IPAC2018 - List of Authors (Grewe, R.)

Paper	Title	Page
THPAL012	Soft Chemical Polishing and Surface Analysis of Niobium Samples	3641
	J. Conrad, L. Alff, M. Arnold, S. Flege, R. Grewe, M. Major, N. Pietralla TU Darmstadt, Darmstadt, Germany F. Hug IKP, Mainz, Germany
	Funding: Work supported by the German Federal Ministry for Education and Research (BMBF) under Grant No. 05H15RDRBA The Superconducting Darmstadt Linear Accelerator S-DALINAC uses twelve Niobium Cavities with a RRR of 280 which are operated at 2 K. The operating frequency is 3 GHz; the design value of the accelerating gradient is 5 MV/m. To achieve the target value of 3 x 10˄9 for Q0, different surface preparation methods were applied and systematically tested using a vertical 2 K cryostat. A well-established technique is the so called Darmstadt Soft Chemical Polishing, which consists of an ultrasonic cleaning of the cavity with ultrapure water followed by oxidizing the inner surface with nitric acid. After rinsing with water the niobium oxide layer is removed with hydrofluoric acid in a separate second step. Finally the structure is rinsed and then dried by a nitrogen flow. Until now each cavity in operation was chemically treated with a proven record of success. In order to understand and to optimize the process on the niobium surface, systematic tests with samples were performed and analyzed using material science techniques like SEM, SIMS and EDX. We will report on the results of our research and we will give a review on our experiences with varied chemical procedures.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2018-THPAL012
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THPAL126	Nitrogen Bake-out Procedures at the Vertical High-Temperature UHV-Furnace of the S-DALINAC	3937
	R. Grewe, L. Alff, M. Arnold, J. Conrad, S. Flege, M. Major, N. Pietralla TU Darmstadt, Darmstadt, Germany F. Hug IKP, Mainz, Germany
	Funding: Work supported by the Federal Ministry of Education and Research through grant No. 05H15RDRBA. As the performance limits of bulk Nb srf cavities are reached, our research is focused on materials with superior srf properties like Nb3Sn and NbN. Research on NbN resulted in the "nitrogen-doping" process used for increasing the quality factors of srf cavities for the LCLS-II project. This process leads to delta-phase Nb-N, a phase with higher critical sc parameters than bulk Nb. This phase is formed at temperatures of 800°C in nitrogen atmospheres of 10^-2 mbar. Other crystalline phases of NbN have even better sc parameters. We concentrate our research on applicability of delta-phase NbN for cavities. The delta-phase forms at temperatures of above 1300°C, which is more than most of the furnaces at accelerator facilites are capable of. Since 2005 the Institute for Nuclear Physics at the Technische Universität Darmstadt operates a high temperature vacuum furnace which has been upgraded to allow temperatures of up to 1750°C and bakeouts of niobium samples and cavities in nitrogen atmospheres. We will report on the current status of our research on nitrogen bake-out procedures on Nb samples. The samples have been analyzed at the Material Science Departement with SIMS, REM and XRD.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2018-THPAL126
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THPAL127	Structural Investigations of Nitrogen-Doped Niobium for Superconducting RF Cavities	3940
	M. Major, L. Alff, M. Arnold, J. Conrad, S. Flege, R. Grewe, M. Mahr, N. Pietralla TU Darmstadt, Darmstadt, Germany F. Hug KPH, Mainz, Germany
	Funding: Work supported by the German Federal Ministry for Education and Research (BMBF) through grant 05H15RDRBA. Niobium is the standard material for superconducting RF (SRF) cavities. Superconducting materials with higher critical temperature or higher critical magnetic field allow cavities to work at higher operating temperatures or higher accelerating fields, respectively. Enhancing the surface properties of the superconducting material in the range of the penetration depth is also beneficial. One direction of search for new materials with better properties is the modification of bulk niobium by nitrogen doping. In the Nb-N phase diagram the cubic delta-phase of NbN has the highest critical temperature (16 K). Already slight nitrogen doping of the alpha-Nb phase results in higher quality factors.* Nb samples were N-doped at the refurbished UHV furnace at IKP Darmstadt. Reference samples were annealed in 1 bar nitrogen atmosphere at different temperatures. In this contribution the results on the structural investigations (x-ray diffraction and pole figure, secondary ion mass spectroscopy, scanning electron microscopy) at the Materials Research Department of TU Darmstadt will be presented. *Grassellino et al., Proc. SRF2015, MOBA06, 48.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2018-THPAL127
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THPML087	First ERL Operation of S-DALINAC and Commissioning of a Path Length Adjustment System	4859
	M. Arnold, C. Burandt, R. Grewe, J. Pforr, N. Pietralla, M. Steinhorst TU Darmstadt, Darmstadt, Germany C. Eschelbach, M. Lösler Frankfurt University of Applied Sciences, Frankfurt am Main, Germany F. Hug KPH, Mainz, Germany
	Funding: Work supported by DFG through GRK 2128 and INST163/383-1/FUGG The S-DALINAC is running in recirculating operation since 1991. In 2015/2016 a major upgrade was performed by adding a third recirculation beam line. The versatility of this recirculation beam line enables a phase shift of the beam of up to 360° of the RF phase. The required range of 10 cm for a 3 GHz RF frequency is realized by a path length adjustment system. A complementary operation in normal scheme (single-pass, once or thrice recirculating with acceleration) or ERL mode (once or twice) is possible by appropriate adjustment of this system. After installation this system was aligned properly and its functionality and stroke was checked without beam. The system was commissioned by measuring the change of the beam phase in dependency of the setting of the path length adjustment system. The complementary usage of the newly installed recirculation for once recirculating with acceleration and once recirculating with ERL mode has been shown successfully in autumn 2017. This contribution will provide an overview on the path length adjustment system and the first run of the once recirculating ERL mode of the S-DALINAC.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2018-THPML087
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Paper

Title

Page

Soft Chemical Polishing and Surface Analysis of Niobium Samples

3641

J. Conrad, L. Alff, M. Arnold, S. Flege, R. Grewe, M. Major, N. Pietralla
TU Darmstadt, Darmstadt, Germany
F. Hug
IKP, Mainz, Germany

Funding: Work supported by the German Federal Ministry for Education and Research (BMBF) under Grant No. 05H15RDRBA
The Superconducting Darmstadt Linear Accelerator S-DALINAC uses twelve Niobium Cavities with a RRR of 280 which are operated at 2 K. The operating frequency is 3 GHz; the design value of the accelerating gradient is 5 MV/m. To achieve the target value of 3 x 10˄9 for Q0, different surface preparation methods were applied and systematically tested using a vertical 2 K cryostat. A well-established technique is the so called Darmstadt Soft Chemical Polishing, which consists of an ultrasonic cleaning of the cavity with ultrapure water followed by oxidizing the inner surface with nitric acid. After rinsing with water the niobium oxide layer is removed with hydrofluoric acid in a separate second step. Finally the structure is rinsed and then dried by a nitrogen flow. Until now each cavity in operation was chemically treated with a proven record of success. In order to understand and to optimize the process on the niobium surface, systematic tests with samples were performed and analyzed using material science techniques like SEM, SIMS and EDX. We will report on the results of our research and we will give a review on our experiences with varied chemical procedures.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2018-THPAL012

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THPAL126

Nitrogen Bake-out Procedures at the Vertical High-Temperature UHV-Furnace of the S-DALINAC

3937

R. Grewe, L. Alff, M. Arnold, J. Conrad, S. Flege, M. Major, N. Pietralla
TU Darmstadt, Darmstadt, Germany
F. Hug
IKP, Mainz, Germany

Funding: Work supported by the Federal Ministry of Education and Research through grant No. 05H15RDRBA.
As the performance limits of bulk Nb srf cavities are reached, our research is focused on materials with superior srf properties like Nb3Sn and NbN. Research on NbN resulted in the "nitrogen-doping" process used for increasing the quality factors of srf cavities for the LCLS-II project. This process leads to delta-phase Nb-N, a phase with higher critical sc parameters than bulk Nb. This phase is formed at temperatures of 800°C in nitrogen atmospheres of 10^-2 mbar. Other crystalline phases of NbN have even better sc parameters. We concentrate our research on applicability of delta-phase NbN for cavities. The delta-phase forms at temperatures of above 1300°C, which is more than most of the furnaces at accelerator facilites are capable of. Since 2005 the Institute for Nuclear Physics at the Technische Universität Darmstadt operates a high temperature vacuum furnace which has been upgraded to allow temperatures of up to 1750°C and bakeouts of niobium samples and cavities in nitrogen atmospheres. We will report on the current status of our research on nitrogen bake-out procedures on Nb samples. The samples have been analyzed at the Material Science Departement with SIMS, REM and XRD.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2018-THPAL126

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THPAL127

Structural Investigations of Nitrogen-Doped Niobium for Superconducting RF Cavities

3940

M. Major, L. Alff, M. Arnold, J. Conrad, S. Flege, R. Grewe, M. Mahr, N. Pietralla
TU Darmstadt, Darmstadt, Germany
F. Hug
KPH, Mainz, Germany

Funding: Work supported by the German Federal Ministry for Education and Research (BMBF) through grant 05H15RDRBA.
Niobium is the standard material for superconducting RF (SRF) cavities. Superconducting materials with higher critical temperature or higher critical magnetic field allow cavities to work at higher operating temperatures or higher accelerating fields, respectively. Enhancing the surface properties of the superconducting material in the range of the penetration depth is also beneficial. One direction of search for new materials with better properties is the modification of bulk niobium by nitrogen doping. In the Nb-N phase diagram the cubic delta-phase of NbN has the highest critical temperature (16 K). Already slight nitrogen doping of the alpha-Nb phase results in higher quality factors.* Nb samples were N-doped at the refurbished UHV furnace at IKP Darmstadt. Reference samples were annealed in 1 bar nitrogen atmosphere at different temperatures. In this contribution the results on the structural investigations (x-ray diffraction and pole figure, secondary ion mass spectroscopy, scanning electron microscopy) at the Materials Research Department of TU Darmstadt will be presented.
*Grassellino et al., Proc. SRF2015, MOBA06, 48.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2018-THPAL127

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THPML087

First ERL Operation of S-DALINAC and Commissioning of a Path Length Adjustment System

4859

M. Arnold, C. Burandt, R. Grewe, J. Pforr, N. Pietralla, M. Steinhorst
TU Darmstadt, Darmstadt, Germany
C. Eschelbach, M. Lösler
Frankfurt University of Applied Sciences, Frankfurt am Main, Germany
F. Hug
KPH, Mainz, Germany

Funding: Work supported by DFG through GRK 2128 and INST163/383-1/FUGG
The S-DALINAC is running in recirculating operation since 1991. In 2015/2016 a major upgrade was performed by adding a third recirculation beam line. The versatility of this recirculation beam line enables a phase shift of the beam of up to 360° of the RF phase. The required range of 10 cm for a 3 GHz RF frequency is realized by a path length adjustment system. A complementary operation in normal scheme (single-pass, once or thrice recirculating with acceleration) or ERL mode (once or twice) is possible by appropriate adjustment of this system. After installation this system was aligned properly and its functionality and stroke was checked without beam. The system was commissioned by measuring the change of the beam phase in dependency of the setting of the path length adjustment system. The complementary usage of the newly installed recirculation for once recirculating with acceleration and once recirculating with ERL mode has been shown successfully in autumn 2017. This contribution will provide an overview on the path length adjustment system and the first run of the once recirculating ERL mode of the S-DALINAC.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2018-THPML087

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