TUIOA04
Fermilab, Batavia, USA
In this talk we will present recent cavity experiments (sequential HF, irradiation, temperature mapping) and sample investigations (positron annihilation, low energy and bulk muSR, Bitter decoration, LCSM cold stage hydride imaging, TEM/STEM and SEM room/cryogenic imaging etc), which are providing new insight on the physical mechanisms behind phenomena leading to Q slope. New findings on relevant mechanisms at work during 120C and 600-800C heat treatments and electropolishing/BCP will also be discussed.
Fermilab, Batavia, USA
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.
Fermilab, Batavia, USA
ISSP, Chernogolovka, Russia
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.
TUP030
Fermilab, Batavia, USA
INFN/LNL, Legnaro (PD), Italy
IIT, Chicago, USA
We investigate the effect of high temperature treatments followed by only high-pressure water rinse (HPR) of superconducting radio frequency (SRF) niobium cavities. The objective is to provide a cost effective alternative to the typical cavity processing sequence, by eliminating the material removal step post furnace treatment while preserving or improving the RF performance. The studies have been conducted in the temperature range 800-1000C for different conditions of the starting substrate: large grain and fine grain, electro-polished (EP) and centrifugal barrel polished (CBP) to mirror finish. An interesting effect of the grain size on the performances is found. Cavity results and samples characterization show that furnace contaminants cause poor cavity performance, and a practical solution is found to prevent surface contamination. Extraordinary values of residual resistances ~ 1 nOhm and below are then consistently achieved for the contamination-free cavities. We explore the addition of a small partial pressure of gas during the anneal to further increase the cavity quality factor by reducing the BCS resistance.
TUP031
Fermilab, Batavia, USA
TRIUMF, Canada's National Laboratory for Particle and Nuclear Physics, Vancouver, Canada
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
IIT, Chicago, USA
In the previous studies [*] magnetic flux penetration into fine and large grain BCP cavity cutouts was investigated using the muon spin rotation (muSR) technique. The technique is based on implanting muons, which serve as sensitive magnetic probes inside the material. Here we report muSR studies on fine grain EP cavity cutouts, both before and after 120C baking, and on the films of new materials.
[*] A. Grassellino et al, Phys. Rev. ST Accel. Beams 16, 062002 (2013)
TUP038
Fermilab, Batavia, USA
For both fundamental understanding of performance limiting processes and practical design of future accelerators a crucial information is the temperature and field dependence of the components in surface resistance. We report an explicit deconvolution of temperature-independent residual (Rres) and temperature-dependent BCS (Rbcs) components in multiple cavities treated by standard processing techniques (EP, BCP, 120C bake, 800C degassing) at all fields up to Brf > 100 mT. Such deconvolution allows to address the nature of the low, medium, and high field Q slopes, and provides input for accelerator parametric design optimization.
TUP039
Fermilab, Batavia, USA
PSI, Villigen PSI, Switzerland
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)
TUP042
UIC, Chicago, USA
Fermilab, Batavia, USA
We present an atomic-resolution study of the effects that 48hr bake at 120 °C in vacuum has on the high-field properties of Nb-based SRF cavities. This bake results a significant increase in high-field Q, reversely, 800 °C bake for 2hr reduces the Hc3/Hc2-ratio. Several mechanisms have been proposed, including an increased NbOx surface layer thickness and the precipitation of NbHy. Using combination of atomic-resolution Z-contrast imaging and electron energy-loss spectroscopy with in-situ heating and cooling experiments, we examine the atomic and electronic structures of Nb and related oxides/hydrides near the cavity surface. We quantify the oxygen diffusion on surface during bake by measuring the local Nb valence using EELS. We demonstrate that hydrogen atoms incorporated into the Nb crystal, forming β-NbH precipitates, can be directly visualized by annular bright field imaging in our aberration-corrected JEOL ARM-200CF. The effects of baking on the local hydrogen and other impurity will be examined by imaging, EEL spectra and strain analysis. Our results will be combined with atom-probe tomography to develop a 3-D impurity and phase profile of Nb near the SRF cavity surface.
"R. Tao, R. F. Klie et al, Journal of Applied Physics, 110, Issue 12, (2011)"
"Y.J. Kim, R. Tao, R.F. Klie and D. Seidman, ACS nano 7 (1), pp 732–739, (2013)"
IIT, Chicago, Illinois, USA
Fermilab, Batavia, USA
*A. Romanenko, F. Barkov, L. D. Cooley, A. Grassellino, Supercond. Sci. Technol. 26 (2013) 035003.
TUP050
Fermilab, Batavia, USA
We report the first test results of several 650 MHz single cell niobium cavities processed at Fermilab. The target for the 5-cell 650 MHz cavities for Project X is CW operation at magnetic peak field ~ 60-70 mT, making high quality factors at medium accelerating fields the main goal of the surface processing R&D. We will discuss how the performance vary with the different surface processing and parameters/criteria of choice for the final surface preparation sequence.
Fermilab, Batavia, USA
IIT, Chicago, USA
Fermilab, Batavia, USA
IHEP, Beijing, People's Republic of China
Fermilab, Batavia, USA
Euclid TechLabs, LLC, Solon, Ohio, USA
*Y. Xie et.al., "Relationship between defects pre-heating and defects size", SRF2009.
**T. Kubo et.al.,"Rf field-attenuation formulae for the multilayer coating model", IPAC2013
TUP101
Fermilab, Batavia, USA
A problem of the medium field Q slope in cavities treated by standard surface processing techniques recently gained a lot of attention due to its importance for CW accelerators. Here we present high resolution thermometry studies of the losses in the medium field range (20-80 mT), and discuss its possible connection to the observations at high fields (>80 mT).
University of Chicago, Chicago, Illinois, USA
MIPT, Dolgoprudniy, Moscow Region, Russia
Fermilab, Batavia, USA
WEIOC03
ANL, Argonne, USA
JLAB, Newport News, Virginia, USA
Fermilab, Batavia, USA
IIT, Chicago, Illinois, USA
I will present a summary of the work done over the last 2 years that encompasses both coupons study of thin superconducting films and multilayers and preliminary superconducting RF cavity tests coated by ALD. I will also present results of Nb onto Copper.