SRF2019 - List of Authors (Gurevich, A.V.)

Paper	Title	Page
TUP049	Maximum Performance of Cavities Affected by the High-field Q-slope (HFQS)	533
	G. Ciovati JLab, Newport News, Virginia, USA A.V. Gurevich, I.P. Parajuli ODU, Norfolk, Virginia, USA
	Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. The work of I. P. and A. G. is supported by NSF Grant PHY 100614-010. The performance of high-purity, bulk niobium SRF cavities treated by chemical processes such as BCP or EP is limited by the so-called high-field Q-slope (HFQS). Several models and experimental studies have been proposed and performed over the years to understand the origin of these anomalous losses but a general consensus on what these orgins are is yet to be established. In this contribution, we present the results from the RF tests of several 1.3 GHz single-cell cavities limited by the HFQS and tested using a variable input coupler. This allowed to maintain close to critical coupling even at high field and the data showed that the HFQS did not saturate and that in some cases a power dissipation of up to 200 W at 2 K could be sustained without quench.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP049
About •	paper received ※ 21 June 2019 paper accepted ※ 30 June 2019 issue date ※ 14 August 2019
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TUP052	Design and Commissioning of a Magnetic Field Scanning System for SRF Cavities	547
SUSP031	use link to see paper's listing under its alternate paper code
	I.P. Parajuli, J.R. Delayen, A.V. Gurevich, J. Nice ODU, Norfolk, Virginia, USA G. Ciovati, W.A. Clemens, J.R. Delayen JLab, Newport News, Virginia, USA
	Funding: Work supported by NSF Grant 100614-010. G. C. is supported by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. Trapped magnetic vortices are one of the leading sources of residual losses in SRF cavities. Mechanisms of flux pinning depend on the materials treatment and cool-down conditions. A magnetic field scanning system using flux-gate magnetometers and Hall probes has been designed and built to allow measuring the local magnetic field of trapped vortices normal to the outer surface of 1.3 GHz single-cell SRF cavities at cryogenic temperatures. Such system will allow inferring the key information about the distribution and magnitude of trapped flux in the SRF cavities for different material, surface preparations and cool-down conditions.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP052
About •	paper received ※ 22 June 2019 paper accepted ※ 30 June 2019 issue date ※ 14 August 2019
Export •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

TUP055	Nonlinear Dynamics and Dissipation of Vortex Lines Driven by Strong RF Fields	560
SUSP033	use link to see paper's listing under its alternate paper code
	M.R.P. Walive Pathiranage, A.V. Gurevich ODU, Norfolk, Virginia, USA
	Trapped vortices can contribute significantly to a residual surface resistance of superconducting radio frequency (SRF) cavities but the nonlinear dynamics of flexible vortex lines driven by strong rf currents has not been well understood. Here we report extensive numerical simulations of large-amplitude oscillations of a trapped vortex line under the strong rf magnetic field. The rf power dissipated by an oscillating vortex segment driven by the rf Meissner currents was calculated by taking into account the nonlinear vortex line tension, vortex mass and a nonlinear Larkin-Ovchinnikov and overheating viscous drag force. We calculated the field dependence of the surface resistance Rs and showed that at low frequencies Rs(H) increases with H but as the frequency increases, Rs(H) becomes a non-monotonic function of H which decreases with H at higher fields. These results suggest that trapped vortices can contribute to the extended Q(H) rise observed on the SRF cavities.
	Poster TUP055 [1.744 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP055
About •	paper received ※ 23 June 2019 paper accepted ※ 05 July 2019 issue date ※ 14 August 2019
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THFUA4	Field-Dependent Nonlinear Surface Resistance and Its Optimization by Surface Nano-Structuring of the SRF Cavities
	T. Kubo KEK, Ibaraki, Japan A.V. Gurevich ODU, Norfolk, Virginia, USA
	Funding: The work of T. K. was supported by JSPS KAKENHI Grant Number JP17H04839 and JP17KK0100. The work of A. G. was supported by NSF under Grant No. PHY-1416051 and DOE under Grant No DE-SC100387-020. We propose a theory of nonlinear surface resistance of a dirty superconductor in a strong RF field (H⁰), taking into account magnetic and nonmagnetic impurities, finite quasiparticle lifetimes, and a thin proximity-coupled normal layer characteristic of the oxide surface of many materials. It is shown that the interplay of the broadening of the quasiparticle density of states (DOS) peaks and a decrease of a quasiparticle gap caused by the RF currents produces a minimum in Rs(H⁰) and an extended rise of the quality factor Q(H⁰) with the RF field. Paramagnetic impurities shift the minimum in Rs(H⁰) to lower fields and can reduce Rs(H⁰) in a wide range of H0. Subgap states in the DOS can give rise to a residual surface resistance while reducing Rs at higher temperatures. A proximity-coupled normal layer can shift the minimum to either low and high fields and can reduce Rs below that of an ideal surface. The field dependence of Q(H⁰) can be very sensitive to the materials processing. The nonlinear RF losses can be minimized by tuning pairbreaking effects using impurity management or surface nanostructuring.
	Slides THFUA4 [7.130 MB]
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Paper

Title

Page

Maximum Performance of Cavities Affected by the High-field Q-slope (HFQS)

533

G. Ciovati
JLab, Newport News, Virginia, USA
A.V. Gurevich, I.P. Parajuli
ODU, Norfolk, Virginia, USA

Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. The work of I. P. and A. G. is supported by NSF Grant PHY 100614-010.
The performance of high-purity, bulk niobium SRF cavities treated by chemical processes such as BCP or EP is limited by the so-called high-field Q-slope (HFQS). Several models and experimental studies have been proposed and performed over the years to understand the origin of these anomalous losses but a general consensus on what these orgins are is yet to be established. In this contribution, we present the results from the RF tests of several 1.3 GHz single-cell cavities limited by the HFQS and tested using a variable input coupler. This allowed to maintain close to critical coupling even at high field and the data showed that the HFQS did not saturate and that in some cases a power dissipation of up to 200 W at 2 K could be sustained without quench.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP049

About •

paper received ※ 21 June 2019 paper accepted ※ 30 June 2019 issue date ※ 14 August 2019

Export •

reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

TUP052

Design and Commissioning of a Magnetic Field Scanning System for SRF Cavities

547

SUSP031

use link to see paper's listing under its alternate paper code

I.P. Parajuli, J.R. Delayen, A.V. Gurevich, J. Nice
ODU, Norfolk, Virginia, USA
G. Ciovati, W.A. Clemens, J.R. Delayen
JLab, Newport News, Virginia, USA

Funding: Work supported by NSF Grant 100614-010. G. C. is supported by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
Trapped magnetic vortices are one of the leading sources of residual losses in SRF cavities. Mechanisms of flux pinning depend on the materials treatment and cool-down conditions. A magnetic field scanning system using flux-gate magnetometers and Hall probes has been designed and built to allow measuring the local magnetic field of trapped vortices normal to the outer surface of 1.3 GHz single-cell SRF cavities at cryogenic temperatures. Such system will allow inferring the key information about the distribution and magnitude of trapped flux in the SRF cavities for different material, surface preparations and cool-down conditions.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP052

About •

paper received ※ 22 June 2019 paper accepted ※ 30 June 2019 issue date ※ 14 August 2019

Export •

reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

TUP055

Nonlinear Dynamics and Dissipation of Vortex Lines Driven by Strong RF Fields

560

SUSP033

use link to see paper's listing under its alternate paper code

M.R.P. Walive Pathiranage, A.V. Gurevich
ODU, Norfolk, Virginia, USA

Trapped vortices can contribute significantly to a residual surface resistance of superconducting radio frequency (SRF) cavities but the nonlinear dynamics of flexible vortex lines driven by strong rf currents has not been well understood. Here we report extensive numerical simulations of large-amplitude oscillations of a trapped vortex line under the strong rf magnetic field. The rf power dissipated by an oscillating vortex segment driven by the rf Meissner currents was calculated by taking into account the nonlinear vortex line tension, vortex mass and a nonlinear Larkin-Ovchinnikov and overheating viscous drag force. We calculated the field dependence of the surface resistance Rs and showed that at low frequencies Rs(H) increases with H but as the frequency increases, Rs(H) becomes a non-monotonic function of H which decreases with H at higher fields. These results suggest that trapped vortices can contribute to the extended Q(H) rise observed on the SRF cavities.

Poster TUP055 [1.744 MB]

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP055

About •

paper received ※ 23 June 2019 paper accepted ※ 05 July 2019 issue date ※ 14 August 2019

Export •

reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

THFUA4

Field-Dependent Nonlinear Surface Resistance and Its Optimization by Surface Nano-Structuring of the SRF Cavities

T. Kubo
KEK, Ibaraki, Japan
A.V. Gurevich
ODU, Norfolk, Virginia, USA

Funding: The work of T. K. was supported by JSPS KAKENHI Grant Number JP17H04839 and JP17KK0100. The work of A. G. was supported by NSF under Grant No. PHY-1416051 and DOE under Grant No DE-SC100387-020.
We propose a theory of nonlinear surface resistance of a dirty superconductor in a strong RF field (H⁰), taking into account magnetic and nonmagnetic impurities, finite quasiparticle lifetimes, and a thin proximity-coupled normal layer characteristic of the oxide surface of many materials. It is shown that the interplay of the broadening of the quasiparticle density of states (DOS) peaks and a decrease of a quasiparticle gap caused by the RF currents produces a minimum in Rs(H⁰) and an extended rise of the quality factor Q(H⁰) with the RF field. Paramagnetic impurities shift the minimum in Rs(H⁰) to lower fields and can reduce Rs(H⁰) in a wide range of H0. Subgap states in the DOS can give rise to a residual surface resistance while reducing Rs at higher temperatures. A proximity-coupled normal layer can shift the minimum to either low and high fields and can reduce Rs below that of an ideal surface. The field dependence of Q(H⁰) can be very sensitive to the materials processing. The nonlinear RF losses can be minimized by tuning pairbreaking effects using impurity management or surface nanostructuring.

Slides THFUA4 [7.130 MB]

Export •

reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)