Kapitza resistance cooling of single crystal (111) niobium for superconducting rf cavities

The use of large grains or single crystal niobium to improve the Q factor of superconducting rf cavities for particle accelerators, is presently under study. Heat extraction which plays a decisive role in the thermomagnetic stability of these devices depends on the thermal conductivity of niobium K and the thermal boundary (Kapitza) resistance R_{K} at the niobium/superfluid helium interface. Here we present the first measurements of R_{K} performed between 1.5–2.1 K with single crystal (111) niobium, having two different surface morphologies, namely, a surface with a damage layer and a chemically polished surface. The thermal conductivity of the single crystal Nb samples is also simultaneously determined. For monocrystalline niobium we demonstrate that R_{K} is an increasing primary limiting factor with temperature, contrary to the behavior found for polycrystalline cavities. The present investigation reveals for the first time that the presence of impurities (metallic particles and oxygen) within the damage layer leads to a stronger R_{K}, although the effective heat exchange area to the superfluid is increased. We further show the importance of dislocations in the thermal conductivity of monocrystalline niobium.