped field over a broad temperature range.

Figure 8

Release of different trapped fields during warm-up for a single crystal sample with $1200°\mathrm{C}$ tempering.

Figure 9

Sketch of the generation of a temperature gradient over one sample.

Figure 10

Progression of the temperature gradient and the magnetic field due to the Seebeck effect superimposed with the heater fields for both current directions. The heater was switched on at $t=120\mathrm{s}$ to establish a temperature gradient. It was switched off at $t=1120\mathrm{s}$.