JLAB, Newport News, Virginia
The College of William and Mary, Williamsburg
A Surface Impedance Characterization (SIC) system has been proposed at the 2005 SRF workshop and recently updated as detailed at the 2009 PAC conference. Currently the SIC system can measure samples in a temperature range from 2K to 20K exposed to an RF magnetic flux density of less than 3mT. We report on new results of a BCP etched large grain Nb sample measured with this system as compared with previous results of a BCP etched polycrystalline Nb sample. The design of an upgraded SIC system for use at higher magnetic flux densities is on the way to more efficiently investigate correlations between local material characteristics and associated SRF properties, both for preparation studies of bulk niobium and also new thin film SRF developments.
AASC, San Leandro, California
NHMFL, Tallahassee, Florida
ASC, Tallahassee, Florida
JLAB, Newport News, Virginia
We report single-step vacuum growth of a stoichiometric superconducting thin film of MgB2, using AASC’s cathodic arc deposition process. Energetic condensation using cathodic arcs produces non-equilibrium fast ions (~50-100eV) that allow growth modes on relatively low temperature substrates. We have demonstrated a film Tc of 34K with MgB2 by depositing at 275 °C in a single step, from a stoichiometric, solid MgB2 source. In a subsequent experiment, the single-step coated samples (on c-plane sapphire) were ex-situ annealed. Post-deposition anneal temperature ranged from 825900K, for 15 minutes. The annealed film also showed a transition at 30K. An MgB2 film was also deposited over a 50mm diameter circle on a Buffered Chemically Polished Niobium substrate for future RF evaluation in a cavity. The depositions spanned a range of substrate temperatures from 550-675 K. The films began to change in appearance form silver to black as substrate temperature was increased, indicating a decreasing magnesium content in the films. Future plans are to reduce oxygen contamination and to use our filtered cathodic arc to deposit smoother films
AASC, San Leandro, California
JLAB, Newport News, Virginia
NSU, Newport News
AASC, JLab and NSU conduct research into SRF thin-film coatings by first characterizing properties such as morphology, grain size, crystalline structure, defects, and impurities, then measuring Tc and RRR, and following this with ‘in-cavity’ RF measurements of the Surface Impedance of the films at cryogenic temperatures. These progressive steps are essential to eventual design and measurement of SRF accelerator structures at high fields. We have recently produced Nb superconducting thin-films with crystal grain sizes ~50μm using our proprietary CED^TM cathodic arc technique. RRR of ~129 at Tc of 9.2K was measured in a film grown on a-plane sapphire heated to 400oC. At 20oC, the RRR dropped to ~4. Energetic condensation using cathodic arcs produces non-equilibrium fast ions (~50-100eV). These ion energies are much higher than typical sputtering energies. When such energetic condensation is complemented by substrate biasing (to ~200-300eV) the incident ion energy is further increased, allowing growth modes that would otherwise require much higher substrate temperatures. Data are presented for pure Nb films using SEM, EBSD, XRD and a Surface Impedance Characterization RF cavity.
AASC, San Leandro, California
JLAB, Newport News, Virginia
NSU, Newport News
Single and large grain Nb films are of interest to reduce the cost of SRF cavities. The structural properties and SRF potential of Nb films obtained by coaxial energetic deposition (CEDTM) in an ultra-high vacuum process are compared and discussed. The CEDTM is a hybrid technique with both energetic ion deposition and implantation phases based on cathodic arc plasma sources, which are copious generators of condensable energetic (20-200 eV), multiply charged ions from metal or alloy cathodes. The X-ray pole figure of the thin films revealed grain orientations on Nb films grown at different substrate temperatures that indicate good structural and electrical properties. Single crystalline 110 epilayers of Nb films are grown on a-plane sapphire substrates at 400 degreeC, but at lower temperature, there are two kinds of twins in which the grains are rotated by ~45 degree about the film normal. RRR of ~129 and Tc=9.2K were measured on a Nb film on a-plane sapphire substrate at 400 degreeC, dropping to ~4 on a room temperature substrate.
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
ODU, Norfolk, Virginia
The College of William and Mary, Williamsburg
NSU, Newport News
In the past years, energetic vacuum deposition methods have been developed in different laboratories to improve Nb/Cu technology for superconducting cavities. JLab is pursuing energetic condensation deposition via Electron Cyclotron Resonance. As part of this study, the influence of the deposition energy on the material and RF properties of the Nb thin film is investigated. The film surface and structure analyses are conducted with various techniques like X-ray diffraction, Transmission Electron Microscopy, Auger Electron Spectroscopy and RHEED. The microwave properties of the films are characterized on 50 mm disk samples with a 7.5 GHz surface impedance characterization system. This paper presents surface impedance measurements in correlation with surface and material characterization for Nb films produced on copper substrates with different bias voltages and also highlights emerging opportunities for developing multi-layer SRF films with a new deposition system.
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
We have EP processed several multi-cell cavities previously heavy BCP etched. With a surprisingly light EP removal of less than 50 micron, all cavities have shown significant gradient and Q improvement. So far three cavities including two fine-grain niobium 7-cell CEBAF upgrade prototype cavities and one large-grain niobium 9-cell ILC cavities have been treated and tested. Both 7-cell cavities reached a quench limit without field emission. Another 7-cell cavity has been treated and is under RF test. We give a summary of the test results.
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 highfield 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
JLAB, Newport News, Virginia
Extending efforts reported at SRF2007, a model of the thermal and hydrodynamic flow conditions internal to an electropolishing niobium cavity has been developed using the commercial code CFDesign®. Building upon parametric studies with small niobium samples that highlighted the process sensitivity to local temperature and fluid flow conditions, we seek to gain predictive insight into processing methods which assure uniform controlled polishing in both the traditional rotating horizontal configuration as well as the potentially more convenient vertical orientation. For present modeling runs, the temperature-dependent viscosity of the standard HF/H2SO4 electrolyte has been included. For the horizontal configuration, we have modeled the recent JLab-adopted reduced temperature and flow conditions used for ILC cavity processing and find improved thermal control and uniformity. For vertical EP, the amplitude and pattern of nominally steady-state internal convective flow under conditions of external wall cooling and no electrolyte circulation has been observed for both single-cell and 9-cell cavities. Opportunities for modeling an integrated next-generation system are being explored.
JLAB, Newport News, Virginia
Particulate contamination on the surface of SRF cavities limits their performance via the enhanced generation of field-emitted electrons. Considerable efforts are expended to actively clean and avoid such contamination on niobium surfaces. The protocols in active use have been developed via feedback from cavity testing. This approach has the risk of over-conservatively ratcheting an ever increasing complexity of methods tied to particular circumstances. A complementary and perhaps helpful approach is to quantitatively assess the effectiveness of candidate methods at removing intentional representative particulate contamination. Toward this end, we developed a standardized contamination protocol using water suspensions of Nb2O5 and SS 316 powders applied to BCP’d surfaces of standardized niobium samples yielding particle densities of order 200 particles/mm2. From these starting conditions, controlled application of high pressure water rinse, ultrasonic cleaning, or CO2 snow jet cleaning was applied and the resulting surfaces examined via SEM/scanning EDS with particle recognition software. Results of initial parametric variations of each will be reported.
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.