Simultaneous laser polishing and N-doping of niobium

Superconducting niobium is the base material for many modern particle accelerators. The high cleanliness requirements in all radio frequency superconductor technology have led to the development of complex cleaning processes in recent decades. High-pressure rinsing, heating processes under vacuum and gas atmospheres as well as chemical and electrochemical polishing are commonly applied procedures, that are required to obtain the properties needed for their application. In order to optimize the surface finish of Nb materials in a more environment-compatible way, i.e., with less energy consumption and avoiding hazardous liquids, we report on a combination of simultaneous N-doping and laser polishing here. A nanosecond laser was employed, and the prepared Nb surfaces were investigated with a combination of electron microscopy, x-ray fluorescence spectroscopy (EDX), optical profilometry, and x-ray absorption spectroscopy (EXAFS) to show the effect of different N2-pressures during the laser polishing procedure in an ultrahigh vacuum chamber. The results show that for N2-pressures above ca. 10−3 mbar, traces of nitrogen can be observed by both EDX and EXAFS. In parallel, a smoothing of the surfaces occur, with slightly different roughnesses and microstructures of the polycrystalline Nb surfaces depending on the N2-pressure.