Köszegi, J.; Knobloch, J.: Advanced study of the thermoelectrically generated magnetiv field in a nine-cell cavity during the superconducting phase transition. Physical Review Accelerators and Beams 22 (2019), p. 052001/1-9
10.1103/PhysRevAccelBeams.22.052001
Open Access Version
Abstract:
Today’s superconducting radio-frequency (srf) cavities approach quality factors up to 1011. With the decrease in overall dissipation the loss contributions previously considered minor gain in importance. One contribution is trapped magnetic flux. Superconducting rf cavities have to be cooled down to their cryogenic operating temperature. During the process, high temperature differences can occur in the system driving thermoelectric currents in the cavity wall and the surrounding liquid helium tank. The associated magnetic field can get trapped in the superconductor during its phase transition from the normal conducting into the superconducting (sc) state and significantly increase dissipation during later operation. The existence of this effect has been proven in general. The study presented here now adds experimental data and simulations on the amplitude and distribution of the trapped magnetic flux after the sc phase transition in the complete nine-cell cavity geometry. Furthermore, the dynamic behavior in the thermoelectrically generated magnetic field during the phase transition is evaluated. DOI: 10.1103/PhysRevAccelBeams.22.052001 I. INTRODUCTION For many modern accelerators, continuous wave superconducting radio-frequency (srf) cavities are a key component. Presently, significant research and development is concentrating on reducing the power dissipation in these cavities to reduce the often prohibitively large cryogenic load. During the process it was found that the surface resistance Rs can be impacted by a number of different treatments. Various annealing techniques for high purity (high residual-resistivity ratio RRR ≈ 300) niobium with titanium or nitrogen were established in the past years [1,2]. They mainly influence the temperature and rf field dependence of the BCS surface resistance. In contrast, we expand here on