Investigation of Trapped Magnetic Flux in Superconducting Niobium Samples
The operation costs of a superconducting radio frequency (SRF) cavity are driven by the losses in the cavity. While losses described by the BCS theory cannot be avoided, residual losses are caused by imperfections and contaminations in the material and need to be minimized. Although called residual, these losses can account for up to 50 % of the total losses so that their minimization can reduce the operation costs significantly.
One known contribution to the residual losses is the so called trapped magnetic flux. In an ideal superconductor an ambient magnetic field will be expelled when the materials becomes superconducting. Imperfections in the crystal lattice as well as contamination suppress this expulsion and trap the ambient field in the form of flux tubes. These flux tubes have normal conducting cores which cause residual losses.
In this thesis a setup was designed to study the effect of flux trapping depending on the treatment history of the material. Collaborators provided fine grain and single crystal niobium samples from SRF cavity production which then underwent typical cavity treatments such as chemical etching and heat treatments at different temperatures.
Figure 1 shows a picture of the set up consisting of a copper plate serving as holder for four samples, a pair of Helmholtz coils, a flux gate magnetometer and heaters attached to the sample holder. The apparatus was mounted in HoBiCaT and cooled down to 6 K. For measuring the trapped flux the samples were warmed up to the normal conducting state and then cooled down in an external magnetic field produced by the pair of Helmholtz coils. After cooling down the coils were switched off and the trapped field was measured directly with a fluxgate magnetometer. For each sample the field strength and the cool down conditions were varied to get more insight into the flux trapping mechanism.