JLAB, Newport News, Virginia
The College of William and Mary, Williamsburg
As the field requirements of niobium SRF cavities approach fundamental material limits, there is increased interest in understanding the details of topographical influences on realized performance limitations. In this study, a set of samples representing 24 different starting conditions used in cavity processing has been assembled. This set includes fine grain, large grain, and single crystal Nb samples under EBW’ed, hand ground, and CBP with a variety of stones, the latter provided by KEK colleagues. Sample topography has been carefully characterized in both the initial condition and after removal of 30 microns via controlled-parameter EP. A power spectral density (PSD) approach based on Fourier analysis of surface topography*, stylus profilometry and atomic force microscopy (AFM) are used to distinguish the scale-dependent smoothing effects. The detailed topographic transformation of Nb surface with the varied starting state Nb surface (CBP/ EBW) is reported. This study will help to identify optimum EP parameter sets for controlled and reproducible surface leveling of Nb for cavity production.
* “Surface Roughness Characterization of Niobium Subjected to Incremental BCP and EP Processing Steps” , Hui Tian, G. Ribeill, Charles E. Reece, and Michael J. Kelley. Proceedings of SRF2007.
JLAB, Newport News, Virginia
NCSU AIF, Raleigh, North Carolina
The College of William and Mary, Williamsburg
Performance of SRF cavities depends on material within the shallow RF penetration depth. C, N, and O are of particular interest as interstitial contaminants and earlier work suggested very high H concentration*. Secondary Ion Mass Spectrometry (SIMS) has the sensitivity to quantitatively measure these species in the region of interest. However, standards for quantitative SIMS analysis of these elements in Nb did not exist. Initial attempts to develop an ion implanted standard were unsuccessful because of the roughness of the Nb surface. In this study, Nb samples were specially chemical mechanical polished and then subsequently treated with a light BCP. The result is a surface finish suitable for SIMS analysis and implantation standards. Ion implants of C, N, O, and deuterium (D) were obtained in Nb (and simultaneously in Si for dose verification). D was implanted to characterize H, and to avoid the high H background. The results show that D is apparently very mobile in Nb, and another approach will be required to quantify this element. This multi-element standard has already been of great benefit in characterization of C, O, and N in polycrystalline and large grain Nb**.
* A. D. Batchelor, et al. Proc. Single Crystal Niobium Technology Workshop, Brazil, AIP Conf. Proc., Melville, NY (2007) 72-83.
** P. Maheshwari, et al. Surface and Interface Analysis (in press)
JLAB, Newport News, Virginia
Future accelerators, such as the envisioned international linear collider (ILC), require unprecedented cavity performance, which is strongly influenced by interior surface nano-smoothness. Electropolishing (EP) is the technique of choice to being developed for high–field SRF cavities. Previous study shows that the EP mechanism of Nb in 1:9 volume ratio of H2SO4/HF acid electrolyte proceeds by formation and dissolution of a compact salt film under F- diffusion-limited mass transport control*. We pursue an improved understanding of the microscopic conditions required for optimum surface leveling. The temperature-dependent viscosity of the standard electrolyte has been measured and, using a rotating Nb disk electrode, the diffusion coefficient of F- was measured at a variety of temperatures from 0C to 50C. In addition, data indicates that above 25C electrode kinetics becoming competitive with the mass transfer current limitation and increase dramatically with temperature. These findings are expected to guide the optimization of EP process parameters for achieving controlled, reproducible and uniform nano-smooth surface finishing for SRF cavities.
*H. Tian, S. G. Corcoran, C. E. Reece, M. J. Kelley, Journal of the Electrochemical Society, v 155, n 9, Sept. 2008, p 563-8