HIGH-TEMPERATURE HEAT TREATMENT STUDY ON A LARGE- GRAIN NIOBIUM CAVITY*

Improvement of the cavity performance by a hightemperature heat-treatment without subsequent chemical etching has been reported for large-grain Nb cavities treated by buffered chemical polishing (BCP), as well as for a fine-grain cavity treated by vertical electropolishing [1]. Changes in the quality factor, Q0, and maximum peak surface magnetic field achieved in a large-grain Nb single-cell cavity have been determined as a function of the heat treatment temperature, between 600 °C and 1200 °C. The highest Q0 improvement of about 30% at a peak surface magnetic field of 90 mT was obtained after heat-treatment at 800 °C – 1000 °C. Measurements by secondary ion mass spectrometry on large-grain samples heat-treated with the cavity showed large reduction of hydrogen concentration after heat treatment, compared to samples treated by BCP.