Effect of high temperature heat treatments on the quality factor of a large-grain superconducting radio-frequency niobium cavity

Large-grain Nb has become a viable alternative to fine-grain Nb for the fabrication of superconducting radio-frequency cavities. In this contribution we report the results from a heat treatment study of a large-grain 1.5 GHz single-cell cavity made of ``medium purity'' Nb. The baseline surface preparation prior to heat treatment consisted of standard buffered chemical polishing. The heat treatment in the range 800--1400^∘C was done in a newly designed vacuum induction furnace. Q₀ values of the order of 2×10¹⁰ at 2.0 K and peak surface magnetic field (Bₚ) of 90 mT were achieved reproducibly. A Q₀ value of (5±1)×10¹⁰ at 2.0 K and Bₚ=90 mT was obtained after heat treatment at 1400^∘C. This is the highest value ever reported at this temperature, frequency, and field. Samples heat treated with the cavity at 1400^∘C were analyzed by secondary ion mass spectrometry, x-ray photoelectron spectroscopy, energy dispersive x ray, point-contact tunneling, and x-ray diffraction, and revealed a complex surface composition which includes titanium oxide, increased carbon, and nitrogen content but reduced hydrogen concentration compared to a non-heat-treated sample.