Valente - SRF thin films produced by energetic condensation


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RF and structural characterization of SRF thin films produced by energetic condensation (AnneMarie Valente - 30')
Speaker: AnneMarie Valente - Jefferson Lab - Newport News (VA) USA | Duration: 30 min.
In the past years, energetic vacuum deposition methods have been developed in different laboratories to further improve thin film technology for superconducting cavities. Energetic condensation produces high density plasma with singly or quasi-singly charged ions and allows growth modes that favour a better structure and performance in thin films. In the framework of a collaboration with surrounding universities, JLab is pursuing energetic condensation deposition particularly via Electron Cyclotron Resonance (ECR). As part of this study, the influence on the material and RF properties of Nb thin film of the deposition parameters like energy, coating temperature, interface with the substrate 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 results on RF and cryogenic measurements in correlation with surface and material characterization for Nb films produced on insulating and metallic substrates. Emerging opportunities for developing multilayer SRF films with a new deposition system will also be highlighted.

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  • A-plane sapphire as a substrate is a suitable proving ground for niobium thin film studies due to the low lattice mismatch (~1.9-12%) and comparable rates of thermal expansion.
  • In hetero-epitaxial growth, the bulk lattice parameters of the film and the substrate are not commensurate.Thus, strained growth occurs wherein the film’s lattice spacing deviates from bulk equilibrium values. As the film grows thicker, the strain may be relieved by the formation of lattice defects such as vacancies or dislocations. RHEED(Reflection High Energy Electron Diffraction) is a surface sensitive technique that was used to characterize strain evolution in this case. The niobium lattice parameter at the surface can be determined by analyzing the spacing of the characteristic streaks.•RHEED images were collected for epitaxial niobium for varying thicknesses and growth parameters.•Image processing with MATLAB allows for a systematic way to abstract information related to surface crystallinity and morphology.•A specific image can be used to calculate the lattice parameter corresponding to the top most layer of the sample using the following equation:•Curve-fitting models are then applied to obtain quantitative information to extract the in-plane strain and lattice parameters
  • Superconductors for SRF Cavities:RF dissipation and nonlinear surface resistance Rs(T,H) in the Meissner state: Q(H) curve-RF breakdown of the Meissner state: maximum critical field Bm and accelerating gradient, Emax-Effect of temperature and impurities on Rs(T,H)-Possibilities to use higher performance superconductors other than Nb
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