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A Method for High-Precision Characterization of the Q-Slope of Superconducting RF Cavities
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics.ORCID iD: 0000-0002-2217-8032
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2016 (English)In: IEEE transactions on microwave theory and techniques, ISSN 0018-9480, E-ISSN 1557-9670, Vol. 64, no 11, p. 3764-3771Article in journal (Refereed) Published
Abstract [en]

We propose a novel method for high-precision determination of a quality factor Q(0) of a superconducting radio-frequency cavity as a function of the strength of the field excited in the cavity, the so-called Q-slope. Usually, the cavity parameters are measured only at resonance for different cavity field strengths, but such a single data point measurement for a given field strength results in a 10%-15% uncertainty in Q(0). In contrast, we propose a method that improves the accuracy of Q(0) determination by an order of magnitude. We vary the phase of an excited stabilized field in the cavity and measure the reflection coefficient of the cavity as a function of the phase. The procedure is repeated for different strengths of the excited field. Given the fact that the complex reflection coefficient of a cavity describes a perfect circle in polar coordinates as a function of the field phase for a constant field strength, we find the coupling coefficient much more accurately by fitting the overdetermined set of measured data to the circle for each value of the cavity field. From the time-decay measurement, which allows least-squares minimization, we accurately find the total (loaded) quality factor and deduce Q(0) with an uncertainty of around 1%.

Place, publisher, year, edition, pages
2016. Vol. 64, no 11, p. 3764-3771
Keywords [en]
Cavity quality factor, minimization procedure, precise microwave measurements, self-excited loop, superconducting (SC) radio-frequency (RF) resonator
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-311203DOI: 10.1109/TMTT.2016.2605671ISI: 000388501900002OAI: oai:DiVA.org:uu-311203DiVA, id: diva2:1059050
Available from: 2016-12-22 Created: 2016-12-22 Last updated: 2018-02-28Bibliographically approved
In thesis
1. From Macroscopic to Microscopic Dynamics of Superconducting Cavities
Open this publication in new window or tab >>From Macroscopic to Microscopic Dynamics of Superconducting Cavities
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Superconducting (SC) radio frequency (RF) cavities are at the heart of many large-scale particle accelerators such as the European Spallation Source (ESS), the X-ray Free Electron Laser (XFEL), the Linac Coherent Light Source (LCLS)-II and the proposed International Linear Collider (ILC). The SC cavities are essentially resonant structures with very high intrinsic quality factors (Q0) of the order of 1010. The high Q0 of the cavities leads to increased reflection during charging of the cavities to nominal voltage because the bandwidth of the signal exceeding that of the cavity. This results in high energy losses in case of pulsed machines. In this thesis I explore and present a novel technique to optimally charge the superconducting cavities with the particular example of the spoke cavities to be used for the ESS project in Lund, Sweden. The analysis reveals that slow charging with hyperbolic sine cavity voltage profile matches the signal bandwidth to that of the cavity which leads to energy efficient filling.

However, a filling rate lower than some particular value is counter-productive. The energy expended in cryogenic cooling to evacuate the heat due to ohmic losses in the cavity starts to dominate the lost energy. Such cryogenic losses are dependent on cavity Q0 through the residual resistance. The residual resistance changes with the applied electromagnetic field due to the pair-breaking mechanism of Cooper-pairs. Hence, methods for accurate measurement of the cavity Q0 are essential for accurate characterization and operation of the superconducting cavities. In this thesis I propose a novel method to accurately measure Q0 as a function of the applied electromagnetic field and present experimental results from the prototype spoke cavity in the Facility for Research Instrumentation and Accelerator Development (FREIA), at Uppsala University.

The cavity quality factor (Q0) is also dependent on the material’s purity and the trapped magnetic flux in the superconducting material. Recent studies have revealed that the rate of cooling of materials through the critical temperature has an effect on the residual flux trapped in the material. In this thesis I use the time-dependent Ginzburg-Landau equations to model the process of state transition from a normal to a superconducting state. This theoretical study may allow an explanation of the experimentally observed results from the basic principles of the general theory of state transitions as proposed by Ginzburg and Landau.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 75
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1638
Keywords
superconducting cavity, superconductivity, self-excited loop, Ginzburg-landau, vortex, optimization, quality factor, microwave
National Category
Accelerator Physics and Instrumentation
Research subject
Physics with specialization in Elementary Particle Physics
Identifiers
urn:nbn:se:uu:diva-343704 (URN)978-91-513-0253-9 (ISBN)
Public defence
2018-04-20, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:30 (English)
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Supervisors
Available from: 2018-03-27 Created: 2018-02-28 Last updated: 2018-04-24

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Goryashko, Vitaliy A.Bhattacharyya, Anirban KrishnaLi, HanDancila, DragosRuber, Roger

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