RF surface resistance tuning of superconducting niobium via thermal diffusion of native oxide

Abstract

Recently, Nb superconducting radio frequency cavities vacuum heat treated between 300 and 400 °C for a few hours have exhibited very high quality factors (∼5 × 1010 at 2.0 K). Secondary ion mass spectrometry measurements of O, N, and C show that this enhancement in RF surface conductivity is primarily associated with interstitial O alloying via dissolution and diffusion of the native oxide. We use a theory of oxide decomposition and O diffusion to quantify previously unknown parameters crucial in modeling this process. RF measurements of a vacuum heat-treated Nb superconducting radio frequency cavity confirm the minimized surface resistance (higher Q0) previously expected only from 800 °C diffusive alloying with N.

SIMS Investigation of Furnace-Baked Nb

“…Our recent SIMS investigation [4] found elevated oxygen content near the surface of samples prepared in the similar way as Ito [1]. RF measurements of a Nb SRF cavity prepared via vacuum heat treatment at 300 °C and subsequently electropolished 180 nm to exclude any ingress of C and N demonstrated that the enhancement in quality factor is mainly due to interstitial oxygen alloying via a thermal diffusion process.…”

“…RF measurements of a Nb SRF cavity prepared via vacuum heat treatment at 300 °C and subsequently electropolished 180 nm to exclude any ingress of C and N demonstrated that the enhancement in quality factor is mainly due to interstitial oxygen alloying via a thermal diffusion process. The measurements made in [4] found that the thermal diffusion process was con-sistent with Ciovati's model [5] of Nb 2 O 5 dissolution and oxygen diffusion.…”

Effect of duration of 120 oC baking on the performance on superconducting radio frequency niobium cavities

“…The oxygen diffusion model proposed by G. Ciovati [13], qualitatively explains the diffusion of oxygen in to the rf surface of SRF cavities. The same model also describes well to the diffusion of oxygen during the mid-T heat treatment (300 -400 • C) [7].…”

“…The calculations based on the oxygen diffusion models described in Refs. [7,13] predicts the dilute oxygen concentration on the rf surface and oxygen diffuse deeper into the bulk. Even though, the final surface preparation may play some role on the initial condition of rf surface and successive baking duration, the cavity TE1-05 showed higher flux trapping sensitivity compared to cavity TE1-06.…”

“…The effect of low temperature baking in the presence of nitrogen also resulted increase in quality factor at medium accelerating gradient and it was extended to higher accelerating gradient over the baseline measurement [4,5]. Most recently, the medium temperature heat treatment also resulted in the increase in quality factor likely due to the decomposition of surface oxide phase and oxygen diffusion within the rf penetration depth [6,7].…”