PLASMA TREATMENT OF BULK Nb SURFACE IN THE Ar/Cl2 DISCHARGE*

The preparation of the cavity walls has been one of the major challenges in the superconducting radio-frequency (SRF) accelerator technology. Therefore, constant research and development effort is devoted to develop surface preparation processes that will improve roughness and lower the level of impurities, like hydrogen or oxygen, embedded in bulk Nb, having in the same time reasonable etching rates. Plasma based surface modification provides an excellent opportunity to achieve these goals. We present Ar/Cl2 discharge treatment of bulk Nb where we achieved etching rates comparable to the rates obtained with the electropolishing method without introducing impurities in Nb. The current experiments were performed on disk shaped Nb samples, exposed to plasma produced in a microwave discharge system. Surface composition and topology measurements were carried out before and after plasma treatment. Upon determining optimal experimental conditions on disk shaped samples, we will apply the same procedure on the single cell cavities, pursuing improvement of their RF performance.