Chemical analysis and field emission study of electropolished niobium surface containing synthesized niobium oxide particles

Abstract

We conducted electropolishing (EP) of a Nb superconducting RF coupon cavity containing coupons at various positions of the cavity, and laboratory EP of Nb samples. The surface of the electropolished coupons was studied by using XPS, SEM, AFM, and EDX. The XPS analysis revealed that the atomic compositions and Nb 3d peak shapes (intensity ratio of the Nb5+ and Nb0 peaks) varied for the different coupons, indicating that the chemical state was not the same over the entire cavity surface. These differences might originate from the particle clusters observed on the coupon surfaces, which mainly existed as Nb2O5 and its hydrated form in different number densities. An individual particle cluster was isolated from the surface for EDX analysis, revealing the presence of C and F impurities, where F existed mostly as CFx with traces of niobium fluoride. These particle clusters were possibly synthesized during the post-EP water rinsing process by the hydrolysis of the niobium fluoride species (H2NbOF5) that were produced in the EP process and existing in the acid viscous layer on the surface. The post-EP water rinsing conditions affected the number density of particle clusters, and strong agitation of the acid reduced the particle content on the laboratory electropolished samples. An individual particle cluster emitted electrons at a DC field of 64 MV/m after being activated at a high field of ~150 MV/m, possibly limiting the performance of the SRF cavity. An optimized post-EP water rinsing followed by HF rinsing or prolonged ultrasonic rinsing of the cavity was effective at removing undesired particles from the surface.