Smoothing RF cavities with gas cluster ions to mitigate high voltage breakdown

Abstract

Many studies have demonstrated that improving the surface smoothness and cleanliness of high voltage electrodes increases the voltage standoff capability, but none have specifically investigated the role of nano-scale and atomic level surface roughness. Using AFM imaging, we have studied the effect of gas cluster ion beams (GCIB) on oxygen-free Cu electrode material that is used in high gradient RF cavities. Using Ar clusters accelerated by 30kV, with a dose of 6×1014ecm−2, we have effectively removed an asperity that was 3500Å wide and 350Å high. Subsequent processing with 5kV acceleration reduced the surface roughness from an Ra value of 13.2Å to 4.8Å. This demonstrates the effectiveness of GCIB for reducing sub-micron roughness to atom level smoothness.