JLAB, Newport News, Virginia
ODU, Norfolk, Virginia
TEM cross-section observation on Nb surfaces has been a challenge to our superconducting radio frequency (SRF) community due to the highly reactive nature of Nb. Although it was demonstrated in an early attempt1 that under a suitable sample preparation procedure reasonably clear cross-section images of Nb could be obtained, to the best of our knowledge good atomically resolved images had never been obtained. In this report, it is shown that by modifying the sample preparation procedure adopted in reference 1 it is possible to obtain good cross-section images of Nb surfaces with atomic resolution routinely. Surface atomic structures of Nb samples prepared by buffered electropolishing (BEP), buffered chemical polishing (BCP), and an untreated sample will be reported and compared.
1: A.T. Wu, Proc. of the 11th SRF Workshop, Germany, (2003), ThP13
JLAB, Newport News, Virginia
ANL, Argonne
Passport Systems Inc, Billerica, Massachusetts
Recently, it was demonstrated1 that significant reductions in field emission on Nb surfaces could be achieved by means of a new surface treatment technique called gas cluster ion beam (GCIB). Further study revealed that GCIB treatments could also modify surface irregularities and remove surface asperities leading to a smoother surface finish as demonstrated through measurements using a 3-D profilometer, an atomic force microscope, and a scanning electron microscope. These experimental observations were supported by computer simulation via atomistic molecular dynamics and a phenomenological surface dynamics. Measurements employing a secondary ion mass spectrometry found that GCIB could also alter Nb surface oxide layer structure. Possible implications of the experimental results on the performance of Nb superconducting radio frequency cavities treated by GCIB will be discussed.
1: A.T. Wu et al, invited contribution to the book “Neural Computation and Particle Accelerators: Research, Technology, and Applications”, NOVA Science Publishers, New York, USA, 2009, In press
JLAB, Newport News, Virginia
This paper reports an on-going surface study of a superconducting radio frequency resonant cavity made of large-grain bulk niobium, which experienced anomalous RF energy loss in the medium field range. "Hot" and "cold" spots were identified via in-situ thermometry mapping of the BCP-etched single-cell cavity. The cavity was cut apart for surface investigation via high resolution electron microscopy (SEM), electron-back scattering diffraction microscopy (EBSD), optical microscopy, and three dimensional profilometry. Etching pits with clearly discernable crystal facets were observed in both "hotspot" and "coldspot" specimens. They were found in-grain, at bi-crystal boundaries, and on tri-crystal junctions. Two types of pits were observed with significantly different geometrical features, as observed with high resolution SEM. All "coldspots" examined had qualitatively low density of etching pits or very shallow pits at tri-crystal bound
JLAB, Newport News, Virginia
CEA, Gif-sur-Yvette
PKU/IHIP, Beijing
ORNL, Oak Ridge, Tennessee
BNL, Upton, Long Island, New York
Buffered electropolishing (BEP) is a Nb surface treatment technique developed at Jefferson Lab1. Experimental results obtained from flat Nb samples show2-4 that BEP can produce a surface finish much smoother than that produced by the conventional electropolishing (EP), while Nb removal rate can be as high as 4.67 μm/min. This new technique has been applied to the treatments of Nb SRF single cell cavity employing a vertical polishing system5 constructed at JLab as well as a horizontal polishing system at CEA Saclay. Preliminary results show that the accelerating gradient can reach 32 MV/m for a large grain cavity and 26.7 MV/m for a regular grain cavity. In this presentation, the latest progresses from the international collaboration between Peking University, CEA Saclay, and JLab on BEP will be summarized.
JLAB, Newport News, Virginia
Passport Systems Inc, Billerica, Massachusetts
Field emission is one of the major obstacles for achieving constantly high accelerating gradient for Nb superconducting radio frequency (SRF) cavities, although various techniques and procedures have been adopted trying to keep the inner surfaces of Nb SRF cavities clean and free from field emission in the past a couple of decades. In this report, it is shown that significant reductions in field emission on Nb surfaces can be achieved by means of a new surface treatment technique called gas cluster ion beam (GCIB). When a relevant treatment agent is selected with optimal treating parameters, it is demonstrated a reduction in field emission as much as 87.5% is possible through measurements using a home-made scanning field emission microscope. Possible mechanism regarding the suppression of field emission on Nb surfaces by GCIB treatments will be discussed.
CEA, Gif-sur-Yvette
JLAB, Newport News, Virginia
Buffered Electrochemical Polishing (abbreviated as BEP) is a process developed at JLAB. It was shown that BEP can produce the smoothest niobium surface ever reported [*,**] with a very fast removal rate. Some encouraging results after BEP on single-cell cavities treated by a vertical set-up have also been reported [***]. Complementary experiments on flat Nb samples have been carried out at CEA Saclay with a rotating disc electrode. Electrochemical Impedance Spectroscopy measurements were also performed. Experimental results obtained on BEP were compared with those achieved with the standard EP [****]. Differences in terms of fluorine diffusion and of surface film resistance between BEP and EP have been put forward. Studies are on going to understand if some poor RF test results obtained after BEP with a horizontal set-up as well as with a vertical one [***] are due to the modified electro-chemical mechanisms or the decreased quality of commercial electrolyte used.
*A.T. Wu et al, Applied Surface Science,253(2007)3041
**S. Jin et al, Proc. of 13th SRF Workshop SRF(2007)
***A.T. Wu et al, Proc. of PAC 2009, Vancouver(2009)
****F. Eozénou et al,This conference