SRF2013 - List of Authors (Barkov, F.L.)

Paper

Title

Page

Fast Table Top Niobium Hydride Investigations Using Direct Imaging in a Cryo-Stage

447

F.L. Barkov, A. Grassellino, A. Romanenko
Fermilab, Batavia, USA

Funding: Fermilab is operated by Fermi Research Alliance, LLC under Contract No. De-AC02-07CH11359 with the United States Department of Energy.
Performance of niobium SRF cavities can be strongly affected by hydrogen segregation into lossy niobium hydrides as known for "hydrogen Q disease" at higher concentration of dissolved H and may be a reason for the "high field Q slope" at lower concentrations. With the use of optical cryostat and laser confocal microscope we have developed a "table top technique" for direct observation of hydride precipitation, and studied formation, morphology, and time evolution of hydrides after different treatments used for cavities. Our results show that hydrides can form at the niobium surface at 90-180K depending mainly on H concentration and the cooldown rate. A lot of H is absorbed by bulk niobium during mechanical polishing, which leads to the formation of very large (>10 microns) hydrides. Both EP and BCP do not influence H concentration significantly provided that temperature during treatments is kept below 15C. 800C degassing reduces H concentration and precludes large hydride precipitation. 120C baking and mechanical deformation do not change H concentration but affect hydride precipitation through their influence on the number of nucleation centers and H binding defects.

TUP015

Bitter Decoration Studies of Magnetic Flux Penetration Into Cavity Cutouts

451

F.L. Barkov, A. Grassellino, A. Romanenko
Fermilab, Batavia, USA
L.Y. Vinnikov
ISSP, Chernogolovka, Russia

Funding: Fermilab is operated by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the United States Department of Energy.
Magnetic flux penetration may produce additional losses in superconducting radio frequency cavities. All the existing models for flux penetration are based on the formation of Abrikosov vortices. Using high resolution Bitter decoration technique we have investigated magnetic flux distribution patterns in cavity cutouts at the perpendicular magnetic fields of 10-80 mT. At low fields <20 mT the magnetic field penetrates in the form of flux bundles and not Abrikosov vortices, the situation characteristic of type-I superconductors. With the increase of the magnetic field up to 30 mT "bundles" first merge into a connected structure and then break up into individual Abrikosov vortices at ~60 mT and a well-known intermediate mixed state is observed. Such magnetic field driven transition from type I to type II superconductivity has never been observed before in any existing superconductor. For the case of flat samples we have observed a coexistence of both "bundles" and Abrikosov vortices in one experiment. Our results show that high-purity cavity grade niobium is a "border-line" material and behaves as a type-I superconductor at lower fields and type-II at higher fields.

TUP039

Meissner Screening at Hot (Unbaked) and Cold (Baked) Spots in Electropolished Cavities Studied by Low Energy Muon Spectroscopy

A. Romanenko, F.L. Barkov, A. Grassellino
Fermilab, Batavia, USA
T. Prokscha, Z. Salman, A. Suter
PSI, Villigen PSI, Switzerland

Funding: Fermilab is operated by Fermi Research Alliance, LLC under Contract No. De-AC02-07CH11359 with the United States Department of Energy.
While there is a number of recent structural investigations, which shed light on possible underlying mechanisms of the high field Q slope and 120C baking effect [], there is fewer explicit superconducting investigations exploring the microscopic superconducting properties at the locations of "hot" spots in unbaked cavities. Furthermore, while the nature of the magnetic field penetration in the Meissner state into bulk niobium is predicted by BCS theory and its strong coupling extensions, it was never directly observed. Here we present a direct measurement of the magnetic field profile B(z) in the Meissner state inside a "hot" spot cutout from the electropolished cavity, and compare it to a "cold" spot from the baked electropolished cavity. We demonstrate the presence of a dead layer, a non-exponential B(z) profile, and a drastic change introduced by the 120C baking.
[*] A. Romanenko, F. Barkov, L.D. Cooley, A. Grassellino, Supercond. Sci. Technol. 26, 035003 (2013)
[**]A. Romanenko, C.J. Edwardson, P.G. Coleman, P.J. Simpson, Appl. Phys. Lett. 102, 232601 (2013)

Paper	Title	Page
TUP014	Fast Table Top Niobium Hydride Investigations Using Direct Imaging in a Cryo-Stage	447
	F.L. Barkov, A. Grassellino, A. Romanenko Fermilab, Batavia, USA
	Funding: Fermilab is operated by Fermi Research Alliance, LLC under Contract No. De-AC02-07CH11359 with the United States Department of Energy. Performance of niobium SRF cavities can be strongly affected by hydrogen segregation into lossy niobium hydrides as known for "hydrogen Q disease" at higher concentration of dissolved H and may be a reason for the "high field Q slope" at lower concentrations. With the use of optical cryostat and laser confocal microscope we have developed a "table top technique" for direct observation of hydride precipitation, and studied formation, morphology, and time evolution of hydrides after different treatments used for cavities. Our results show that hydrides can form at the niobium surface at 90-180K depending mainly on H concentration and the cooldown rate. A lot of H is absorbed by bulk niobium during mechanical polishing, which leads to the formation of very large (>10 microns) hydrides. Both EP and BCP do not influence H concentration significantly provided that temperature during treatments is kept below 15C. 800C degassing reduces H concentration and precludes large hydride precipitation. 120C baking and mechanical deformation do not change H concentration but affect hydride precipitation through their influence on the number of nucleation centers and H binding defects.

TUP015	Bitter Decoration Studies of Magnetic Flux Penetration Into Cavity Cutouts	451
	F.L. Barkov, A. Grassellino, A. Romanenko Fermilab, Batavia, USA L.Y. Vinnikov ISSP, Chernogolovka, Russia
	Funding: Fermilab is operated by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the United States Department of Energy. Magnetic flux penetration may produce additional losses in superconducting radio frequency cavities. All the existing models for flux penetration are based on the formation of Abrikosov vortices. Using high resolution Bitter decoration technique we have investigated magnetic flux distribution patterns in cavity cutouts at the perpendicular magnetic fields of 10-80 mT. At low fields <20 mT the magnetic field penetrates in the form of flux bundles and not Abrikosov vortices, the situation characteristic of type-I superconductors. With the increase of the magnetic field up to 30 mT "bundles" first merge into a connected structure and then break up into individual Abrikosov vortices at ~60 mT and a well-known intermediate mixed state is observed. Such magnetic field driven transition from type I to type II superconductivity has never been observed before in any existing superconductor. For the case of flat samples we have observed a coexistence of both "bundles" and Abrikosov vortices in one experiment. Our results show that high-purity cavity grade niobium is a "border-line" material and behaves as a type-I superconductor at lower fields and type-II at higher fields.

TUP039	Meissner Screening at Hot (Unbaked) and Cold (Baked) Spots in Electropolished Cavities Studied by Low Energy Muon Spectroscopy
	A. Romanenko, F.L. Barkov, A. Grassellino Fermilab, Batavia, USA T. Prokscha, Z. Salman, A. Suter PSI, Villigen PSI, Switzerland
	Funding: Fermilab is operated by Fermi Research Alliance, LLC under Contract No. De-AC02-07CH11359 with the United States Department of Energy. While there is a number of recent structural investigations, which shed light on possible underlying mechanisms of the high field Q slope and 120C baking effect [], there is fewer explicit superconducting investigations exploring the microscopic superconducting properties at the locations of "hot" spots in unbaked cavities. Furthermore, while the nature of the magnetic field penetration in the Meissner state into bulk niobium is predicted by BCS theory and its strong coupling extensions, it was never directly observed. Here we present a direct measurement of the magnetic field profile B(z) in the Meissner state inside a "hot" spot cutout from the electropolished cavity, and compare it to a "cold" spot from the baked electropolished cavity. We demonstrate the presence of a dead layer, a non-exponential B(z) profile, and a drastic change introduced by the 120C baking. [] A. Romanenko, F. Barkov, L.D. Cooley, A. Grassellino, Supercond. Sci. Technol. 26, 035003 (2013) [*]A. Romanenko, C.J. Edwardson, P.G. Coleman, P.J. Simpson, Appl. Phys. Lett. 102, 232601 (2013)