Abstract
Microroughness is viewed as a critical issue for attaining optimum performance of superconducting radio frequency accelerator cavities. The principal surface smoothing methods are buffered chemical polish (BCP) and electropolish (EP). The resulting topography is characterized by atomic force microscopy (AFM). The power spectral density (PSD) of AFM data provides a more thorough description of the topography than a single-value roughness measurement. In this work, one dimensional average PSD functions derived from topography of BCP and EP with different controlled starting conditions and durations have been fitted with a combination of power law, $K$ correlation, and shifted Gaussian models to extract characteristic parameters at different spatial harmonic scales. While the simplest characterizations of these data are not new, the systematic tracking of scale-specific roughness as a function of processing is new and offers feedback for tighter process prescriptions more knowledgably targeted at beneficial niobium topography for superconducting radio frequency applications.
Highlights
Particle accelerators play a steadily increasing role in an expanding range of scientific research
This suggests that buffered chemical polish (BCP) etching establishes a characteristic topography which remains substantially constant as the surface recedes
The absence of steps is consistent with the roughness value and the notion that steps are associated with grain boundaries, which are absent in single crystals
Summary
Particle accelerators play a steadily increasing role in an expanding range of scientific research. Their greater capabilities and superior cost for performance in many instances result in even more rapid growth for accelerators using superconducting radio frequency (SRF) cavities to power the beam. The shallow penetration ($ 40 nm) of the rf into superconducting niobium lends great importance to SRF cavity interior surface chemistry and topography. These in turn are strongly influenced by the chemical etching ‘‘surface clean-up’’ that follows fabrication. EP uses a 1:10 (volume) mixture of hydrofluoric (49%) and sulfuric acid
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