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Title Surface Characterization: what has been done, what has been learnt?
Type Talk  
Abstract Electromagnetic fields penetrate only a distance of about 60 nm into the surface of a superconductor such as niobium. Therefore it is obvious that the surface condition of a cavity surface will effect in a major way the performance of this cavity. For at least the last 30 years niobium surfaces as used in superconducting accelerating cavities have been investigated by surface characterization techniques such as scanning electron microscopy (SEM), Auger spectroscopy (AES), X-ray photon spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX), electron spectroscopy for chemical analysis (ESCA) and secondary ion mass spectrometry (SIMS). The objective of all these investigations was to establish correlations between surface conditions and cavity performances such as surface resistance and accelerating gradients. Much emphasis was placed on investigating surface topography and the oxidation states of niobium under varying conditions such as buffered chemical polishing, electropolishing, oxipolishing, high temperature heat treatment, post-purification heat treatment and in-situ baking. Additional measurements were conducted to characterize the behavior of a niobium surface more relevant to rf cavities such as resonant (multipacting) and non-resonant (field emission) electron loading. A large amount of knowledge has been extracted by all these investigations; nevertheless, there is still a lack of reproducibility in cavity performance when applying the best process to a cavity surface and no clear correlation has been established between niobium surface features and cavity performance. This contribution gives a review of the attempts to characterize niobium surfaces over the last three decades and tries to extract the white spots in our knowledge.

List of authors...

Principle author first.

Last (Family) Name First Name (Initials only) Affiliation or Organization (abbreviations if possible)
Kneisel P. JLAB



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