Unprecedented quality factors at accelerating gradients up to 45 MVm⁻¹in niobium superconducting resonators via low temperature nitrogen infusion

TLDR

The work helps deepen the understanding of the physics of the RF niobium cavity surface. The study reports new surface treatments that allow controlled manipulation of niobium resonator nitrogen content in the first few nanometers, thereby affecting the fundamental Mattis‑Bardeen surface resistance and residual resistance. The new surface infusion conditions more than double the state‑of‑the‑art quality factor at 2 K for fields above 35 MV/m, enable accelerating gradients up to ~45 MV/m with peak magnetic fields of 190 mT, and open the possibility to tailor surface impurity distributions to maximize Q and gradients, with important implications for future SRF accelerator performance and cost.

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.