Optimizing Superconducting Nb Film Cavities by Mitigating Medium-Field Q-Slope Through Annealing

TL;DR

This study improves niobium film superconducting cavities by annealing treatments that increase quench fields and reduce Q-slope, providing insights into microstructural factors affecting performance.

Contribution

It demonstrates that specific annealing protocols can mitigate the medium-field Q-slope in Nb film cavities, a challenge in coating-based superconducting devices.

Findings

Annealing at 600-800°C increases quench fields significantly.

Higher annealing temperature (900°C) causes Q-switch phenomenon.

Microstructure and impurities influence the Q-slope evolution.

Abstract

Niobium films are of interest in applications in various superconducting devices, such as superconducting radiofrequency cavities for particle accelerators and superconducting qubits for quantum computing. In this study, we addressed the persistent medium-field Q-slope issue in Nb film cavities, which, despite their high-quality factor at low RF fields, exhibit a significant Q-slope at medium RF fields compared to bulk Nb cavities. Traditional heat treatments, effective in reducing surface resistance and mitigating the Q-slope in bulk Nb cavities, are challenging for niobium-coated copper cavities. To overcome this challenge, we employed DC biased high-power impulse magnetron sputtering to deposit niobium film onto a 1.3 GHz single-cell elliptical bulk niobium cavity, followed by annealing treatments aimed at modifying the properties of the niobium film. In-situ annealing at 340 {\deg}C increased the quench field from 10.0 to 12.5 MV/m. Vacuum furnace annealing at 600 {\deg}C and 800 {\deg}C for 3 hours resulted in a quench field increase of 13.5 and 15.3 MV/m, respectively. Further annealing at 800 {\deg}C for 6 hours boosted the quench field to 17.5 MV/m. Additionally, the annealing treatments significantly reduced the field dependence of the surface resistance. However, increasing the annealing temperature to 900 {\deg}C induced a Q-switch phenomenon in the cavity. The analysis of RF performance and material characterization before and after annealing has provided critical insights into how the microstructure and impurity levels in Nb films influence the evolution of the Q-slope in Nb film cavities. Our findings highlight the significant roles of hydrides, high local misorientation, and lattice and surface defects in driving field-dependent losses.