Purification of niobium by multiple electron beam melting for superconducting RF cavities

In this work, purification of commercial grade (~99.9 %) niobium by multiple Electron Beam Melting (EBM) is reported. Impurity removal of carbon, oxygen, nitrogen, aluminum, iron, molybdenum, zirconium and tungsten in niobium matrix is presented as a part of the ingot melting stages. The minor material loss of niobium during melting is attributed to the amount of decarburization based on the ratio of initial and critical content of oxygen to carbon. The analysis of nearly 60 impurity elements in niobium was carried out by glow discharge mass spectrometry. In the end, the purity of niobium after multiple electron beam meltings was found to be 4N while the purity enhanced from 99.9 % (3N) to 99.993 % (4N3), including gaseous impurities, and 99.98 % (3N8) to 99.998 % (4N8) without gaseous impurities. The glow discharge mass spectrometry analytical results of purified niobium indicated that the material is suitable as input material for further processing of fabricated superconducting radio-frequency cavities.