Surface Characterization: what has been done , what has been learnt?
Electromagnetic fields penetrate only a distance of {approx} 60 nm into the surface of a superconductor such as niobium. Therefore it is obvious that the condition of a cavity surface will affect the performance of this cavity. In 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.
- Research Organization:
- Thomas Jefferson National Accelerator Facility (TJNAF), Newport News, VA (United States)
- Sponsoring Organization:
- USDOE Office of Energy Research (ER) (US)
- DOE Contract Number:
- AC05-84ER40150
- OSTI ID:
- 822207
- Report Number(s):
- JLAB-ACC-03-213; DOE/ER/40150-2604; TRN: US0401182
- Resource Relation:
- Conference: 11th Workshop on RF Superconductivity, Travenmunde, Lubeck (DE), 09/08/2003--09/12/2003; Other Information: PBD: 1 Sep 2003
- Country of Publication:
- United States
- Language:
- English
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