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.
Physical Review Accelerators and Beams
Dhakal, P; Ciovati, G; Myneni, G R.; Gray, K E.; Groll, N; Maheshwari, P; McRae, D M.; Pike, Robert D.; Proslier, T; Stevie, F; Walsh, R P.; Yang, Q; and Zasadzinzki, J, Effect of high temperature heat treatments on the quality factor of a large-grain superconducting radio-frequency niobium cavity (2013). Physical Review Accelerators and Beams, 16(042001).