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

• Published in: Physical review special topics. PRST-AB. Accelerators and beams Vol. 16; no. 4; p. 042001
• Main Authors: Dhakal, P., Ciovati, G., Myneni, G. R., Gray, K. E., Groll, N., Maheshwari, P., McRae, D. M., Pike, R., Proslier, T., Stevie, F., Walsh, R. P., Yang, Q., Zasadzinzki, J.
• Format: Journal Article
• Language: English
• Published: College Park American Physical Society 01.04.2013; American Physical Society (APS)
• Subjects: Chemical polishing; CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; Heat treating; Heat treatment; High temperature effects; Holes; MATERIALS SCIENCE; Niobium; Photoelectrons; Q factors; Secondary ion mass spectrometry; Superconductivity; Surface preparation; Titanium oxides; Vacuum induction furnaces
• ISSN: 1098-4402; 1098-4402; 2469-9888
• DOI: 10.1103/PhysRevSTAB.16.042001

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=90mT 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.