HIGH-PRECISION MEASUREMENTS OF THE QUALITY FACTOR OFSUPER CONDUCTING CAVITIES AT THE FREIA LABORATORY
2015 (English)Conference paper (Refereed)
The dependence of cavity quality factor Q0 on accelerating gradient gives insight into the intrinsic limit of RFsurface impedance that determines the cavity performance. In this paper we propose a high-precision method of measuringQ0 of SRF cavities. A common way to study the performance of an SRF cavity is to build an oscillator around it that is referred to as a self-excited loop. In the standard approach, by tuning the loop phase for a maximum field level in thecavity and measuring forward and reflected waves, one finds the cavity coupling. Then, performing a time-decay measurement and finding the total quality factor, one gets Q0. However, this approach suffers from a deficiency originating from a single data-point measurement of the reflection coefficient. In our method by varying the loop phase shift, one obtains amplitudes of the reflection coefficient of the cavity as a function of its phases. The complex reflection coefficient describes a perfect circle in polar coordinates. Fitting the overdetermined set of data to that circle allows more accurate calculation of Q0 via the least-squares procedure. The method has been tested at the FREIA Laboratory on two cavities from IPN Orsay: a single spoke and a prototype ESS double spoke.
Place, publisher, year, edition, pages
Accelerator Physics and Instrumentation
IdentifiersURN: urn:nbn:se:uu:diva-291289ISBN: 978-3-95450-178-6OAI: oai:DiVA.org:uu-291289DiVA: diva2:925226
Proc. of SRF2015, Whistler, Canada, pp. 810-813, TUPB089