Paper | Title | Page |
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TUPPO069 | Energetic Condensation Growth of Nb Thin-Films for SRF Applications | 396 |
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AASC, JLab and NSU conduct research into SRF thin-film coatings by first characterizing properties such as morphology, grain size, crystalline structure, defects, and impurities, then measuring Tc and RRR, and following this with ‘in-cavity’ RF measurements of the Surface Impedance of the films at cryogenic temperatures. These progressive steps are essential to eventual design and measurement of SRF accelerator structures at high fields. We have recently produced Nb superconducting thin-films with crystal grain sizes ~50μm using our proprietary CED^TM cathodic arc technique. RRR of ~129 at Tc of 9.2K was measured in a film grown on a-plane sapphire heated to 400oC. At 20oC, the RRR dropped to ~4. Energetic condensation using cathodic arcs produces non-equilibrium fast ions (~50-100eV). These ion energies are much higher than typical sputtering energies. When such energetic condensation is complemented by substrate biasing (to ~200-300eV) the incident ion energy is further increased, allowing growth modes that would otherwise require much higher substrate temperatures. Data are presented for pure Nb films using SEM, EBSD, XRD and a Surface Impedance Characterization RF cavity. |
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TUPPO080 | X-ray Pole Figure Analysis on Fiber Textured Epitaxial Niobium Films for SRF Cavities | 418 |
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Single and large grain Nb films are of interest to reduce the cost of SRF cavities. The structural properties and SRF potential of Nb films obtained by coaxial energetic deposition (CEDTM) in an ultra-high vacuum process are compared and discussed. The CEDTM is a hybrid technique with both energetic ion deposition and implantation phases based on cathodic arc plasma sources, which are copious generators of condensable energetic (20-200 eV), multiply charged ions from metal or alloy cathodes. The X-ray pole figure of the thin films revealed grain orientations on Nb films grown at different substrate temperatures that indicate good structural and electrical properties. Single crystalline 110 epilayers of Nb films are grown on a-plane sapphire substrates at 400 degreeC, but at lower temperature, there are two kinds of twins in which the grains are rotated by ~45 degree about the film normal. RRR of ~129 and Tc=9.2K were measured on a Nb film on a-plane sapphire substrate at 400 degreeC, dropping to ~4 on a room temperature substrate. |
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TUPPO084 | RF and Structural Characterization of New SRF Films | 431 |
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In the past years, energetic vacuum deposition methods have been developed in different laboratories to improve Nb/Cu technology for superconducting cavities. JLab is pursuing energetic condensation deposition via Electron Cyclotron Resonance. As part of this study, the influence of the deposition energy on the material and RF properties of the Nb thin film is investigated. The film surface and structure analyses are conducted with various techniques like X-ray diffraction, Transmission Electron Microscopy, Auger Electron Spectroscopy and RHEED. The microwave properties of the films are characterized on 50 mm disk samples with a 7.5 GHz surface impedance characterization system. This paper presents surface impedance measurements in correlation with surface and material characterization for Nb films produced on copper substrates with different bias voltages and also highlights emerging opportunities for developing multi-layer SRF films with a new deposition system. |
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TUPPO086 | Surface Characterization of Niobium Samples Electropolished Together with Real Cavities | 441 |
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We report the results of surface characterization of niobium samples electropolished together with single-cell niobium cavities. These witness samples were located in three regions of the cavity, namely at the equator, the iris and the beam-pipe respectively. Auger electron spectroscopy (AES) was utilized to probe the chemical composition of the topmost four atomic layers. Scanning electron microscopy with energy dispersive x-ray for elemental analysis (SEM/EDX) was used to observe the surface topography and chemical composition at the micrometer scale. A few atomic layers of sulfur (S) were found covering the samples non-uniformly. Niobium oxide granules with a sharp geometry were observed on every sample. Some Nb-O granules appeared to also contain sulfur. |
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TUPPO087 | Study of Etching Pits in a Large-Grain Single Cell Bulk Niobium Cavity | 446 |
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This paper reports an on-going surface study of a superconducting radio frequency resonant cavity made of large-grain bulk niobium, which experienced anomalous RF energy loss in the medium field range. "Hot" and "cold" spots were identified via in-situ thermometry mapping of the BCP-etched single-cell cavity. The cavity was cut apart for surface investigation via high resolution electron microscopy (SEM), electron-back scattering diffraction microscopy (EBSD), optical microscopy, and three dimensional profilometry. Etching pits with clearly discernable crystal facets were observed in both "hotspot" and "coldspot" specimens. They were found in-grain, at bi-crystal boundaries, and on tri-crystal junctions. Two types of pits were observed with significantly different geometrical features, as observed with high resolution SEM. All "coldspots" examined had qualitatively low density of etching pits or very shallow pits at tri-crystal bound |
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