JLAB, Newport News, Virginia, USA
The performance of the operational SRF-based electron linacs CEBAF, ELBE, and S-DALINAC continues to evolve positively. These facilities are exploiting opportunities to improve operational capability by both remediation of past limitations and also construction of new capacity using state-of-the-art designs and processes. A project to rework the weakest ten cryomodules in CEBAF was completed and enabled robust operation for physics at 6 GeV. The 12 GeV Upgrade of CEBAF is now underway and involves construction of ten new CW >100 MV cryomodules with 80 new 7-cell low-loss cell shaped fine-grained niobium cavities, all electropolished. The technical challenges associated with the preparation of these cavities will be reviewed and their performance in individual acceptance and cryomodule testing to date will be summarized. The ELBE facility at Helmhotz Zentrum Dresden Rossendorf continues to develop its source with an SRF gun system. The long-running S-DALINAC at Technische Universität Darmstadt is operating reliably, and in the framework of the injector upgrade program, a new cryomodule featuring a waveguide power coupler with low transversal fields was developed and is assembled.
AASC, San Leandro, California, USA
NCSU AIF, Raleigh, North Carolina, USA
JLAB, Newport News, Virginia, USA
NSU, Newport News, Virginia, USA
ASC, Tallahassee, Florida, USA
This paper describes Energetic Condensation Growth of Nb films using a cathodic arc plasma, whose 40-120eV ions enable sufficient surface mobility to ensure that the lowest energy state (crystalline structure with minimal defects) is accessible to the film. Hetero-epitaxial films of Nb were grown on a-plane sapphire and MgO crystals with good superconducting properties and crystal size (10mm × 20mm) limited only by substrate size. The substrates were heated to 700 deg C and coated at 300, 500 and 700 deg C. Film thickness varied from ~0.25μm up to >3μm. Residual resistivity ratio (RRR) values (up to a record RRR-554 on MgO and RRR-328 on a-sapphire) vary with substrate annealing and deposition temperatures. XRD spectra and pole figures reveal that RRR increases as the crystal structure of the Nb film becomes more ordered, consistent with fewer defects and hence longer electron mean free path. A transition from Nb(110) to Nb(100) orientation on the MgO(100) lattice occurs at higher temperatures. SIMS depth profiles, EBSD and SEM images complement the XRD data. Crystalline structure in Nb on amorphous borosilicate substrates has implications for future, lower-cost SRF cavities.
Faraday Technology, Inc., Clayton, USA
JLAB, Newport News, Virginia, USA
* Hui Tian and Charles E. Reece, Evaluation of the diffusion coefficient of fluorine during the electropolishing of niobium, Phys. Rev. ST Accel. Beams, 13, 083502 (2010)
The College of William and Mary, Williamsburg, USA
JLAB, Newport News, Virginia, USA
Electropolishing (EP) in sulfuric/hydrofluoric acid mixtures affords significantly greater surface smoothness than the incumbent buffered chemical polishing (BCP), making it attractive as the future baseline technology for SRF cavity manufacture. However, reported observations of particulate sulfur residues raise concern. One hypothesis is sulfate reduction to elemental sulfur at the cathode, where the measured potential drop is thermodynamically sufficient. Alternatively, the low effectiveness of the cathode’s aluminum oxide surface as a hydrogen recombination catalyst could lead to accumulation of atomic hydrogen, a powerful reductant. We explored these possibilities under standard EP conditions in a small three-electrode laboratory cell. We varied aluminum cathode area to obtain different current densities (and thus overpotentials) at constant cell current. We substituted platinum, an excellent hydrogen recombination catalyst, for aluminum in some experiments. Surface of cathodes were examined with Scanning Electron Microscope (SEM). Surface composition was analyzed by Energy Dispersive X-Ray Spectroscopy (EDS) and X-Ray Photoelectron Spectroscopy (XPS).
JLAB, Newport News, Virginia, USA
A project is underway at Jefferson Lab to fully renovate all of the SRF research, development, fabrication, processing and assembly facilities. Initiated in 2009 and funded by the US Department of Energy’s Science Laboratory Infrastructure program, this work together with construction of a new building to house JLab’s Engineering Division and detector electronics group is collectively known as the Technology and Engineering Development Facility (TEDF) Project. The majority of the SRF facilities will be consolidated in a new building with 30,000 square feet (3300 m2) of work space attached to the existing Test Lab. The purpose-built facility integrates fabrication, chemistry, and cleanroom suites and cryomodule assembly lines for convenient, yet flexible operations serving multiple projects in parallel. A robust ultra-pure water system and integrated hazardous materials transfer and neutralization system are included in the project. Construction is underway and move-in is scheduled for early 2012. Project details will be presented.
JLAB, Newport News, Virginia, USA
80 new 7-cell, LL cell-shaped cavities are required for the CEBAF 12 GeV Upgrade project. In addition to ten pre-production units fabricated at JLab, the full set of commercially-produced cavities have been delivered. An efficient processing routine, which includes a controlled 30 micron EP, has been established to transform these cavities into qualified 8-cavity strings. This work began in 2010 and will run through the end of 2011. The realized cavity performance consistently exceeds project requirements and also the maximum useful gradient in CEBAF: 25 MV/m. We will describe the cavity processing and preparation protocols and summarize test results obtained to date.
JLAB, Newport News, Virginia, USA
JLAB, Newport News, Virginia, USA
University of Houston, Houston TX, USA
NSU, Newport News, Virginia, USA
AASC, San Leandro, California, USA
JLAB, Newport News, Virginia, USA
JLAB, Newport News, Virginia, USA
ODU, Norfolk, Virginia, USA
NSU, Newport News, Virginia, USA
An energetic condensation thin film coating technique with an electron cyclotron resonance (ECR) induced plasma ion source is used to deposit Nb thin films on crystalline insulating substrates, such as a-plane and c-plane sapphire (Al2O3) and on magnesium oxide, MgO (100), (110), and (111). Heteroepitaxial Nb films were produced by ECR deposition with regulated substrate temperature. The residual resistance ratio (RRR) of about 1 micron thick films reach unprecedented values (350 - 450) on a-plane (11-20) sapphire substrates. The epitaxial relationship of Nb/crystalline substrate is found to be strongly influenced by the substrate bias voltage (adding to the initial Nb+ kinetic energy), the substrate crystalline orientation, and heating conditions. At low substrate temperature, the Nb films demonstrated non-epitaxial crystalline textures, revealed by XRD Pole Figure technique and Electron Backscattering Diffraction (EBSD). The texture might be caused by “Volmer-Weber” growth mode, i.e. island growth, at low surface adatom mobility. This study shows that the film’s crystal structural character has great impact on its RRR/Tc value.
The College of William and Mary, Williamsburg, USA
JLAB, Newport News, Virginia, USA
Micro-roughness is attributed to be a critical issue for realizing optimum performance of Superconducting Radio Frequency (SRF) cavities. Several surface processing methods such as chemical, mechanical and plasma, are used to obtain relatively smooth surfaces. Among those process methods, Buffered Chemical Polish (BCP) and Electro-Polishing (EP) are most commonly used in current niobium cavity production. The Power Spectral Density (PSD) of surface height data provides a more thorough description to the topography than a simple Rq (RMS) measurement and reveals useful information including fractal and superstructure contributions. Polishing duration and temperature can have predictable effects on the evolution of such features at different scale regions in PSD spectrum. 1 dimensional average PSD functions derived from morphologies of niobium surfaces treated by BCP and EP with different controlled starting conditions and durations have been fitted with a combination of fractal, K-correlation and shifted Gaussian models, to extract characteristic parameters at different spatial harmonic scales.
The College of William and Mary, Williamsburg, USA
JLAB, Newport News, Virginia, USA
Superconducting thin films and multilayers have attracted the attention of the scientific community due to the promise of overcoming the maximum field gradients that SRF cavities can withstand, pushing them above 100 MeV/m *. Nevertheless, in order to achieve the desired properties, special attention needs to be devoted to the epitaxy and growth mode of such thin films, taking into account multiple aspects such as crystalline quality, lattice strain, grain size, etc. We present a complete correlation between morphology, structure and superconducting properties such as critical field, critical temperature and complex susceptibility for single crystal Nb(110) thin films sputter deposited on a-plane sapphire substrates. The influence of strain and grain boundaries in the superconducting transition is analyzed in detail, since the lattice mismatch between Nb and sapphire induces strain in the first atomic layers and may affect the superconducting properties of the thin films. AC susceptibility techniques allow us to identify the dissipative effects in the lattice associated with the presence of defects, thus allowing us to tune the growth conditions to minimize their effect.
*A. Gurevich, Applied Physics Letters 88 (1), 012511 (2006).
The College of William and Mary, Williamsburg, USA
JLAB, Newport News, Virginia, USA
LANL, Los Alamos, New Mexico, USA
The Surface Impedance Characterization (SIC) system in Jefferson Lab can presently make direct calorimetric RF surface impedance measurements on the central 0.8 cm2 area of 5 cm diameter disk samples from 2 to 20 K exposed to RF magnetic fields up to 14 mT at 7.5 GHz. MgB2 thin films from STI/LANL were deposited on 5 cm diameter Nb disks using reactive evaporation technique. We will report the results of measurements on these samples using the SIC system. The data will be interpreted based on BCS theory as the temperature-dependent properties suggest evaluation of the Tc, energy gap, penetration depth, mean free path and coherence length.
The College of William and Mary, Williamsburg, USA
JLAB, Newport News, Virginia, USA
SRF technology used in linear accelerators is based on bulk Nb cavities that have high cost and are approaching the maximum field gradients they can withstand*. Thus, development of a suitable alternative to bulk Nb is needed. Attempts have been made to implement Nb-coated Cu cavities since the thermal conductivity of Cu is better than bulk Nb**. Our studies show that the transport properties of Nb, in particular the residual resistance ratio (RRR), are better when epitaxially grown on crystalline ceramics (i.e. MgO and Al2O3) compared to Cu templates. Since grain boundaries are one of the main obstacles to superconducting transport, we show how the increased number of crystallographic domains that can occur during epitaxial Nb growth onto Cu surfaces leading to higher density of grain boundaries can explain our results. We propose a route to improved performance while maintaining thermal efficiency by using seed-layers on Cu templates that can decrease grain boundary density. We will show our correlated studies of microstructure and surface morphology and the resulting transport/susceptibility properties illustrating possible mechanisms to improve cavity performance of such films.
* P. Kneisel et al., Proceedings of 2005 Particle Accelerator Conference, Knoxville, TN, TPPT076 (2005).
** S. Calatroni, Physica C 441, 95 (2006).
JLAB, Newport News, Virginia, USA
ODU, Norfolk, Virginia, USA
NSU, Newport News, Virginia, USA
Particle accelerator technologies rely on SRF cavities to create the accelerating gradient for beam lines. Solid niobium cavities are widely employed throughout the community despite high material, fabrication, and operation cost. New thin film technologies are being explored for the suitability of niobium coatings for accelerating cavities. Thin layers of high-quality niobium would be deposited on a base material that has lower material and fabrication cost. Copper is a strong candidate for the cavity base due to availability, cost, machinability, and potentially improved performance characteristics of the niobium SRF surface. Initial results of TEM, EBSD and XRD analyses of niobium thin films grown on copper substrates under controlled conditions are presented to demonstrate the feasibility of the technology and establish lower limits of performance characteristics. Correlation of RRR data with the structure of niobium thin films will demonstrate the importance of thin film structural quality.
The College of William and Mary, Williamsburg, USA
JLAB, Newport News, Virginia, USA
Surface and interface roughness are critical factors in determining the technological viability of many systems, in particular the development of next-generation superconducting radio frequency (SRF) cavities. Thus, we have undertaken a systematic effort to investigate the surface evolution of epitaxially grown Nb thin films under specific deposition conditions. This is important since ongoing efforts to improve cavity’s performance have considered the possibility of multi-layered thin film coatings* as an alternative to the current bulk Nb technology. We examined the surface morphology of epitaxial Nb films grown on MgO at different stages during growth and applied dynamical scaling analysis to the surface features. Our thin film nucleation and growth kinetics studies are relevant since thin films may differ from bulk systems due to limited material supply as well as stress contributions from lattice mismatch with the substrate. This can induce significant surface roughness which can in turn lead to undesirable effects for SRF applications. Our studies may offer a venue to minimize these drawbacks by suitable choice of thin film growth parameters and substrates.
* A. Gurevich, Appl. Phys. Lett. 88, 012511 (2006).
JLAB, Newport News, Virginia, USA
We performed Centrifugal Barrel Polishing (CBP) on single crystal niobium samples housed in a stainless steel sample holder following a polishing recipe recently developed at FNAL [*]. We were able to obtain a mirror-like finish after the final stage of tumbling, although some defects and imbedded particles remain. Our presentation will discuss the initial results from the coupon study, including qualitative and quantitative analysis of the surface characteristics from each step in the CBP process, followed by HPR and well controlled incremental EP. These will include surface roughness, size and character of contaminants, surface crystal structure, and overall finish. We will discuss how the surface characteristics should guide the SRF community in exploiting or adapting the Fermi recipe; including why minimal subsequent EP is needed, and possible places for modification of the recipe to reduce polishing time.
* CA Cooper, LD Cooley , “Mirror Smooth Superconducting RF Cavities by MechanicalPolishing with Minimal Acid Use,” http://lss.fnal.gov/archive/2011/pub/fermilab-pub-11-032-td.pdf, (May 31, 2011)
JLAB, Newport News, Virginia, USA
NCSU AIF, Raleigh, North Carolina, USA
The College of William and Mary, Williamsburg, USA
NSU, Newport News, Virginia, USA
For the past three decades, bulk niobium has been the material of choice for SRF cavities applications. In the recent years, RF cavities performances have approached the theoretical limit for bulk niobium. For further improvement of RF cavity performance for future accelerator projects, an interesting alternative has been recently proposed by Alex Gurevich with the Superconductor-Insulator-Superconductor multilayer approach, using the benefit of the higher critical field Hc2 of higher-Tc superconductors without being limited with their lower Hc1. JLab is pursuing this approach with the development of multilayer structures based on NbTiN via magnetron sputtering and High Power Impulse Magnetron Sputtering (HiPIMS). Insulators such as, AlN, Al2O3 and MgO are being investigated as candidates for the insulator layers. This paper present the preliminary results on the characteristics of NbTiN and insulator layers and a first attempt of a NbTiN-based multilayer structure on bulk Nb and thick Nb films.
Texas A&M University, College Station, Texas, USA
JLAB, Newport News, Virginia, USA
A high-gradient test cavity is being developed to test wafer samples of advanced SRF surfaces at gradients to or beyond the BCS limit of Nb. The cavity design employs dielectric loading by a large high-purity sapphire crystal. As a first step towards construction we set out to measure the loss tangent of such a large HEMEX-grade sapphire crystal. The crystal was inserted into a single-cell CEBAF cavity equipped with couplers to operate in the TE01 mode so that the electric field was localized within the sapphire. Cold testing of the cavity, without the sapphire and numerical simulations, verified that the unloaded Q of the cavity was adequate to accurate measurement of the sapphire loss tangent down to 10-10. Several Q measurements were made of the sapphire-loaded cavity in a variety of conditions. The temperature dependence of the sapphire’s loss tangent and heat capacity were measured. The implications of these results for the high-gradient wafer test cavity design are reported.