SRF2015 - List of Authors (Knobloch, J.)

Paper	Title	Page
MOBA04	High-Q Operation of SRF Cavities: The Impact of Thermocurrents on the RF Surface Resistance	37
	J.M. Köszegi, J. Knobloch, O. Kugeler HZB, Berlin, Germany
	For CW applications much effort is being expended to minimize the power dissipation (surface resistance) of niobium cavities. Previous studies have shown that residual resistance can be reduced by performing a thermal cycle, a procedure of warming up a cavity after initial cooldown to about 20K and cooling it down again. It was postulated that thermocurrents during cooldown generate additional trapped magnetic flux that impacts the cavity quality factor. Here, we present a more extensive study that includes measurements of two additional passband modes and that confirms the effect. A change in surface resistance of more than a factor seven was observed. In this paper, we also discuss simulations that support the claim. While the layout of the cavity LHe tank system is cylindrically symmetric, we show that the temperature dependence of the material parameters results in a non-symmetric current distribution.
	Slides MOBA04 [2.830 MB]
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MOPB019	Horizontal Testing and Thermal Cycling of an N-Doped Tesla Type Cavity	125
	O. Kugeler, J. Knobloch, J.M. Köszegi HZB, Berlin, Germany A. Grassellino, O.S. Melnychuk, A. Romanenko, D.A. Sergatskov Fermilab, Batavia, Illinois, USA
	An N-doped TESLA type cavity treated at FERMILAB has been tested in the HoBiCaT horizontal test stand. Temperatures and magnetic fields occuring during the superconducting transition were recorded at various positions and directions on the outer cavity surface. Several thermal cycling runs were performed yielding different Q0 factors just like in undoped cavities. The resulting residual and BCS resistance values were correlated to the thermal and magnetic conditions during cooldown.
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TUAA03	BESSY VSR: A Novel Application of SRF for Synchrotron Light Sources	462
	A.V. Vélez, H.-W. Glock, P. Goslawski, A. Jankowiak, J. Knobloch, A. Neumann, M. Ries, G. Wüstefeld HZB, Berlin, Germany
	CW SRF Cavities have been used very successfully in the past in synchrotron light sources to provide high power acceleration. Here we present a novel application of higher harmonic systems of two frequencies (1.5 GHz and 1.75 GHz) to generate a beating of accelerating voltage. With such a system it is possible to store "standard" (some 10 ps long) and "short" (ps and sub-ps long) pulses simultaneously in the light source. This opens up brand new possibilities for light source users to perform dynamic and high-resolution experiments at the same facility. The demands on the SRF system and RF control are substantial and a new design, based on waveguide damping, is currently being developed. This system will be used for a major upgrade of the BESSY-II facility to the BESSY Variable Pulse Storage Ring (BESSY-VSR) for a next-generation storage-ring light source. We will discuss the concept, challenges and designs for BESSY-VSR.
	Slides TUAA03 [2.103 MB]
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TUBA03	On the Understanding of Q-Slope of Niobium Thin Films	494
	S. Aull, T. Junginger, A. Sublet, W. Venturini Delsolaro, P. Zhang CERN, Geneva, Switzerland J. Knobloch HZB, Berlin, Germany J. Knobloch University of Siegen, Siegen, Germany A-M. Valente-Feliciano JLab, Newport News, Virginia, USA
	The Q-slope of niobium coated copper cavities at medium fields is still the limiting factor for the application the Nb/Cu technology in accelerators. This paper presents a dedicated study of a niobium coating with bulk-like characteristics which shows a Q-slope comparable to bulk Nb at 400 MHz and 4 K. Combining the bulk like film with recent findings of the HIE Isolde indicates that the film microstructure and the Nb/Cu interface are the key aspects to understanding the Q-slope.
	Slides TUBA03 [3.414 MB]
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TUPB050	Secondary Electron Yield of SRF Materials	686
	S. Aull, T. Junginger, H. Neupert CERN, Geneva, Switzerland S. Aull, J. Knobloch University of Siegen, Siegen, Germany J. Knobloch HZB, Berlin, Germany
	The secondary electron yield (SEY) describes the number of electrons emitted to the vacuum per arriving electron at the surface. For a given geometry, the SEY is the defining factor for multipacting activity. In the quest of superconducting RF materials beyond bulk niobium, we studied the SEY of the currently most important candidates for future SRF applications: Nb3Sn, NbTiN and MgB2. All studies were done on clean but technical surfaces, i.e. on clean surfaces exposed to air and with their native oxides as it would be the case for SRF cavities.
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TUPB067	Design and First Measurements of an Alternative Calorimetry Chamber for the HZB Quadrupole Resonator	739
	S. Keckert, R. Kleindienst, J. Knobloch, O. Kugeler HZB, Berlin, Germany
	Funding: The work is part of EuCARD-2, partly funded by the European Commission, GA 312453. The systematic research on superconducting thin films requires dedicated testing equipment. The Quadrupole Resonator (QPR) is a specialized tool to characterize the superconducting properties of circular samples. A calorimetric measurement of the RF surface losses allows the surface resistance to be measured with sub nano-ohm resolution. This measurement can be performed over a wide temperature and magnetic field range, at frequencies of 433, 866 and 1300 MHz. The system at Helmholtz-Zentrum Berlin (HZB) is based on a resonator built at CERN and has been optimized to lower peak electric fields and an improved resolution. An alternative calorimetry chamber has been designed in order to provide flat samples for coating and to ease changing of samples. Furthermore it enables exchangeability of samples between the QPRs at HZB and CERN. With this poster the design and first measurements of the new calorimetry chamber is presented.
	Poster TUPB067 [1.776 MB]
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TUPB078	Developments on a Cold Bead-Pull Test Stand for SRF Cavities	770
	A.V. Vélez, A. Frahm, J. Knobloch, A. Neumann HZB, Berlin, Germany
	Final tuning and field profile characterization of SRF cavities always takes place at room temperature. However, important questions remains as to what happens when the cavity is cooled to LHe temperature, in particular with multi cell systems. To enable the characterization of cavities in the cold, we have designed and commissioned a "cold bead-pull" test stand at HZB. The present test stand is designed to be integrated in HoBiCaT (Horizontal bi-cavity testing facility) with the ability to provide electric field profile measurements under realistic superconducting conditions (T=1.8K). In this paper mechanical and operational details of the apparatus will be described as well as future plans for the development and usage of this facility.
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WEA1A04	Commissioning Results of the HZB Quadrupole Resonator	930
	R. Kleindienst, A. Burrill, S. Keckert, J. Knobloch, O. Kugeler HZB, Berlin, Germany
	Funding: The research leading to these results has received funding from the European Commission under the FP7 Research Infrastructures project EuCARD-2, grant agreement no.312453 Recent cavity results with niobium have demonstrated the necessity of a good understanding of both the BCS and residual resistance. For a complete picture, and comparison with theory, it is essential that one can measure the RF properties as a function of applied magnetic field, temperature, frequency and ambient magnetic field. Standard cavity measurements are limited in their ability to change all parameters freely and in a controlled manner. On the other hand, most sample measurement setups operate at fairly high frequency, where the surface resistance is always BCS dominated. The quadrupole resonator, originally developed at CERN, is ideally suited for RF characterization of samples at frequencies of 400 and 1300 MHz, between which many of today’s SRF cavities operate. We report on a modified version of the QPR with improved RF figures of merit for high-field operation. Experimental challenges in the commissioning run and alternate designs towards a simpler sample change are shown alongside measurement results of a large grain niobium sample.
	Slides WEA1A04 [5.611 MB]
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THPB026	Update on SRF Cavity Design, Production and Testing for BERLinPro	1127
	A. Neumann, W. Anders, A. Burrill, A. Frahm, H.-W. Glock, J. Knobloch, O. Kugeler HZB, Berlin, Germany K. Brackebusch, T. Galek, J. Heller, U. van Rienen Rostock University, Faculty of Computer Science and Electrical Engineering, Rostock, Germany G. Ciovati, W.A. Clemens, C. Dreyfuss, D. Forehand, T. Harris, P. Kneisel, R.B. Overton, L. Turlington JLab, Newport News, Virginia, USA E.N. Zaplatin FZJ, Jülich, Germany
	Funding: Work supported by German Bundesministerium für Bildung und Forschung, Land Berlin, and grants of Helmholtz Association. The BERLinPro Energy Recovery Linac (ERL) is currently being built at Helmholtz-Zentrum Berlin in order to study the accelerator physics of operating a high current, 100 mA, 50 MeV low emittance ERL utilizing all SRF cavity technology. For this machine three different types of SRF cavities are being developed. For the injector section, consisting of an SRF photoinjector and a three two cell booster cavity module, fabrication is completed. The cavities were designed at HZB and manufactured, processed and vertically tested at Jefferson Laboratory. In this paper we will review the design and production process of the two structures and show the latest horizontal acceptance tests at HZB prior to installation into the newly designed cryo-module. For the Linac cavity the latest cavity and module design studies are being shown.
	Poster THPB026 [1.535 MB]
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Paper

Title

Page

High-Q Operation of SRF Cavities: The Impact of Thermocurrents on the RF Surface Resistance

37

J.M. Köszegi, J. Knobloch, O. Kugeler
HZB, Berlin, Germany

For CW applications much effort is being expended to minimize the power dissipation (surface resistance) of niobium cavities. Previous studies have shown that residual resistance can be reduced by performing a thermal cycle, a procedure of warming up a cavity after initial cooldown to about 20K and cooling it down again. It was postulated that thermocurrents during cooldown generate additional trapped magnetic flux that impacts the cavity quality factor. Here, we present a more extensive study that includes measurements of two additional passband modes and that confirms the effect. A change in surface resistance of more than a factor seven was observed. In this paper, we also discuss simulations that support the claim. While the layout of the cavity LHe tank system is cylindrically symmetric, we show that the temperature dependence of the material parameters results in a non-symmetric current distribution.

Slides MOBA04 [2.830 MB]

Export •

reference for this paper to ※ BibTeX, ※ LaTeX, ※ Text, ※ RIS/RefMan, ※ EndNote (xml)

MOPB019

Horizontal Testing and Thermal Cycling of an N-Doped Tesla Type Cavity

125

O. Kugeler, J. Knobloch, J.M. Köszegi
HZB, Berlin, Germany
A. Grassellino, O.S. Melnychuk, A. Romanenko, D.A. Sergatskov
Fermilab, Batavia, Illinois, USA

An N-doped TESLA type cavity treated at FERMILAB has been tested in the HoBiCaT horizontal test stand. Temperatures and magnetic fields occuring during the superconducting transition were recorded at various positions and directions on the outer cavity surface. Several thermal cycling runs were performed yielding different Q0 factors just like in undoped cavities. The resulting residual and BCS resistance values were correlated to the thermal and magnetic conditions during cooldown.

Export •

reference for this paper to ※ BibTeX, ※ LaTeX, ※ Text, ※ RIS/RefMan, ※ EndNote (xml)

TUAA03

BESSY VSR: A Novel Application of SRF for Synchrotron Light Sources

462

A.V. Vélez, H.-W. Glock, P. Goslawski, A. Jankowiak, J. Knobloch, A. Neumann, M. Ries, G. Wüstefeld
HZB, Berlin, Germany

CW SRF Cavities have been used very successfully in the past in synchrotron light sources to provide high power acceleration. Here we present a novel application of higher harmonic systems of two frequencies (1.5 GHz and 1.75 GHz) to generate a beating of accelerating voltage. With such a system it is possible to store "standard" (some 10 ps long) and "short" (ps and sub-ps long) pulses simultaneously in the light source. This opens up brand new possibilities for light source users to perform dynamic and high-resolution experiments at the same facility. The demands on the SRF system and RF control are substantial and a new design, based on waveguide damping, is currently being developed. This system will be used for a major upgrade of the BESSY-II facility to the BESSY Variable Pulse Storage Ring (BESSY-VSR) for a next-generation storage-ring light source. We will discuss the concept, challenges and designs for BESSY-VSR.

Slides TUAA03 [2.103 MB]

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TUBA03

On the Understanding of Q-Slope of Niobium Thin Films

494

S. Aull, T. Junginger, A. Sublet, W. Venturini Delsolaro, P. Zhang
CERN, Geneva, Switzerland
J. Knobloch
HZB, Berlin, Germany
J. Knobloch
University of Siegen, Siegen, Germany
A-M. Valente-Feliciano
JLab, Newport News, Virginia, USA

The Q-slope of niobium coated copper cavities at medium fields is still the limiting factor for the application the Nb/Cu technology in accelerators. This paper presents a dedicated study of a niobium coating with bulk-like characteristics which shows a Q-slope comparable to bulk Nb at 400 MHz and 4 K. Combining the bulk like film with recent findings of the HIE Isolde indicates that the film microstructure and the Nb/Cu interface are the key aspects to understanding the Q-slope.

Slides TUBA03 [3.414 MB]

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reference for this paper to ※ BibTeX, ※ LaTeX, ※ Text, ※ RIS/RefMan, ※ EndNote (xml)

TUPB050

Secondary Electron Yield of SRF Materials

686

S. Aull, T. Junginger, H. Neupert
CERN, Geneva, Switzerland
S. Aull, J. Knobloch
University of Siegen, Siegen, Germany
J. Knobloch
HZB, Berlin, Germany

The secondary electron yield (SEY) describes the number of electrons emitted to the vacuum per arriving electron at the surface. For a given geometry, the SEY is the defining factor for multipacting activity. In the quest of superconducting RF materials beyond bulk niobium, we studied the SEY of the currently most important candidates for future SRF applications: Nb3Sn, NbTiN and MgB2. All studies were done on clean but technical surfaces, i.e. on clean surfaces exposed to air and with their native oxides as it would be the case for SRF cavities.

Export •

reference for this paper to ※ BibTeX, ※ LaTeX, ※ Text, ※ RIS/RefMan, ※ EndNote (xml)

TUPB067

Design and First Measurements of an Alternative Calorimetry Chamber for the HZB Quadrupole Resonator

739

S. Keckert, R. Kleindienst, J. Knobloch, O. Kugeler
HZB, Berlin, Germany

Funding: The work is part of EuCARD-2, partly funded by the European Commission, GA 312453.
The systematic research on superconducting thin films requires dedicated testing equipment. The Quadrupole Resonator (QPR) is a specialized tool to characterize the superconducting properties of circular samples. A calorimetric measurement of the RF surface losses allows the surface resistance to be measured with sub nano-ohm resolution. This measurement can be performed over a wide temperature and magnetic field range, at frequencies of 433, 866 and 1300 MHz. The system at Helmholtz-Zentrum Berlin (HZB) is based on a resonator built at CERN and has been optimized to lower peak electric fields and an improved resolution. An alternative calorimetry chamber has been designed in order to provide flat samples for coating and to ease changing of samples. Furthermore it enables exchangeability of samples between the QPRs at HZB and CERN. With this poster the design and first measurements of the new calorimetry chamber is presented.

Poster TUPB067 [1.776 MB]

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reference for this paper to ※ BibTeX, ※ LaTeX, ※ Text, ※ RIS/RefMan, ※ EndNote (xml)

TUPB078

Developments on a Cold Bead-Pull Test Stand for SRF Cavities

770

A.V. Vélez, A. Frahm, J. Knobloch, A. Neumann
HZB, Berlin, Germany

Final tuning and field profile characterization of SRF cavities always takes place at room temperature. However, important questions remains as to what happens when the cavity is cooled to LHe temperature, in particular with multi cell systems. To enable the characterization of cavities in the cold, we have designed and commissioned a "cold bead-pull" test stand at HZB. The present test stand is designed to be integrated in HoBiCaT (Horizontal bi-cavity testing facility) with the ability to provide electric field profile measurements under realistic superconducting conditions (T=1.8K). In this paper mechanical and operational details of the apparatus will be described as well as future plans for the development and usage of this facility.

Export •

reference for this paper to ※ BibTeX, ※ LaTeX, ※ Text, ※ RIS/RefMan, ※ EndNote (xml)

WEA1A04

Commissioning Results of the HZB Quadrupole Resonator

930

R. Kleindienst, A. Burrill, S. Keckert, J. Knobloch, O. Kugeler
HZB, Berlin, Germany

Funding: The research leading to these results has received funding from the European Commission under the FP7 Research Infrastructures project EuCARD-2, grant agreement no.312453
Recent cavity results with niobium have demonstrated the necessity of a good understanding of both the BCS and residual resistance. For a complete picture, and comparison with theory, it is essential that one can measure the RF properties as a function of applied magnetic field, temperature, frequency and ambient magnetic field. Standard cavity measurements are limited in their ability to change all parameters freely and in a controlled manner. On the other hand, most sample measurement setups operate at fairly high frequency, where the surface resistance is always BCS dominated. The quadrupole resonator, originally developed at CERN, is ideally suited for RF characterization of samples at frequencies of 400 and 1300 MHz, between which many of today’s SRF cavities operate. We report on a modified version of the QPR with improved RF figures of merit for high-field operation. Experimental challenges in the commissioning run and alternate designs towards a simpler sample change are shown alongside measurement results of a large grain niobium sample.

Slides WEA1A04 [5.611 MB]

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reference for this paper to ※ BibTeX, ※ LaTeX, ※ Text, ※ RIS/RefMan, ※ EndNote (xml)

THPB026

Update on SRF Cavity Design, Production and Testing for BERLinPro

1127

A. Neumann, W. Anders, A. Burrill, A. Frahm, H.-W. Glock, J. Knobloch, O. Kugeler
HZB, Berlin, Germany
K. Brackebusch, T. Galek, J. Heller, U. van Rienen
Rostock University, Faculty of Computer Science and Electrical Engineering, Rostock, Germany
G. Ciovati, W.A. Clemens, C. Dreyfuss, D. Forehand, T. Harris, P. Kneisel, R.B. Overton, L. Turlington
JLab, Newport News, Virginia, USA
E.N. Zaplatin
FZJ, Jülich, Germany

Funding: Work supported by German Bundesministerium für Bildung und Forschung, Land Berlin, and grants of Helmholtz Association.
The BERLinPro Energy Recovery Linac (ERL) is currently being built at Helmholtz-Zentrum Berlin in order to study the accelerator physics of operating a high current, 100 mA, 50 MeV low emittance ERL utilizing all SRF cavity technology. For this machine three different types of SRF cavities are being developed. For the injector section, consisting of an SRF photoinjector and a three two cell booster cavity module, fabrication is completed. The cavities were designed at HZB and manufactured, processed and vertically tested at Jefferson Laboratory. In this paper we will review the design and production process of the two structures and show the latest horizontal acceptance tests at HZB prior to installation into the newly designed cryo-module. For the Linac cavity the latest cavity and module design studies are being shown.

Poster THPB026 [1.535 MB]

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reference for this paper to ※ BibTeX, ※ LaTeX, ※ Text, ※ RIS/RefMan, ※ EndNote (xml)