New Evidence for Thermal Boundary Resistance Effects in Superconducting Cavities for Particle Accelerator
Kapitza thermal resistance and, more in general, thermal boundary resistance effects have been often considered as a possible source of non ideal superconducting accelerating cavity behavior, through the formation of a temperature difference between the inner cavity superconducting surface and the helium bath.
However the general assessment reported in the literature is that such effects can be generally neglected, at least at low or moderate input power. Here we present new data on small test bulk Nb 6Ghz cavities, showing that when the cavity surface resistance (or the Q) is plotted as a function of the temperature at constant input power, a clear anomaly occurs at the Helium superfluid transition point reflecting an abrupt change of the thermal boundary resistance at that temperature.
The data analysis shows that this anomaly is consistent with independently measured values of the thermal boundary/Kapitza resistance. It is then shown that appropriate cavity external surface treatments can lower the Kapitza resistance, improving the overall cavity performance.