Thermal Link Design for Conduction Cooling of SRF Cavities Using Cryocoolers

Abstract

Superconducting radio frequency (SRF) cavities with quality factors ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> near 4 K have potential to be cooled using regenerative-cycle cryocoolers. Contrary to using liquid helium, cryogen-free operation can be realized by conductively linking a cryocooler to a cavity for extracting the cavity RF dissipation. The cavity-cryocooler thermal link needs careful design as its thermal conductance will control the temperatures of the cavity and the cryocooler. We present a thermal analysis of a conduction-cooled SRF cavity that identifies the link thermal conductance requirement. The analysis uses published or expected RF dissipation characteristics of an Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn coated niobium cavity and measured cooling capacity of a pulse tube cryocooler. We describe the mechanical design of a link that is constructed using commercial high-purity aluminum and facilitates bolted-connection to elliptical SRF cavities. The thermal performance of the link is assessed by finite element simulations, taking into account temperature dependent thermal conductivities and measured thermal contact resistance of aluminum and niobium. The link is shown to support operation at an accelerating gradient of 10 MV/m with the lowest-known `perfect' Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn residual surface resistance (~10 nΩ) and also under non-ideal cases that assume certain static heat leak into the system and non-perfect Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn coatings.