Advanced Surface Polishing For Accelerator Technology Using Ion Beams

Abstract

Surface erosion problems are common in the development of TeV accelerators, fusion and fission reactors, semiconductor, optical and magnetic storage devices, and Extreme Ultra-Violet (EUV) lithography devices. We have reviewed various erosion mechanisms of ion interactions with the surfaces studied by experiment and computer simulation. Nanoscale surface roughness in rf-linacs and contamination cause field emission of electrons, field evaporation of ions and fragments, plasma formation, and lead to high-gradient rf vacuum breakdown of electrodes which is a limiting factor in the development of high-gradient rf technology for future TeV accelerators. A few mechanisms of nanoscale surface fracture under a high-gradient electric field were developed and will be discussed. A Gas Cluster Ion Beam (GCIB) technology was successfully applied to surface treatment of Cu, Stainless steel, Ti and Nb samples and to Nb rf-cavities by using accelerated cluster ion beams of Ar, O2, N2, and NF3, and combinations of them, with accelerating voltages up to 35 kV. DC field emission (dark current) measurements and electron microscopy were used to investigate metal surfaces treated by GCIB. The experimental results showed that GCIB technique can significantly reduce the number of field emitters and also change the structure of the Nb oxide layer on the surface. The RF tests on the GCIB treated Nb rf-cavities showed improvement of the quality factor Q at 4.5 K. The superconducting gap was also enhanced by using the oxygen GCIB irradiation exposure. GCIB may become a standard technique to modify and control the oxygen content on the surface and a promising surface treatment technique for Nb SRF cavities in particle accelerators. Computer simulation of bombardment of Nb surfaces with Ar and O2 clusters by molecular dynamics and phenomenological surface dynamics equations confirms experimental results.