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

Abstract

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 $800–1400°\mathrm{C}$ was done in a newly designed vacuum induction furnace. $Q_0$ values of the order of $2\times {10}^{10}$ at 2.0 K and peak surface magnetic field ($B_p$) of 90 mT were achieved reproducibly. A $Q_0$ value of $(5\pm 1)\times {10}^{10}$ at 2.0 K and $B_p=90\mathrm{mT}$ was obtained after heat treatment at $1400°\mathrm{C}$. This is the highest value ever reported at this temperature, frequency, and field. Samples heat treated with the cavity at $1400°\mathrm{C}$ 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.