Investigation of the surface resistance of superconducting niobium using thermometry in superfluid helium

For future application in high energy linear colliders superconducting accelerator structures must exhibit gradients of at least Eacc= 30 MV/m at Q-values above 10¹⁰. Even though Q-values as high as 10¹¹have been obtained in smaller assemblies, the Q-value of niobium accelerator structures is usually limited by temperature independent residual losses. The corresponding average residual resistance is typically a few orders of magnitude higher than the theoretical surface resistance at 1.5 K predicted by the BCS-theory. In order to systematically investigate the spatial distribution of the residual resistance and its dependence on surface conditions of the niobium cavity surface, we have built a TE011- S-band cavity and a thermometry system, which allows the detection of losses as low as a few microwatt in superfluid helium using a lock-in technique. The thermometry system consists of 100 Allen-Bradley carbon resistors, which are pressed against the endplate of the cavity and which are well shielded against the surrounding helium bath. A computerized data acquisition system is used to measure the loss distribution on the endplate of the cavity. First measurements indicate the presence of three kinds of losses: joint losses caused by poor rf contact at the indium gasket between cavity body and endplate, large patches of moderate resistance and a few localized spots of enhanced losses. With the present system we have been able to measure at a bath temperature of 1.5 K surface resistances of about 50 nOhm at an rf magnetic field of about 20 Oersted, which in turn indicates that at some higher field levels the system will be capable of detecting loss distributions equivalent to Q-values above 10¹¹.