NIOBIUM REACTION KINETICS: AN INVESTIGATION INTO THE REACTIONS BETWEEN BUFFERED CHEMICAL POLISH AND NIOBIUM AND THE IMPACT ON SRF CAVITY ETCHING*

In the SRF community, there is no definitive agreement on the precise reaction mechanism in the etching of niobium cavities by buffered chemical polish (BCP) mixtures. As a consequence, it is difficult to predict the heat produced during cavity etching. To gain a better understanding of the reaction kinetics of niobium and BCP, calorimetry experiments were performed to establish an experimental heat of reaction, and research was performed to determine a reasonable reaction scheme to allow for calculation of the theoretical heat of reaction. The results of the calorimetry experiments were in excellent agreement with one another and with the theoretical value. These results will allow for more accurate estimation of etch removal amounts on cavities without the need to perform time-intensive etch rate tests. Applying the experimental data to pre and post cavity etch ultrasonic thickness measurements has shown a significant improvement in the predictability of etch removal amounts in Facility for Rare Isotope Beams (FRIB) half-wave and quarter-wave resonators, and will allow for more reliable heat removal and prevention of Qdisease during other cavity etching procedures.