To study the RF properties of superconducting A15 compounds

The purpose of the work was to study the low RF-field loss of A15 superconducting compounds. This type of superconducting material has potential for application to the construction of microwave energy storage cavities whose energy can be dumped in a very short time so as to produce a very large microwave power pulse. An apparatus, using a calorimetric technique, was developed to measure the surface resistance of small-area superconducting samples deposited on sapphire substrates. Both a Nb and a Nb3Sn sample (the latter in the A15 structure) were prepared, characterized and measured. These initial measurements revealed additional losses that are most likely due to the region near the superconductor-sapphire interface. Measurements of the superconducting transition temperature were made as a function of film thickness to study the effect of the interface. The interface losses can be avoided in practice, and in our measurements they can be circumvented by employing a circularly cylindrical geometry which shields the interface region from the rf fields. In the course of developing the surface resistance apparatus, dielectric loss measurements were made on pure sapphire samples.