Unprecedented Quality Factors at Accelerating Gradients up to 45 MV/m in Niobium Superconducting Resonators via Low Temperature Nitrogen Infusion

TL;DR

This paper introduces a novel low-temperature nitrogen infusion process that significantly enhances the quality factor and maximum accelerating gradient of niobium superconducting resonators, reaching unprecedented performance levels.

Contribution

It demonstrates a controlled surface treatment method that manipulates nitrogen content to improve SRF cavity performance beyond current state-of-the-art.

Findings

Achieved over two times the Q factor at 2K for fields > 35 MV/m

Repeatedly reached gradients of ~45 MV/m with high magnetic fields

Enhanced understanding of surface impurity effects on cavity performance

Abstract

We report the finding of new surface treatments that permit to manipulate the niobium resonator nitrogen content in the first few nanometers in a controlled way, and the resonator fundamental Mattis-Bardeen surface resistance and residual resistance accordingly. In particular, we find surface infusion conditions that systematically a) increase the quality factor of these 1.3 GHz superconducting radio frequency (SRF) bulk niobium resonators, up to very high gradients; b) increase the achievable accelerating gradient of the cavity compared to its own baseline with state-of-the-art surface processing. Cavities subject to the new surface process have larger than two times the state of the art Q at 2K for accelerating fields > 35 MV/m. Moreover, very high accelerating gradients ~ 45 MV/m are repeatedly reached, which correspond to peak magnetic surface fields of 190 mT, among the highest measured for bulk niobium cavities. These findings open the opportunity to tailor the surface impurity content distribution to maximize performance in Q and gradients, and have therefore very important implications on future performance and cost of SRF based accelerators. They also help deepen the understanding of the physics of the RF niobium cavity surface.