Thermal Transport and Thermal-Magnetic Breakdown in Superconducting Cavities Made of High Thermal Conductivity Niobium.
Kathleen Rempel Krafft, Ph.D.
Cornell University 1983
Abstract
Thermal-magnetic breakdown is the mechanism which ultimately limits field strengths in superconducting cavities whose 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 lossy area to exceed the superconducting critical temperature and lead to cavity breakdown. The objective of this investigation was to systematically investigate 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 niobium at 4.2K was over one hundred times higher than that of typical reactor grade niobium. To investigate the thermal transport processes surface temperature profiles were measured when dc heater power was applied locally to a cavity surface; the experimental results were compared with the equilibrium surface temperatures calculated from functions for the thermal conductivity of the Nb and the thermal boundary resistance between superconducting Nb and liquid He I or superfluid HE II; good agreement between the experimental results and the calculations was found when reasonable values for the thermal boundary resistance were used in the calculations. The microwave performance of the Nb cavities at X-band was found to be considerably improved by the use of high thermal conductivity niobium; the high thermal conductivity Nb cavities sustained field levels over five times higher than for low thermal conductivity Nb cavities, without exhibiting breakdown.
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