SRF2013 - List of Authors (Knobloch, J.)

Paper

Title

Page

High Q0 Research: The Dynamics of Flux Trapping in Superconducting Niobium

374

J.M. Vogt
HZB, Berlin, Germany
J. Knobloch, O. Kugeler
BESSY GmbH, Berlin, Germany

The quality factor Q0 that can be obtained in a superconducting cavity is known to depend on various factors like niobium material properties, treatment history and magnetic shielding. We believe that cooling conditions have an additional impact, as they appear to influence the amount of trapped flux and hence the residual resistance. We have constructed a test stand using niobium rods to study flux trapping. Here we can precisely control the temperature and measure the dynamics of flux trapping at the superconducting phase transition. We learned that magnetic flux can be generated when a temperature gradient exists along the rod as the niobium transitions to the superconducting state, which subsequently remains trapped. It was also shown that the cooling rate can influence the amount of externally applied flux which is trapped. Furthermore, we also were able to demonstrate that flux lines become mobile if the superconductor is warmed close to below Tc. The acquired knowledge may be used to modify the cooldown procedure of SRF cavities leading to a reduced level of trapped flux and hence operation closer to the BCS limit.

Slides TUIOA02 [5.774 MB]

MOIOB02

Towards a 100mA Superconducting RF Photoinjector for BERLinPro

42

A. Neumann, W. Anders, A. Burrill, A. Jankowiak, T. Kamps, J. Knobloch, O. Kugeler, P. Lauinger, A.N. Matveenko, M. Schmeißer, J. Völker
HZB, Berlin, Germany
G. Ciovati, P. Kneisel
JLAB, Newport News, Virginia, USA
R. Nietubyć
NCBJ, Świerk/Otwock, Poland
S.G. Schubert, J. Smedley
BNL, Upton, Long Island, New York, USA
J.K. Sekutowicz
DESY, Hamburg, Germany
V. Volkov
BINP SB RAS, Novosibirsk, Russia
I. Will
MBI, Berlin, Germany
E.N. Zaplatin
FZJ, Jülich, Germany

For BERLinPro, a 100 mA CW-driven SRF energy recovery linac demonstrator facility, HZB needs to develop a photo-injector superconducting cavity which delivers a at least 1mm*mr emittance beam at high average current. To address these challenges of producing a high peak brightness beam at high repetition rate, at first HZB tested a fully superconducting injector with a lead cathode*,followed now by the design of a SC cavity allowing operation up to 4 mA using CW-modified TTF-III couplers and inserting a normal conducting high quantum efficiency cathode using the HZDR-style insert scheme. This talk will present the latest results and an overview of the measurements with the lead cathode cavity and will describe the design and optimization process, the first production results of the current design and an outlook to the further development steps towards the full power version.
*T. Kamps et al., Proceedings of the 2nd International Particle Accelerator Conference, San Sebastián, Spain, 2011.

Slides MOIOB02 [7.574 MB]

TUIOA01

Influence of the Couldown at the Transition Temperature on the SRF Cavity Quality Factor

370

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

The quality factor Q0 that can be obtained in a superconducting cavity is known to depend on various factors like niobium material properties, treatment history and magnetic shielding. We believe that cooling conditions have an additional impact, as they appear to influence the amount of trapped flux and hence the residual resistance. We constructed a test stand using a niobium rod shorted out by a titanium rod to mimic a cavity in its helium tank to study flux trapping. Here we can precisely control the temperature and measure the dynamics of flux trapping at the superconducting phase transition. We learned that magnetic flux can be generated when a temperature gradient exists along the rod and when the niobium transitions into the superconducting state it subsequently remains trapped. Furthermore, it was shown that the cooling rate during isothermal cooldown through the transition temperature can influence the amount of externally applied flux which remains trapped. The acquired knowledge may be used to modify the cooldown procedure of SRF cavities leading to a reduced level of trapped flux and hence operation closer to the BCS limit.

Slides TUIOA01 [1.276 MB]

TUP074

Development of an Optimized Quadrupole Resonator at HZB

614

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

Funding: EuCARD II
Current superconducting cavities are generally made of solid niobium. A possibility to reduce cost as well as increase accelerating fields and, essential for CW applications, the quality factor is to use thin-film coated cavities. Measuring and understanding the RF-properties of superconducting thin films, specifically the surface resistance at the operating field and frequency, is needed to drive forward this development. Presently, only few facilities exist capable of measuring the surface resistance of thin films samples with a resolution in the nano-ohm range at L-Band. We describe here a dedicated test stand consisting of a quadrupole resonator that was constructed at the Helmholtz Zentrum Berlin. Starting with 400-MHz quadrupole resonator developed at CERN, the design was adapted and optimized for resolution and reduced peak electric field to 433 MHz (making available the higher harmonic mode at 1,3GHz) using simulation data obtained with CST Microwave Studio as well as ANSYS. The relevant figures of merit have been improved, giving the possibility to perform measurements with high resolution at high field levels.

TUP096

High Power Processing at a High Order Mode Frequency

697

V. Volkov
BINP SB RAS, Novosibirsk, Russia
J. Knobloch, A.N. Matveenko, A. Neumann
HZB, Berlin, Germany

Regular High Power Processing (HPP) at fundamental frequency in a superconducting cavity usually carried out to increase maximal RF field in the cavity that is limited by Field Emission (FE). HPP at a High Order Mode (HOM) frequency allow significantly increasing FE threshold of fundamental RF field. In the paper we give proof of this prediction and give the concrete proposal of such HPP design for Rossendorf 3.5-cell RF gun structure. Expected RF over field is about 100% (from 17 up to 34 MV/m) as compared with a regular HPP.

WEIOC01

High Resolution Surface Resistance Studies

785

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

Funding: Work supported by the German Doctoral Students program of the Federal Ministry of Education and Research (BMBF).
The attempt to reach quality factors beyond 10¹¹ and pushing the accelerating gradients of SRF cavities to the theoretical limit, the treatment depending loss mechanisms in niobium need better understanding. CERNs Quadrupole Resonator enables sub-nΩ-resolution measurements of the surface resistance. The available parameters cover resonant modes at 400, 800 and 1200 MHz, any temperature up to 15 K and rf fields up to 60 mT. Recently the setup has been extended with a coil creating a dc magnetic field for trapped flux studies. Overall, much more information about the rf performance is accessible compared to regular cavity measurements. Since the samples are flat disks of 75 mm diameter geometric fabrication issues are simplified which makes the Quadrupole Resonator also the perfect tool to study alternative materials or new coating techniques. In this contribution in depth studies of a heat treated bulk niobium sample exploiting the complete parameter range of the setup are presented.

Slides WEIOC01 [2.724 MB]

WEIOC01

High Resolution Surface Resistance Studies

785

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

Funding: Work supported by the German Doctoral Students program of the Federal Ministry of Education and Research (BMBF).
The attempt to reach quality factors beyond 10¹¹ and pushing the accelerating gradients of SRF cavities to the theoretical limit, the treatment depending loss mechanisms in niobium need better understanding. CERNs Quadrupole Resonator enables sub-nΩ-resolution measurements of the surface resistance. The available parameters cover resonant modes at 400, 800 and 1200 MHz, any temperature up to 15 K and rf fields up to 60 mT. Recently the setup has been extended with a coil creating a dc magnetic field for trapped flux studies. Overall, much more information about the rf performance is accessible compared to regular cavity measurements. Since the samples are flat disks of 75 mm diameter geometric fabrication issues are simplified which makes the Quadrupole Resonator also the perfect tool to study alternative materials or new coating techniques. In this contribution in depth studies of a heat treated bulk niobium sample exploiting the complete parameter range of the setup are presented.

Slides WEIOC01 [2.724 MB]

Paper	Title	Page
TUIOA02	High Q0 Research: The Dynamics of Flux Trapping in Superconducting Niobium	374
	J.M. Vogt HZB, Berlin, Germany J. Knobloch, O. Kugeler BESSY GmbH, Berlin, Germany
	The quality factor Q0 that can be obtained in a superconducting cavity is known to depend on various factors like niobium material properties, treatment history and magnetic shielding. We believe that cooling conditions have an additional impact, as they appear to influence the amount of trapped flux and hence the residual resistance. We have constructed a test stand using niobium rods to study flux trapping. Here we can precisely control the temperature and measure the dynamics of flux trapping at the superconducting phase transition. We learned that magnetic flux can be generated when a temperature gradient exists along the rod as the niobium transitions to the superconducting state, which subsequently remains trapped. It was also shown that the cooling rate can influence the amount of externally applied flux which is trapped. Furthermore, we also were able to demonstrate that flux lines become mobile if the superconductor is warmed close to below Tc. The acquired knowledge may be used to modify the cooldown procedure of SRF cavities leading to a reduced level of trapped flux and hence operation closer to the BCS limit.
	Slides TUIOA02 [5.774 MB]

MOIOB02	Towards a 100mA Superconducting RF Photoinjector for BERLinPro	42
	A. Neumann, W. Anders, A. Burrill, A. Jankowiak, T. Kamps, J. Knobloch, O. Kugeler, P. Lauinger, A.N. Matveenko, M. Schmeißer, J. Völker HZB, Berlin, Germany G. Ciovati, P. Kneisel JLAB, Newport News, Virginia, USA R. Nietubyć NCBJ, Świerk/Otwock, Poland S.G. Schubert, J. Smedley BNL, Upton, Long Island, New York, USA J.K. Sekutowicz DESY, Hamburg, Germany V. Volkov BINP SB RAS, Novosibirsk, Russia I. Will MBI, Berlin, Germany E.N. Zaplatin FZJ, Jülich, Germany
	For BERLinPro, a 100 mA CW-driven SRF energy recovery linac demonstrator facility, HZB needs to develop a photo-injector superconducting cavity which delivers a at least 1mmmr emittance beam at high average current. To address these challenges of producing a high peak brightness beam at high repetition rate, at first HZB tested a fully superconducting injector with a lead cathode,followed now by the design of a SC cavity allowing operation up to 4 mA using CW-modified TTF-III couplers and inserting a normal conducting high quantum efficiency cathode using the HZDR-style insert scheme. This talk will present the latest results and an overview of the measurements with the lead cathode cavity and will describe the design and optimization process, the first production results of the current design and an outlook to the further development steps towards the full power version. *T. Kamps et al., Proceedings of the 2nd International Particle Accelerator Conference, San Sebastián, Spain, 2011.
	Slides MOIOB02 [7.574 MB]

TUIOA01	Influence of the Couldown at the Transition Temperature on the SRF Cavity Quality Factor	370
	O. Kugeler, J. Knobloch, J.M. Vogt HZB, Berlin, Germany
	The quality factor Q0 that can be obtained in a superconducting cavity is known to depend on various factors like niobium material properties, treatment history and magnetic shielding. We believe that cooling conditions have an additional impact, as they appear to influence the amount of trapped flux and hence the residual resistance. We constructed a test stand using a niobium rod shorted out by a titanium rod to mimic a cavity in its helium tank to study flux trapping. Here we can precisely control the temperature and measure the dynamics of flux trapping at the superconducting phase transition. We learned that magnetic flux can be generated when a temperature gradient exists along the rod and when the niobium transitions into the superconducting state it subsequently remains trapped. Furthermore, it was shown that the cooling rate during isothermal cooldown through the transition temperature can influence the amount of externally applied flux which remains trapped. The acquired knowledge may be used to modify the cooldown procedure of SRF cavities leading to a reduced level of trapped flux and hence operation closer to the BCS limit.
	Slides TUIOA01 [1.276 MB]

TUP074	Development of an Optimized Quadrupole Resonator at HZB	614
	R. Kleindienst, J. Knobloch, O. Kugeler HZB, Berlin, Germany
	Funding: EuCARD II Current superconducting cavities are generally made of solid niobium. A possibility to reduce cost as well as increase accelerating fields and, essential for CW applications, the quality factor is to use thin-film coated cavities. Measuring and understanding the RF-properties of superconducting thin films, specifically the surface resistance at the operating field and frequency, is needed to drive forward this development. Presently, only few facilities exist capable of measuring the surface resistance of thin films samples with a resolution in the nano-ohm range at L-Band. We describe here a dedicated test stand consisting of a quadrupole resonator that was constructed at the Helmholtz Zentrum Berlin. Starting with 400-MHz quadrupole resonator developed at CERN, the design was adapted and optimized for resolution and reduced peak electric field to 433 MHz (making available the higher harmonic mode at 1,3GHz) using simulation data obtained with CST Microwave Studio as well as ANSYS. The relevant figures of merit have been improved, giving the possibility to perform measurements with high resolution at high field levels.

TUP096	High Power Processing at a High Order Mode Frequency	697
	V. Volkov BINP SB RAS, Novosibirsk, Russia J. Knobloch, A.N. Matveenko, A. Neumann HZB, Berlin, Germany
	Regular High Power Processing (HPP) at fundamental frequency in a superconducting cavity usually carried out to increase maximal RF field in the cavity that is limited by Field Emission (FE). HPP at a High Order Mode (HOM) frequency allow significantly increasing FE threshold of fundamental RF field. In the paper we give proof of this prediction and give the concrete proposal of such HPP design for Rossendorf 3.5-cell RF gun structure. Expected RF over field is about 100% (from 17 up to 34 MV/m) as compared with a regular HPP.

WEIOC01	High Resolution Surface Resistance Studies	785
	S. Aull, S. Döbert, T. Junginger CERN, Geneva, Switzerland S. Aull, J. Knobloch University of Siegen, Siegen, Germany J. Knobloch HZB, Berlin, Germany
	Funding: Work supported by the German Doctoral Students program of the Federal Ministry of Education and Research (BMBF). The attempt to reach quality factors beyond 10¹¹ and pushing the accelerating gradients of SRF cavities to the theoretical limit, the treatment depending loss mechanisms in niobium need better understanding. CERNs Quadrupole Resonator enables sub-nΩ-resolution measurements of the surface resistance. The available parameters cover resonant modes at 400, 800 and 1200 MHz, any temperature up to 15 K and rf fields up to 60 mT. Recently the setup has been extended with a coil creating a dc magnetic field for trapped flux studies. Overall, much more information about the rf performance is accessible compared to regular cavity measurements. Since the samples are flat disks of 75 mm diameter geometric fabrication issues are simplified which makes the Quadrupole Resonator also the perfect tool to study alternative materials or new coating techniques. In this contribution in depth studies of a heat treated bulk niobium sample exploiting the complete parameter range of the setup are presented.
	Slides WEIOC01 [2.724 MB]

WEIOC01	High Resolution Surface Resistance Studies	785
	S. Aull, S. Döbert, T. Junginger CERN, Geneva, Switzerland S. Aull, J. Knobloch University of Siegen, Siegen, Germany J. Knobloch HZB, Berlin, Germany
	Funding: Work supported by the German Doctoral Students program of the Federal Ministry of Education and Research (BMBF). The attempt to reach quality factors beyond 10¹¹ and pushing the accelerating gradients of SRF cavities to the theoretical limit, the treatment depending loss mechanisms in niobium need better understanding. CERNs Quadrupole Resonator enables sub-nΩ-resolution measurements of the surface resistance. The available parameters cover resonant modes at 400, 800 and 1200 MHz, any temperature up to 15 K and rf fields up to 60 mT. Recently the setup has been extended with a coil creating a dc magnetic field for trapped flux studies. Overall, much more information about the rf performance is accessible compared to regular cavity measurements. Since the samples are flat disks of 75 mm diameter geometric fabrication issues are simplified which makes the Quadrupole Resonator also the perfect tool to study alternative materials or new coating techniques. In this contribution in depth studies of a heat treated bulk niobium sample exploiting the complete parameter range of the setup are presented.
	Slides WEIOC01 [2.724 MB]