Abstract
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.
Highlights
The superconducting rf (SRF) cavity quality factor is expressed as Q 1⁄4 !U=P, where U is the stored energy and (P=!) the power loss in the inner walls in one rf radian
For monocrystalline niobium we demonstrate that RK is an increasing primary limiting factor with temperature, contrary to the behavior found for polycrystalline cavities
To show the rather complex behavior of the thermal conductivity of our single crystal niobium, we indicate in Fig. 5 [curve a] the theoretical phonon thermal conductivity of an undeformed superconducting single niobium crystal, given by the Casimir equation
Summary
The superconducting rf (SRF) cavity quality factor is expressed as Q 1⁄4 !U=P, where U is the stored energy and (P=!) the power loss in the inner walls in one rf radian. For type II superconductors like Nb, in the clean limit (i.e. when the phonon mean-free path is greater than the coherence length) and at low frequency, the high field nonlinear correction increases exponentially with field and temperature, and can give rise to thermal runaway [1,2]. This model can, in particular, explain the hot spots observed on cavities where bundles of trapped vortices can produce localized dissipative regions from which heat spreads over several tens of mm. The magnetic/vortex origin of some of the hot spots have been
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