C-AD Accelerator Physics Seminar

Abstract:
Superconducting radio frequency (SRF) technology is widely adopted in particle accelerators. There remain many open questions, however, in developing a systematic understanding of the fundamental behavior of SRF materials. A facility that can measure the SRF properties of small samples in a range of 2~40 K temperature is needed in order to fully answer these questions. The Jefferson Lab surface impedance characterization (SIC) system has been designed to attempt to meet this requirement. It consists of a sapphire-loaded cylindrical Nb TE011 cavity at 7.4 GHz with a 50 mm diameter flat sample placed on a non-contacting end plate and uses a calorimetric technique to measure the radio frequency (RF) induced heat on the sample. Tests with polycrystalline and large grain bulk Nb samples have been done at <15 mT magnetic field. Microstructure analyses and SRF measurements of large scale epitaxial MgB2 films have been reported. The SRF properties of different film thicknesses (200 nm and 350 nm) were evaluated using SIC system under different temperatures and applied fields at 7.4 GHz. A surface resistance of 9±2 μΩ has been observed at 2.2 K.
Based on BCS theory with moving Cooper pairs, the electron states distribution at 0K and the probability of electron occupation with finite temperature have been derived and applied to anomalous skin effect theory to obtain the surface impedance of a superconductor under RF field. We present the numerical results for Nb.