Thermocurrents and their role in highQcavity performance

Abstract

Over the past years it became evident that the quality factor of a superconducting cavity is not only determined by its surface preparation procedure, but is also i n f l u e n c e d b y t h e w a y t h e c a v i t y i s c o o l e d d o w n . Moreover, different data sets exists, some of which indicate that a slow cool-down through the critical temperature is favourable while other data states the exact opposite. Even though there were speculations and some models about the role of thermo-currents and fluxpinning, the difference in behaviour remained a mystery.I n t h i s p a p e r w e w i l l f o r t h e f i r s t t i m e p r e s e n t a consistent theoretical model which we confirmed by data that describes the role of thermo-currents, driven by temperature gradients and material transitions. We will c le arly s ho w ho w the y imp ac t t he qu ali ty f ac to r o f a cavity, discuss our findings, relate it to findings at other labs and develop mitigation strategies which especially address the issue of achieving high quality factors of socalled nitrogen doped cavities in horizontal test.

Advanced study of the thermoelectrically generated magnetic field in a nine-cell cavity during the superconducting phase transition

“…While the overall thermocurrent is mainly defined by the material properties and the temperatures of the two joints, previous studies have demonstrated that the magnetic field at the rf surface is strongly dependent on the temperature distribution [4,5]. Figure 2 depicts the distribution of the thermocurrent for two different temperature configurations.…”

“…It has been demonstrated that large temperature differences between the two cavity (niobium) and tank (titanium) joints drive thermoelectric currents during the cool down process in a horizontal cavity test assembly or cryomodule. The currents generate a magnetic field which can get trapped during the superconducting phase transition [3][4][5]. Any asymmetry in the temperature profile is important in determining the additional surface resistance, which can be as high as 10 nΩ, thus degrading significantly the quality factor of typical cavities [6].…”

Study of Possible Frequency Dependence of Small AC Fields on Magnetic Flux Trapping in Niobium by Polarized Neutron Imaging

“…In particular, it was found that the size of trapped flux is affected by several different parameters, such as the cooldown conditions or material properties like RRR (residual resistivity ratio). This laid the basis for further, systematic investigations [1][2][3][4][5][6][7][8][9][10][11]. These methods have in common that the magnetic flux is observed indirectly-either by measuring its impact on the cavity performance or by measuring the impact of the incomplete Meissner transition on the ambient magnetic field or by small angle neutron scattering.…”