UBC Theses and Dissertations
Computational annealing of superconducting radio-frequency cavities Muller, Norman
The performance of Niobium (Nb) superconducting radio frequency (SRF) cavities is extremely sensitive to defects near the surface. These imperfections include dislocations, interstitial solutes, or grain boundaries which have been shown to limit the material’s ability to expel magnetic flux during the transitions to its superconducting state. This gives rise to detrimental thermal effects that limit the quality factor, and hence performance, of an SRF cavity. However, beside its importance in the performance of SRF cavities, little is known about the effect of annealing on the distribution of dislocations, specially near the surface. In this work, a mixed multiscale computational and experimental approach is developed to study the evolution dislocation density in pure Nb samples during annealing. The model is based on a Discrete Dislocation Dynamics (D.D.D.) technique that couples glide and climb motion of edge dislocations to simulate the evolution of dislocation density within the bulk of a Nb substrate as well as near the surface. The D.D.D. model integrates information from multiple scales. Accelerated molecular dynamics simulations were used to calculate the required activation energies for vacancy migration and hence fed our dislocation climb constitutive laws. Finite element modelling and experimental techniques were used as inputs to the D.D.D. implementation which accounted for the presence of residual stresses for both kink-pair nucleation dominated glide motion and vacancy migration driven climb. The model showed good agreement with other empirical methods. Furthermore, experimental results showed that the resistance of Nb samples at temperatures slightly above the critical temperature (T = 10K) can be coupled to the dislocation density. Hence, together the D.D.D. framework provided herein, can serve as a useful tool to determine an optimal annealing recipe for a given initial state of material condition.
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