Development of vertical electropolishing process applied on 1300 and 704 MHz superconducting niobium resonators

An advanced setup for vertical electropolishing of superconducting radio-frequency niobium elliptical cavities has been installed at CEA Saclay. Cavities are vertically electropolished with circulating standard HF-${\mathrm{HF}\text{\ensuremath{-}}\mathrm{H}}_{2}{\mathrm{SO}}_{4}$ electrolytes. Parameters such as voltage, cathode shape, acid flow, and temperature have been investigated. A low voltage (between 6 and 10 V depending on the cavity geometry), a high acid flow ($25\text{ }\text{ }\mathrm{L}/\mathrm{min}$), and a low acid temperature ($20\ifmmode^\circ\else\textdegree\fi{}\text{ }\mathrm{C}$) are considered as promising parameters. Such a recipe has been tested on single-cell and nine-cell International Linear Collider (ILC) as well as 704 MHz five-cell Super Proton Linac (SPL) cavities. Single-cell cavities showed similar performances at 1.6 K being either vertically or horizontally electropolished. The applied baking process provides similar benefit. An asymmetric removal is observed with faster removal in the upper half-cells. Multicell cavities (nine-cell ILC and five-cell SPL cavities) exhibit a standard ${Q}_{0}$ value at low and medium accelerating fields though limited by power losses due to field emitted electrons.