| Paper | Title | Page |
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| WE5PFP057 | Integrated Surface Topography Characterization of Variously Polished Niobium for Superconducting Particle Accelerators | 2132 |
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Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. As SRF cavities approach fundamental material limits, there is increased interest in understanding the details of topographical influences on performance limitations. Micro-and nano-roughness are implicated in direct geometrical field enhancements and complications of the composition of the 50 nm surface layer in which the super-currents flow. Interior surface etching (BCP/EP) to remove mechanical damage leaves surface topography, including pits and protrusions of varying sharpness. These may promote RF magnetic field entry, locally quenching superconductivity, so as to degrade cavity performance. A more incisive analysis of surface topography than the widely-used average roughness is needed. In this study, a power spectral density (PSD) approach based on Fourier analysis of surface topography data acquired by both stylus profilometry and atomic force microscopy (AFM) is being used to distinguish the scale-dependent smoothing effects. The topographical evolution of the varied starting state Nb surface (CBP/ EBW) as a function of applied etching, polishing steps and conditions is reported, resulting in a novel qualitative and quantitative description of Nb surface topography. |
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| WE5PFP058 | Basic Electropolishing Process Research and Development in Support of Improved Reliable Performance SRF Cavities for the Future Accelerators | 2135 |
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Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. Future accelerators require unprecedented cavity performance, which is strongly influenced by interior surface nanosmoothness. Electropolishing is the technique of choice to be developed for high-field superconducting radiofrequency cavities. Electrochemical impedance spectroscopy (EIS) and related techniques point to the electropolishing mechanism of Nb in a sulfuric and hydrofluoric acid electrolyte of controlled by a compact surface salt film under F- diffusion-limited mass transport control. These and other findings are currently guiding a systematic characterization to form the basis for cavity process optimization, such as flowrate, electrolyte composition and temperature. This integrated analysis is expected to provide optimum EP parameter sets for a controlled, reproducible and uniform surface leveling for Nb SRF cavities. |
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| WE5PFP061 | Commissioning of the SRF Surface Impedance Characterization System at Jefferson Lab | 2144 |
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Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. Much remains to be learned regarding the details of SRF performance effects with material variation, including niobium treated in different ways, and different bulk/thin film materials that are fabricated under different conditions. A facility that can measure small samples’ RF properties in a range of 0~180mT magnetic field and 2~20k temperature is necessary in order to answer this question. The Jefferson Lab surface impedance characterization (SIC) system has been designed to attempt to meet this requirement. The SIC system uses a sapphire-loaded cylindrical Nb cavity at 7.5GHz with 50mm diameter flat sample placed on a non-contacting end plate and a calorimetric technique to directly measure the rf dissipation in the sample in response to known rf fields over ~1 cm2. We report on the commissioning of this system and its first uses for characterizing materials. Preliminary tests with Nb thin film sample sputtered on Cu substrate, and bulk Nb sample have been done at low field. The presently available hardware is expected to enable tests up to 20 mT peak magnetic field on the sample CW. Paths to higher field tests have been identified. |