The Fabrication and Characterization of a Superconducting Birdcage Resonator

Abstract

In this thesis we presents the fabrication and characterization of a superconducting Birdcage style radiofrequency (RF) coil, a commonly used coil design in nuclear magnetic resonance (NMR) spectrometers. The Birdcage was fabricated from a series of superconducting Niobium thin film resonators, deposited onto sapphire wafer by sputtering deposition. The resonators were assembled into the 3 dimensional birdcage coil, and housed in a custom machined RF package. The fabrication, assembly tools, and procedures are presented. A numerical model, based on lumped-element eigenanalysis, was used to describe the resonance structure of the coil. We show that misalignment in the assembly can lead to an increase in the resonant frequency of the transverse mode when compared to the analytical description. A set of low temperature measurements were performed to characterize the quality factor, magnetic field, and power dependence. A modest improvement in the $Q$, over non-superconducting Birdcage coil, is shown. Simulations, in HFSS, of the field and current distribution, as well as changes in the quality factor from experimental measurements of the coil are presented. The coil was perturbed with an aluminum ball bearing and aluminum sheet in order to demonstrate the existence circularly polarized magnetic field within the Birdcage coil.