Study of gas cluster ion beam surface treatments for mitigating RF breakdown

Surface processing with high-energy gas cluster ion beams (GCIB) is investigated for increasing the high voltage breakdown strength of RF cavities and electrodes in general. Various GCIB treatments were studied for Nb, Cu, Stainless Steel and Ti electrode materials using beams of Ar, Ar + H2, O2, N2, Ar + CH4, or O2 + NF3 clusters with accelerating potentials up to 35 kV. Etching using chemically active clusters such as NF3 reduces the grain structure of Nb used for SRF cavities. Smoothing effects on stainless steel and Ti substrates were evaluated using SEM and AFM imaging and show that 200 nm wide polishing scratch marks are greatly attenuated. Using a combination of Ar and O2 processing for stainless steel electrode material, the oxide thickness and surface hardness are dramatically increased. The DC field emission of a 150-mm diameter sample of GCIB processed stainless steel electrode material was a factor of 106 less than a similar untreated sample.