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

TL;DR

This study investigates how high-temperature heat treatments affect the quality factor of large-grain niobium superconducting radio-frequency cavities, achieving record Q0 values through optimized heat treatment processes.

Contribution

It presents the first detailed analysis of high-temperature heat treatments on large-grain Nb cavities, revealing optimal conditions for maximizing Q0 and surface quality.

Findings

Achieved a record Q0 of (5+-1)×10^10 at 2.0 K and 90 mT after 1400°C treatment.

Identified complex surface composition changes including titanium oxide, increased carbon and nitrogen, and reduced hydrogen.

Demonstrated that high-temperature heat treatments significantly improve cavity performance.

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 C was done in a newly designed vacuum induction furnace. Q0 values of the order of 2x1010 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, 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.