An investigation of thermal transport in superconducting cavities made of high thermal conductivity niobium

Thermal-magnetic breakdown is the mechanism which ultimately limits field strengths in superconducting cavities whose microwave performance is not affected by multipactoring or field emission. Thermal-magnetic breakdown is thought to arise from localized heating of an isolated lossy area on the cavity surface; at a certain power level the excess heating may cause the temperature near the lassy area to exceed the superconducting critical temperature and lead to cavity breakdown. The objective of this investigation was to investigate systematically the two mechanisms of thermal transport in the cavity-cooling bath system: the thermal conductivity of the metal and heat transport across the metal to liquid helium interface. For this investigation, cavities were prepared with high thermal conductivity Nb; the thermal conductivity of this Nb at 4.2K was over 100 times higher than that of typical reactor grade Nb. To investigate the thermal transport processes, cavity surface temperature profiles were measured with dc heater power applied locally to the surface. The results agreed well with calculated equilibrium surface temperatures when reasonable values for the thermal boundary resistance between superconducting Nb and liquid He I or superfluid He II were used in the calculations. The microwave performance of the Nb cavities at X-band was considerably improved by the use of high thermal conductivity Nb; the high thermal conductivity Nb cavities consistently reached field levels over five times higher than the low thermal conductivity Nb cavities and sustained over 100 times as much dissipated power. These cavities never exhibited breakdown. Theoretical calculations showed that the performance of the low thermal conductivity Nb cavities was limited by the large temperature gradients at defects, whereas the performance of the high thermal conductivity Nb cavities was limited by transport of heat across the Nb-liquid He interface.