JLAB, Newport News, Virginia, USA
Most of the current R&D in SRF is focused on ways to reduce the construction and operating cost of SRF-based accelerators as well as on the development of new or improved cavity processing techniques. The increase in quality factors is the result of the reduction of the surface resistance of the materials. A recent test [*] on a 1.5 GHz single cell cavity made from ingot niobium of medium purity and heat treated at 1400 C in a ultra-high vacuum induction furnace resulted in a residual resistance of ~ 1nanoohm and a quality factor increasing with field up to ~ 5×1010 at a peak magnetic field of 90 mT. In this contribution, we present some results on the investigation of the origin of the extended Q0-increase, obtained by multiple HF rinses, oxypolishing and heat treatment of “all Nb” cavities.
[*] P. Dhakal et al., Phys. Rev. ST Accel. Beams 16, 042001 (2013).
JLAB, Newport News, Virginia, USA
The Hebrew University of Jerusalem, The Racah Institute of Physics, Jerusalem, Israel
In an attempt to correlate the SRF performance of niobium cavities with the superconducting properties, we present the results of the magnetization and ac susceptibility of the niobium used in the superconducting radiofrequency cavity fabrications which were subjected to buffer chemical polishing surface and high temperature heat treatments, typically applied to the SRF cavities fabrications. The analysis of the results show the different surface and bulk ac conductivity for the samples subjected to BCP and HT. Furthermore, the RF surface impedance is measured on the sample using the TE011 microwave cavity for a comparison to the low frequency measurements.
TUP034
NU, Evanston, Illinois, USA
JLAB, Newport News, Virginia, USA
Fermilab, Batavia, USA
UIC, Chicago, USA
NUCAPT, Evanston,, USA
Niobium is the metal of choice for SRF cavities for a linear particle accelerator because it has the highest critical temperature of any element in the periodic table and can be deformed plastically into complex geometries. Differences in the sub-surface chemistry from bulk niobium are believed to determine the high-field Q-drop. In this study, the subsurface chemistry of niobium was characterized utilizing ultraviolet laser-assisted local-electrode atom-probe (LEAP) tomography employing picosecond laser pulsing. The superior spatial resolution and analytical sensitivity of a LEAP tomograph permits us to determine the subsurface composition on an atom-by-atom and atomic {hkl} plane-by-plane basis. The 3-D reconstructions from the LEAP tomographic analyses demonstrate different behaviors for Nb-oxides and Nb-hydrides in pure niobium as well as interactions with structural imperfections, dislocations and grain boundaries in SRF-grade Nb coupon material. Additionally, the chemistry and crystallographic structure of subsurface interstitial atoms were analyzed based on energy shifts of electron energy-loss spectroscopy in conjunction with a scanning transmission electron microscopy.