Analysis of the DC Field Emission Properties of Laser-Polished Niobium Samples

The field emission properties of laser-polished, fine-grained niobium sheets as the base material for an application in superconducting radio frequency cavities were investigated using a specially developed field emission scanning microscope. To investigate the influence of laser polishing on field emission properties, two surfaces were examined: an unpolished surface and a laser-polished surface, with different conditions for the field emission measurements. Scanning electron microscopy was used to characterize the surfaces before and after the field emission measurements. The field emission field strengths (onset-field EE) were measured to quantify the electron emission over large areas. For smooth, defect-free surfaces, emission field strength values up to 1 GV/m were achieved, while untreated or defect-rich areas remained limited to around 100 MV/m or less. A local reduction of the onset field strength could be attributed to defects such as carbon contamination, pores and cracks. The field emission data obtained was categorized into four classes. This work shows that laser-polished niobium surfaces only exhibit electron field emission at field strengths above 700 MV/m, a tenfold increase over chemically or electrochemically polished samples, thus demonstrating the potential for future superconducting radio frequency cavity applications. At the same time, laser polishing needs further improvements to enable a reliable large-scale manufacturing process of superconducting radio frequency cavities.