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TY - CPAPER AU - Köszegi, J.M. AU - Knobloch, J. AU - Kugeler, O. TI - High-Q Operation of SRF Cavities: The Impact of Thermocurrents on the RF Surface Resistance J2 - Proc. of International Conference on RF Superconductivity (SRF2015), Whistler, BC, Canada, Sept. 13-18, 2015 AB - For CW applications much effort is being expended to minimize the power dissipation (surface resistance) of niobium cavities. Previous studies have shown that residual resistance can be reduced by performing a thermal cycle, a procedure of warming up a cavity after initial cooldown to about 20K and cooling it down again. It was postulated that thermocurrents during cooldown generate additional trapped magnetic flux that impacts the cavity quality factor. Here, we present a more extensive study that includes measurements of two additional passband modes and that confirms the effect. A change in surface resistance of more than a factor seven was observed. In this paper, we also discuss simulations that support the claim. While the layout of the cavity LHe tank system is cylindrically symmetric, we show that the temperature dependence of the material parameters results in a non-symmetric current distribution. PB - JACoW CY - Geneva, Switzerland SP - 37 EP - 40 KW - cavity KW - simulation KW - shielding KW - niobium KW - operation DA - 2015/12 PY - 2015 SN - 978-3-95450-178-6 DO - 10.18429/JACoW-SRF2015-MOBA04 UR - http://srf2015.vrws.de/papers/moba04.pdf ER -