Q0 IMPROVEMENT OF LARGE-GRAIN MULTI-CELL NIOBIUM CAVITIES BY USING JLAB'S STANDARD ILC PROCESSING*

As reported previously at the Berlin SRF conference, applying the JLab standard ILC electropolishing (EP) recipe on initially buffered chemical polishing (BCP) etched fine-grain multi-cell niobium cavities results in improvement both in gradient and Q0. We recently had the opportunity to further experiment with two 1300 MHz 9cell large-gain niobium cavities, one manufacture by JLab and the other by Peking University. Both cavities were initially BCP etched and further processed by using JLab’s standard ILC EP recipe. Due to fabrication defects, these two cavities only reached a gradient in the range of 20-30 MV/m. Interestingly, both cavities exhibited significant Q0 improvement in the gradient range of 15-20 MV/m. At 2K, a Q0 value of 2E10 is achieved at 20 MV/m. At a reduced temperature of 1.8K, a Q0 value of 3E10 is achieved at 20 MV/m. These results suggest that a possible path toward raising Q0 in the medium gradient range is to use the large-grain material for cavity fabrication and ILC-style recipe, namely furnace heat treatment, light EP and low temperature bake for final processing.