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
ODU, Norfolk, Virginia
We are pursuing the use of low cost, environmentally friendly dry etching of SRF cavities in an Ar/Cl2 discharge. We have successfully demonstrated on flat samples that etching rates are comparable to the commonly used wet processes, such as BCP or EP. The geometry of bulk Nb SRF cavities involves the use of an asymmetric RF discharge configuration for plasma etching. The asymmetry in the surface area of driven and grounded electrodes creates a difference in the voltage drop over the plasma sheath attached to the driven electrode and the plasma sheath attached to the cavity surface. To study these asymmetric discharges, a single cell cavity has been specially designed with 20 holes symmetrically placed over the cell. This setup can be used for both diagnostics and sample etching purposes. Radially and spectrally resolved profiles of optical intensity of the discharge are combined with direct etched surface diagnostics to obtain an optimum combination of etching rates, roughness and homogeneity in a variety of discharge types, conditions, and sequences.
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.
ODU, Norfolk, Virginia
JLAB, Newport News, Virginia
Accelerator performance, in particular the average accelerating field and the cavity quality factor, depends on the physical and chemical characteristics of the superconducting radio-frequency (SRF) cavity surface. Plasma based surface modification provides an excellent opportunity to eliminate non-superconductive pollutants in the penetration depth region and to remove mechanically damaged surface layer improving surface roughness. Here we show that plasma treatment of bulk Nb presents surface preparation method alternative to the commonly used BCP and EP methods. We have optimized the experimental conditions in the microwave glow discharge system and their influence on the Nb removal rate on the flat samples. We have achieved etching rates of 1.5 micro-m/min using only 3% Cl2 in the reactive mixture. Combining fast etching step with the moderate one, we have improved a surface roughness without exposing fresh sample surface to the environment. We will apply optimized experimental conditions to single cell cavities, in pursuing improvement of their RF performance.