Fermilab, Batavia, Illinois, USA
Illinois Institute of Technology, Chicago, Illlinois, USA
IIT, Chicago, Illinois, USA
The cool down details of superconducting accelerating cavities are crucial parameters that have to be optimize in order to obtain very high quality factors. The temperature all around the cavity is monitored during its cool down across the critical temperature, in order to visualize the different dynamics of fast and slow cool-down, which determine considerable difference in terms of magnetic field expulsion and cavity performance. The study is performed placing a single cell 1.3 GHz elliptical cavity perpendicularly to the helium cooling flow, which is representative of how SRF cavities are cooled in an accelerator. Hence, the study involves geometrical considerations regarding the cavity horizontal configuration, underling the different impact of the various magnetic field components on the surface resistance. Experimental data also proves that under established conditions, flux lines are concentrated at the cavity top, in the equatorial region, leading to temperature rise.
Fermilab, Batavia, Illinois, USA
Illinois Institute of Technology, Chicago, Illlinois, USA
IIT, Chicago, Illinois, USA
The trapped flux surface resistance is one of the main contributions on cavity losses which appears when cavities are cooled in presence of external magnetic field. The study is focused on the understanding of the different parameters which determine the trapped flux surface resistance, and how this change as a function of different surface treatments. The study is performed on 1.3 GHz niobium cavities processed with different surface treatments after the 800 C bake: electro-polishing (EP), 120 C baking, and N-doping varying the time of the Nitrogen exposure. The trapped flux surface resistance normalized for the trapped magnetic flux is then analyzed as a function of the mean free path in order to find the surface treatment which minimized the trapped flux sensitivity.
Fermilab, Batavia, Illinois, USA
Illinois Institute of Technology, Chicago, Illlinois, USA
IIT, Chicago, Illinois, USA
In this article a calculation of the trapped flux surface resistance is presented. The two main mechanisms considered in such approach are the oscillation of the magnetic flux trapped in the superconductor due to the Lorentz force, and the static resistance associated to the normal conducting vortex core. The model derived shows a good description of the available experimental data, highlighting that the radio frequency vortex dissipation is mostly due to the static part of the surface resistance. We show that the surface resistance for 100% trapped flux normalized to the trapped field (expressed in nOhm/mG) can be approximated to R/B=18.3*(l f)1/2/(50.1+l) with l the mean free path in nm and f the frequency in GHz.
TRIUMF, Canada's National Laboratory for Particle and Nuclear Physics, Vancouver, Canada
CERN, Geneva, Switzerland
PSI, Villigen PSI, Switzerland
ANL, Argonne, Illinois, USA
Sheffield University, Sheffield, United Kingdom
IIT, Chicago, Illinois, USA
