Record Q0 value in a heat treated 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 \degree C was done in a newly designed vacuum induction furnace. $Q_0$ values of the order of $2\times10^{10}$ at 2.0 K and peak surface magnetic field ($B_p$) of 90 mT were achieved reproducibly. A record $Q_0$-value of $4.67\times10^{10}$ at 2.0 K and $B_p$ = 90 mT was obtained after heat treatment at 1400 \degree C. Samples heat treated with the cavity at 1400 \degree C were analyzed by secondary ion mass spectrometry, secondary electron microscopy, 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.