Hydrogen Absorption in Electropolishing of Niobium.

Electropolishing process had been considered to cause hydrogen absorption that could result in hydrogen Q-disease in niobium superconducting RF cavities. Recently, however, we found that hydrogen Q-disease did not always occur in a L-band niobium RF cavity treated by continuous electropolishing (EP) only. We consider mechanical grinding as a very powerful method to eliminate surface defects. Therefore, our surface treatment process is a combination of mechanical grinding and EP. We developed a simple and fast mechanical grinding method, centrifugal barrel polishing method (CBP), as the pre-treatment for the EP. We applied a combination of CBP and EP to a L-band niobium cavity and found the result showing a heavy hydrogen Q-disease. We found out that hydrogen absorption occurred during the CBP, and the absorbed hydrogen came from the water used during the CBP. Hence, we replaced the water by a liquid without hydrogen as its component. As the result, we successfully prevented hydrogen absorption in the CBP. However, the combination of hydrogen-free CBP and EP resulted in a heavy hydrogen Q-disease again. The hydrogen might have been absorbed through surface defects caused by the CBP. We found hydrogen Q-disease did not occur by the combination of hydrogen-free CBP and chemical polishing (CP). CP acid contains nitric acid functioning as an oxidiser. EP acid doesn't contain chemicals that function as oxidiser. This suggested that oxidation helps preventing hydrogen absorption. In the EP, oxidization occurs only when electric voltage is applied. There are moments during the EP, when electric voltage can not be applied (at the beginning and at the end), which in turn stops oxidation so that hydrogen absorption would occur. To assure the effect of the continuous oxidization, we put a small amount of nitric acid in the EP acid, and the additional oxidizer prevented hydrogen absorption successfully. [ABSTRACT FROM AUTHOR]