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For many new accelerator applications, superconducting radio frequency (SRF)systems are the enabling technology. In particular for CW applications, mucheffort is being expended to minimize the power dissipation (surface resistance)of niobium cavities. Starting in 2009, we suggested a means of reducing theresidual resistance by performing a thermal cycle [1], a procedure of warmingup a cavity after initial cooldown to about 20K and cooling it down again. Insubsequent studies [2], this technique was used to manipulate the residualresistance by more than a factor of 2. It was postulated that thermocurrentsduring cooldown generate additional trapped magnetic flux that impacts thecavity quality factor. Here, we present a more extensive study that includesmeasurements of two additional passband modes and that confirms the effect. Inthis paper, we also discuss simulations that support the claim. While thelayout of the cavity LHe tank system is cylindrically symmetric, we show thatthe temperature dependence of the material parameters results in anon-symmetric current distribution. Hence a significant amount of magnetic fluxcan be generated at the RF surface.

High-Q operation of SRF cavities: The potential impact of thermocurrents on the RF surface resistance