KAPITZA CONDUCTANCE AND THERMAL CONDUCTIVITY OF MATERIALS USED FOR SRF CAVITIES FABRICATION

Abstract

A dedicated apparatus was developed for measuring at low temperature the thermal conductivity of different materials actually used for the fabrication of SRF cavities. This device allows the test of four samples simultaneously. Several samples of different materials (Nb sheets, Ti…) were either tested as received or/and subjected to various Heat Treatment (H.T) then tested. Another test facility was used for the characterization of heat transfer between different materials and superfluid helium in the Kapitza regime. Kapitza resistance measurements were performed on several niobium specimen either uncoated or coated with thermally sprayed layers of different materials ( Cu, Ti .....) in the temperature range 1.5 K - 2.1 K. The influence of different parameters such as Nb initial purity (RRR) , Nb heat treatment at 800° C or at 1200 ° C with Ti gettering , Nb and/or coating surface preparation and treatment on both thermal conductivity and Kapitza conductance was investigated. We report also the effect of the coating on Kapitza conductance. Finally, our experimental data are compared to results previously reported and the effect of the coating on the SRF cavities ultimate RF performance and thermal stability is discussed.