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Kelley, Michael J.

Publications and source records attributed to Kelley, Michael J..

18 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Analysis of furnace contamination on superconducting radio frequency niobium using secondary-ion mass spectrometry

Detection of surface contamination on niobium materials used in superconducting radio frequency (SRF) applications is difficult due to quantitative sensitivity and near-atomic depth resolution needed. Inspection of samples known to have experienced surface contamination was found to have inconsistent nitride coverage after high-temperature nitrogen gas exposure (“doping”). Here we compare contaminating species found on samples treated in several different vacuum furnaces, both “exposed” directly in the chamber and “protected” by containment shielding from evaporative sources with “furnace caps.” Typically, furnace caps are used to impede contamination from reaching the interior surface of cavities during the high-temperature vacuum bake that immediately precedes exposure to nitrogen gas. Although, to date, little is known about the effectiveness of these caps, SIMS results showed that they were effective in limiting contamination arising from the furnace environment. Inspection of sample surfaces by SEM showed a lack of nitrides present on contaminated specimens. TEM with energy dispersive spectroscopy performed on these samples revealed that a carbon-rich layer now existed, indicating that a relatively high contaminant load prevents the nucleation and growth of surface nitrides, while thus inhibiting interstitial nitrogen uptake. Except in extreme cases, subsequent removal of the top several micrometers of the surface via electropolishing appears to effectively eliminate any strong influence on the subsequent SRF cavity performance. With the absence of furnace cleaning, carbon contamination was found to be nearly 10× higher for protected nitrogen-doped and electropolished samples, with minimal metallic contamination detected for both processes. SIMS analysis was also performed to compare the cleanliness of samples fully prepared by such nitrogen “doping” with those prepared by a related process, involving the dissolution of niobium surface oxide and diffusion of oxygen into the surface. This oxygen doping or alloying process offers attractive advantages.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Improved quantitation of SIMS depth profile measurements of niobium via sample holder design improvements and characterization of grain orientation effects

The importance of SIMS analyses for “N-doped” impurity alloyed niobium and other surface-alloyed materials continues to increase. A major hurdle is the uncertainty of instrument calibration due to changes in sample height either from sample surface topography or from the sample holder itself. The CAMECA sample holder design allows for many types of samples to be analyzed. However, a drawback is that the holder faceplate can bend, contributing an uncertainty in the relative sensitivity factor (RSF) used to quantify the SIMS results. Here, we describe an improved sample holder having a reinforced faceplate, which prevents deflection and reduces uncertainty. Simulations show that the new design significantly reduces deflection from 10 μm to 5 nm. Sample measurements show a reduction of RSF uncertainty from this source from 4.1% to 0.95%. Grain orientation has long been suspected to affect RSF measurement as well. A bicrystal implant standard, consisting of randomly oriented and [001] grains, was successively rotated 15° between analyses. It was observed that 20% of the analyses performed on the randomly oriented grain exhibited anomalously high RSF values as well as slow sputter rates. These features were associated with the changing grain normal orientation with respect to the primary Cs + beam. The grain orientation associated with the rise in RSF was simulated and determined to be the [101] crystallographic plane, thus indicating that ion channeling was responsible for the significantly increased RSF. Focused ion beam analysis confirmed slower sputter rates for the cardinal crystallographic orientations, indicating that ion channeling occurred for each.

43 PARTICLE ACCELERATORS↗
DOE OSTI2021

Advances in secondary ion mass spectrometry for N-doped niobium

Accurate secondary ion mass spectroscopy measurement of nitrogen in niobium relies on the use of closely equivalent standards, made by ion implantation, to convert nitrogen signal intensity to nitrogen content by determination of relative sensitivity factors (RSFs). Accurate RSF values for ppm-range nitrogen contents are increasingly critical, as more precision is sought in processes for next-generation superconducting radiofrequency (SRF) accelerator cavities. Furthermore, factors influencing RSF value measurements were investigated with the aim of reliably attaining better than 10% accuracy in nitrogen concentrations at various depths into the bulk. This has been accomplished for materials typical of SRF cavities at the cost of increased attention to all aspects.

47 OTHER INSTRUMENTATION↗
DOE OSTI2020

Analysis of RF losses and material characterization of samples removed from a Nb 3 Sn-coated superconducting RF cavity

Nb 3 Sn (T c ≈18 K and H sh ≈400 mT) is a prospective material to replace Nb (T c ≈9 K and H sh ≈200 mT) in SRF accelerator cavities for significant cost reduction and performance enhancement. Because of its material properties, Nb3Sn is best employed as a thin film (coating) inside an already built RF cavity structure. A particular test cavity noted as C3C4 was a 1.5 GHz single-cell Nb cavity, coated with Nb3Sn using Sn vapor diffusion process at Jefferson Lab. Cold measurements of the coated cavity indicated the superconducting transition temperature of about 18 K. Subsequent RF measurements indicated field-dependent surface resistance both at 4.3 K and 2.0 K. After initial cold measurements, the cavity RF loss distribution was studied with a thermometry mapping system. Loss regions were identified with thermometry and were cut out for material analysis. The presence of significantly thin patchy regions and other carbon-rich defects is associated with strong local field-dependent surface resistance. This paper summarizes RF and thermometry results correlated with material science findings.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Genomic Analysis of Metastatic Solid Tumors in Veterans: Findings From the VHA National Precision Oncology Program

The Veterans Health Administration (VHA) is the largest cancer care provider in the United States, with the added challenge of serving more than twice the percentage of patients with cancer in rural areas than the national average. The VHA established the National Precision Oncology Program in 2016 to implement and standardize the practice of precision oncology across the diverse VHA system. Tumor or peripheral blood specimens from veterans with advanced solid tumors who were eligible for treatment were submitted for next-generation sequencing (NGS) at two commercial laboratories. Annotated results were generated by the laboratories and independently using IBM Watson for Genomics. Levels-of-evidence treatment recommendations were based on OncoKB criteria. From July 2016 to June 2018, 3,698 samples from 72 VHA facilities were submitted for NGS testing, of which 3,182 samples (86%) were successfully sequenced. Most samples came from men with lung, prostate, and colorectal cancers. Thirty-four percent of samples were from patients who lived in a rural area. TP53, ATM, and KRAS were among the most commonly mutated genes. Approximately 70% of samples had at least one actionable mutation, with clinical trials identified as the recommended option in more than 50%. Mutations in genes associated with a neuroendocrine prostate cancer phenotype were expressed at increased frequency among veterans than in the general population. The most frequent therapies prescribed in response to NGS testing were immune checkpoint inhibitors, EGFR kinase inhibitors, and PARP inhibitors. Clinical implementation of precision oncology is feasible across the VHA health care system, including rural sites. As a result, veterans have unique occupational exposures that might inform the nature of the mutational signatures identified here. Importantly, these results underscore the importance of increasing clinical trial availability to veterans.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2019

Growth of Nb 3 Sn coating in tin vapor-diffusion process

The potential of Nb 3 Sn for superconducting radio frequency (SRF) cavities is widely recognized and renewed R&D efforts continue to bring new insights about the material's structure and properties. Here, we have systematically coated niobium with Nb 3 Sn for different durations, and several of them were coated over ("overcoat") multiple times, to elucidate the growth kinetics of Nb 3 Sn during tin vapor diffusion process. Analysis of coated samples is consistent with the model in which tin diffuses via grain boundaries to the Nb 3 Sn-Nb interface, where the growth of Nb 3 Sn into the Nb bulk takes place. Similar scaling laws were found for Nb 3 Sn grain growth and layer thickness. Non-parabolic layer growth is consistent with significant grain growth, which reduces the number of Sn transport channels. Examination of patchy region in Nb 3 Sn coating, revealing large single crystalline grains, points to impeded Nb 3 Sn layer growth due to low grain boundary density, resulting in a significantly thinner coating in those areas.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Electron backscatter diffraction of Nb 3 Sn coated niobium: Revealing structure as a function of depth

Over the last two decades, advances in Electron Backscatter Diffraction (EBSD) have moved the technique from a research tool to an essential characterization technique in many fields of material research. EBSD is the best suited technique for determining structure as a function of depth. This characterization is critically important, but has been previously absent from Nb 3 Sn efforts. While EBSD is the technique of choice, obtaining quality data can be difficult. Sample preparation in particular is non-trivial. Here we summarize the general principles of EBSD, discuss specific sample preparation techniques for Nb 3 Sn coated SRF cavity material, and give examples of how EBSD is being used to understand fundamental growth mechanisms for Nb 3 Sn coatings.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Initial growth of tin on niobium for vapor diffusion coating of Nb 3 Sn

Nb 3 Sn offers significant potential to exceed the performance of niobium for superconducting radio frequency accelerator cavities. The most promising path toward deployment is by tin vapor diffusion coating of Nb cavity interiors via a two step nucleation-then-growth sequence. Reported here is a materials science study of the nucleation process. We manipulated the accessible range of process variables and determined the effect on composition and microstructure using an array of materials characterization tools. Broadly, nucleation deposits tin as a thin surface phase and, under some conditions, as near-micron sized particles as well, resembling Stranski–Krastanov growth. Conditions that impair nucleation promote the formation of defects, such as patches, in subsequent coating growth. Furthermore no significant effect on the subsequently grown coating was found for structures produced during nucleation.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Secondary ion mass spectrometry for superconducting radiofrequency cavity materials

Historically, many advances in superconducting radio frequency (SRF) cavities destined for use in advanced particle accelerators have come empirically, through the iterative procedure of modifying processing and then performance testing. However, material structure is directly responsible for performance. Understanding, the link between processing, structure, and performance will streamline and accelerate the research process. In order to connect processing, structure, and performance, accurate and robust materials characterization methods are needed. Here in this paper, one such method, SIMS, is discussed with focus on analysis of SRF materials. In addition, several examples are presented, showing how SIMS is being used to further understanding of materials based SRF technologies.

43 PARTICLE ACCELERATORS↗
DOE OSTI2018

Materials path toward improved energy efficiency for High Energy Physics accelerators

For nearly fifty years researchers working with particle accelerators have envisioned reducing their initial and operating costs by substituting the intermetallic compound Nb 3 Sn for Nb as the cavity interior surface. The goal has remained elusive, stymied especially by the fall of cavity quality factor (representing increasing energy consumption) with increasing gradient (the amount of accelerator need to reach an energy goal). While one research group reported making a few cavities free of the problem, no one has reported how this comes about or has reproduced the improvement. We believe that insufficient understanding is responsible. We report here findings of a detailed, systematic study of the formation of Nb 3 Sn coatings on Nb, relating structure and properties of the coating to the process parameters and substrate materials on the one hand, and RF surface impedance and SRF cavity performance on the other was carried out, focusing on the two step (nucleation – deposition) tin vapor diffusion process. Major outcomes are: The choice of initial surface preparation (BCP etc) has no discernable impact on any quality of the final coating; Though tin chloride serves as the coating nucleant, no trace could be found by any means of chlorine after nucleation or coating; No evidence was found for any substantive impact of the details of the nucleation on the ultimate coating, as long as it was done within the frequently used range; The coating forms as Nb 3 Sn crystals by tin from the vapor diffusing down grain boundaries to the Nb interface and growing there. Higher temperature and longer time lead to larger, but otherwise substantially identical crystals; All coatings exhibited roughness on the micron scale. Testing the effect of coating surface modification technologies in use by the community, only a carefully tailored electropolish showed significant benefit; and, Preliminary results indicate that Ti contamination may be associated with falling gradient. Two particular item appear promising for future work. The effect of surface topography in the range seen here on performance is significant for niobium. The nature and impact on Nb 3 Sn, if any, needs to be understood and managed. At what level Ti, Ta or other impurities affects the performance of Nb 3 Sn needs to be understood.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Surface Characterization of NbTiN Films for Accelerator Applications

The development of next-generation SRF cavities requires the deployment of innovative material solutions with RF performance beyond bulk Nb. Theoretical interest has stimulated efforts to grow and characterize thin multi-layer superconductor/insulator/superconductor (SIS) structures for their potential capability of supporting otherwise inaccessible surface magnetic fields in SRF cavities *. The ternary B1-compound NbTiN is among the candidate superconducting materials for SIS structures. Single crystal NbTiN films with thicknesses below 15 nm are also of interest for the development of high resolution, high sensitivity (SNSPD) detectors for particle physics application. Using DC reactive magnetron sputtering, NbTiN can be deposited with nominal superconducting parameters. This contribution presents the on-going material surface and superconducting properties characterization in order to optimize the NbTiN films for each application.

Beverstock, David R.↗
DOE OSTI2018

Nb3Sn multicell cavity coating at JLAB

Following encouraging results with Nb3Sn-coated R&D cavities, the existing coating system was upgraded to allow for Nb3Sn coating of CEBAF accelerator cavities. The upgrade was designed to allow Nb3Sn coating of original CEBAF 5-cell cavities with the vapor diffusion technique. Several CEBAF cavities were coated in the upgraded system to investigate vapor diffusion coatings on extended structures. Witness samples coated along with the cavities were characterized with material science techniques, while coated cavities were measured at 4 and 2 K. The progress, lessons learned, and the pathforward are discussed.

Pudasaini, Uttar↗
DOE OSTI2018

Nb3Sn multicell cavity coating at JLAB

The Nb3Sn-coated cavities aim to enhance perfor-mance and significantly reduce cost. Their fabrication in-volves tin vapor diffusion coating of Nb3Sn on the interior surface of a Nb cavity. Controlled removal of first few lay-ers to obtain a smoother and cleaner surface could be de-sirable to improve the high field RF performance. Our first results from the application of electropolishing and oxy-polishing techniques on Nb3Sn-coated surfaces indicated reduced surface roughness, and the surface composition appeared nominally unchanged. Systematic studies explore the effect of different polishing parameters into the rough-ness and composition. We present the latest results from SEM/EDS and AFM studies of Nb3Sn-coated samples treated with electropolishing and oxypolishing.

Pudasaini, Uttar↗
DOE OSTI2018

Enhancement of effective linear RF surface resistance of superconducting surfaces by microscopic topography

Minimization of Radio-Frequency (RF) dissipation on superconducting surfaces is of interest for many applications. A prominent one is the use of Superconducting RF (SRF) cavities for charged particle acceleration. In addition to our previous investigation which characterized the occurrence of high-field non-linear losses by microscopic surface topography, the topic of increased linear losses as a function of surface topographic character merits consideration. Surfaces with isotropic homogeneous surface topography may be well characterized by power spectral density (PSD) derived from systematic height measurements. PSD characterizations of representative niobium cavity surface treatments have been developed: Electro-Polishing (EP), Nano-Mechanical Polishing (NMP) and Centrifugal Barrel Polishing (CBP) A perturbation model based on PSD statistical analysis is used to calculate additional RF loss on these surfaces when superconducting. The model assesses the penetration depth effects for a superconductor. We thus estimate the RF power dissipation ratio between these rough surfaces and an ideal smooth surface.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Progress With Multi-Cell Nb3Sn Cavity Development Linked With Sample Materials Characterization

Exploiting both the new Nb3Sn coating system and the materials characterization tools nearby, we report our progress in low-loss Nb3Sn films development. Nb3Sn films a few micrometers thick were grown on Nb coupons as well as single- and multi-cell cavities by the Sn-diffusion technique. Films structure and composition were investigated on coated samples and cavity cutouts with characterization tools including SEM/EDS/EBSD, AFM, XPS, SIMS towards correlating film growth and RF loss to material properties and deposition parameters. Cavity coating efforts focused on establishing techniques for coating progressively more complicated RF structures, and understanding limiting mechanisms in coated cavities. Nb3Sn coated 1.5 GHz 1-cell and 1.3 GHz 2-cell cavities have shown quality factors of 1010 at 4.3 K, with several cavities reaching above Eacc = 10 MV/m. The dominant limiting mechanisms were low field quenches and quality factor degradation above 8 MV/m. The surface data indicates a near-stoichiometric Nb3Sn consistent with the transition temperature and gap measurements. The Nb3Sn layer is covered with Nb2O5 and SnO2 native oxides and has little memory of the pre-coating surface.

Eremeev, Grigory V.↗
DOE OSTI2015

Structure and composition of Nb3Sn diffusion coated films on Nb

The structure and composition of Nb3Sn films obtained by diffusion coating niobium coupons and SRF cavities were investigated by x-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS) and scanning electron microscopy (SEM) with energy-dispersive x-ray spectroscopy (EDS) and electron back-scatter diffraction (EBSD), including native surfaces, depth profiles and cross-sections. We find that the native surface oxide is significantly tin-rich, we have measured depth profiles. We find that the grains apparent in the SEM images are individual crystallites having no evident relationship to the substrate or each other.

Tuggle, Jay↗