Influence Of Ta Content In High Purity Niobium On Cavity Performance

In a previous paper [1] we have reported about initial tests of single cell 1500 MHz cavities made from high purity niobium with three different Ta contents of 160 ppm, similar to 600 ppm and similar to 1400 ppm. These cavities had been treated by buffered chemical polishing several times and 100 pin, 200 mu m and 300 mu m of material had been removed from the surfaces. This contribution reports about subsequent tests following post purification heat treatments with Ti and '' in situ '' baking. As a result, all cavities exhibited increased quench fields due to the improved thermal conductivity after the heat treatment. After the '' in situ '' baking at 120 C for similar to 40 hrs the always present Q-drop at high fields disappeared and further improvements in accelerating gradient could be realized. Gradients as high as E(acc) = 35 MV/m were achieved and there were no clear indications that the cavity performance was influenced by the Ta content in the material. A multi-cell cavity from the high Ta content material has been fabricated and initial results are reported.