Texas A&M University, College Station, Texas, USA
JLAB, Newport News, Virginia, USA
A high-gradient test cavity is being developed to test wafer samples of advanced SRF surfaces at gradients to or beyond the BCS limit of Nb. The cavity design employs dielectric loading by a large high-purity sapphire crystal. As a first step towards construction we set out to measure the loss tangent of such a large HEMEX-grade sapphire crystal. The crystal was inserted into a single-cell CEBAF cavity equipped with couplers to operate in the TE01 mode so that the electric field was localized within the sapphire. Cold testing of the cavity, without the sapphire and numerical simulations, verified that the unloaded Q of the cavity was adequate to accurate measurement of the sapphire loss tangent down to 10-10. Several Q measurements were made of the sapphire-loaded cavity in a variety of conditions. The temperature dependence of the sapphire’s loss tangent and heat capacity were measured. The implications of these results for the high-gradient wafer test cavity design are reported.
