| Paper | Title | Page |
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| THP010 | Exploiting New Electrochemical Understanding of Niobium Electropolishing for Improved Performance of SRF Cavities for CEBAF | 779 |
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Recent incorporation of analytic electrochemistry into the development of protocols for electropolishing niobium SRF cavities has yielded new insights for optimizing this process for consistent, high-performance results. Use of reference electrodes in the electrolyte, electrochemical impedance spectroscopy (EIS), rotating disk electrodes (RDE), and controlled sample temperatures has greatly clarified the process dynamics over the empirical understanding developed via years of practice. Minimizing rf losses at high operational gradients is very valuable for CW linacs. Jefferson Lab is applying these new insights to the low-loss 7-cell cavity design developed for the CEBAF 12 GeV Upgrade. Together with controlled cleaning and assembly techniques to guard against field-emission causing particulates, the resulting process is yielding consistent cavity performance that exceeds project requirements. Cavity tests show BCS-limited Q well above 30 MV/m. Detailed process data, interpretation, and resulting rf performance data will be presented. |
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| THP038 | Ultimate-Gradient Srf Test Cavity and Low Loss Tangent Measurements in Ultra Pure Sapphire | 842 |
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A 1.3 GHz superconducting test cavity is being developed to test wafer samples of advanced SRF materials with surface fields at or beyond the Nb BCS limit. The mushroom-shaped Nb cavity is dielectric-loaded, with a hemisphere of high-purity sapphire located just above a detachable end flange. Wafer samples are mounted on the end flange. The cavity is operated in the TE011 mode, so no currents flow from the end flange to the side walls. Fields are concentrated on the wafer sample so that the peak surface field there is 4 times greater than anywhere else on the cavity walls. The loss tangent of ultrapure sapphire is critical to the performance of the test cavity. A separate first experiment has been conducted in a special 1.8 GHz cavity to measure this loss tangent in L band as a function of temperature for the first time. Results of the measurement and the final design of the ultimate-gradient test cavity will be presented. |