arXiv subjects

Charles E. Reece

Publications and source records attributed to Charles E. Reece.

9 recordsLinked to original sources

arXiv2026

Niobium's intrinsic coherence length and penetration depth revisited using low-energy muon spin spectroscopy and secondary-ion mass spectrometry

We report direct, simultaneous measurements of the London penetration depth ($λ_L$) and Bardeen-Cooper-Schrieffer (BCS) coherence length ($ξ_0$) in oxygen-doped niobium, with impurity concentrations spanning the "clean" to "dirty" limits. Two depth-resolved techniques - low-energy muon spin spectroscopy (LE-$μ$SR) and secondary-ion mass spectrometry (SIMS) - were used to quantify the element's Meissner screening profiles, analyzed within a framework that accounts for nonlocal electrodynamics. The analysis indicates intrinsic length scales of $λ_L = 29.1(10)$ nm and $ξ_0 = 39.9(25)$ nm, corresponding to a Ginzburg-Landau (GL) parameter of $κ= 0.70(5)$. The obtained $λ_L$ and $κ$ values, accurately quantified at the nanoscale, are smaller than values commonly used in applications and modeling, and indicate that clean niobium lies at the boundary between type-I and type-II superconductivity, supporting the contemporary view that its intrinsic state may be type-I.

cond-mat.supr-con↗

arXiv2022

Effect of substrate temperature on the growth of Nb3Sn film on Nb by multilayer sputtering

Nb3Sn films were fabricated by multilayer sequential sputtering on Nb at substrate temperatures ranging from room temperature to 250 °C. The multilayers were then annealed inside a separate vacuum furnace at 950 °C for 3h. The films material properties were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, atomic force microscopy, and transmission electron microscopy. The films superconducting properties were studied by four-point probe resistivity measurements from room temperature to below the superconducting critical temperature Tc. The highest film Tc was 17.76 K, obtained when the multilayers were deposited at room temperature. A superconducting Nb3Sn thin film with a smoother surface morphology but a lower Tc of 17.58 K was obtained on the film deposited at a substrate temperature of 250 °C.

cond-mat.supr-con↗

arXiv2020

Advances in secondary ion mass spectroscopy

Accurate SIMS 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 (RSF). Accurate RSF values for ppm-range nitrogen contents are increasingly critical, as more precision is sought in processes for next-generation superconducting radio-frequency (SRF) accelerator cavities. Factors influencing RSF value measurements were investigated with the aim of reliably attaining better than 10% accuracy in N concentrations at various depths into the bulk. This has been accomplished for materials typical of SRF cavities at the cost of great attention to all aspects.

physics.acc-ph↗

arXiv2020

Properties of Nb3Sn films fabricated by magnetron sputtering from a single target

Superconducting Nb3Sn films were fabricated on sapphire and fine grain Nb substrates by magnetron sputtering from a single stoichiometric Nb3Sn target. The structural, morphological and superconducting properties of the films annealed for 24 h at temperatures of 800-1000 °C were investigated. The effect of the annealing time at 1000 °C was examined for 1, 12, and 24 h. The film properties were characterized by X-ray diffraction, scanning electron microscopy, atomic force microscopy, energy dispersive X-ray spectroscopy, and Raman spectroscopy. The DC superconducting properties of the films were characterized by a four-point probe measurement down to cryogenic temperatures. The RF surface resistance of films was measured over a temperature range of 6-23 K using a 7.4 GHz sapphire-loaded Nb cavity. As-deposited Nb3Sn films on sapphire had a superconducting critical temperature of 17.21 K, which improved to 17.83 K when the film was annealed at 800 °C for 24 h. For the films annealed at 1000 °C, the surface Sn content was reduced to ~11.3 % for an annealing time of 12 h and to ~4.1 % for an annealing time of 24 h. The Raman spectra of the films confirmed the microstructural evolution after annealing. The RF superconducting critical temperature of the as-deposited Nb3Sn films on Nb was 16.02 K, which increased to 17.44 K when the film was annealed at 800 °C for 24 h.

physics.app-ph↗

arXiv2019

Analysis of RF losses and material characterization of samples removed from a Nb3Sn-coated superconducting RF cavity

Nb3Sn (Tc ~ 18 K and Hsh ~ 400 mT) is a prospective material to replace Nb (Tc ~ 9 K and Hsh ~ 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.

physics.acc-ph↗

arXiv2016

Continuous wave superconducting radio frequency electron linac for nuclear physics research

CEBAF, the Continuous Electron Beam Accelerator Facility, has been actively serving the nuclear physics research community as a unique forefront international resource since 1995. This CW electron linear accelerator (linac) at the U.S. Department of Energy's Thomas Jefferson National Accelerator Facility (Jefferson Lab) has continued to evolve as a precision tool for discerning the structure and dynamics within nuclei. Superconducting RF (SRF) technology has been the essential foundation for CEBAF, first as a 4 GeV machine, then 6 GeV, and currently capable of 12 GeV. We review the development, implementation, and performance of SRF systems for CEBAF from its early beginnings to the commissioning of the 12 GeV era.

physics.acc-ph↗

arXiv2015

Simulation of Non-linear SRF losses derived from characteristic topography of etched and electropolished niobium surfaces

A simplified numerical model has been developed to simulate non-linear superconducting radiofrequency (SRF) losses on Nb surfaces. This study focuses exclusively on excessive surface resistance (Rs) losses due to the microscopic topographical magnetic field enhancements. When the enhanced local surface magnetic field exceeds the superconducting critical transition magnetic field Hc, small volumes of surface material may become normal conducting and increase the effective surface resistance without inducing a quench. Using topographic data from typical Buffered Chemical Polish (BCP) and Electropolish (EP) treated fine grain niobium , we have estimated the resulting field-dependent losses and extrapolated this model to the implications for cavity performance. The model predictions correspond well to the characteristic BCP versus EP high field Q0 performance differences for fine grain niobium. We describe the algorithm of the model, its limitations, and the effects of this non-linear loss contribution on SRF cavity performance.

physics.acc-ph↗

arXiv2014

A new first-principles calculation of field-dependent RF surface impedance of BCS superconductor and application to SRF cavities

There is a need to better understand the intrinsic limit of radiofrequency (RF) surface impedance that determines the performance of superconducting RF cavities in particle accelerators. Here we present a field-dependent derivation of Mattis-Bardeen (M-B) theory of the RF surface impedance of BCS superconductors based on the shifted Density of States (DoS) resulting from coherently moving Cooper pairs [1].The surprising reduction in resistance with increasing field is explained to be an intrinsic effect. Using this analysis coded in MathematicaTM, survey calculations have been completed which examine the sensitivities of this surface impedance to variation of the BCS material parameters and temperature.Our theoretical prediction of the effective BCS RF surface resistance (Rs) of niobium as a function of peak surface magnetic field amplitude agrees well with recently reported record low loss resonant cavity measurements from Jefferson Lab (JLab) and Fermi National Accelerator Lab (FNAL) with carefully, yet differently, prepared niobium material. The results present a refined description of the "best theoretical" performance available to potential applications with corresponding materials. [1]Xiao, B.P., C.E. Reece, and M.J. Kelley, Superconducting surface impedance under radiofrequency field. Physica C: Superconductivity, 2013. 490(0): p. 26-31.

cond-mat.supr-con↗

arXiv2012

Superconducting RF Technology R&D for Future Accelerator Applications

Superconducting rf technology (SRF) is evolving rapidly as are its applications. While there is active exploitation of what one may term the current state-of-the-practice, there is also rapid progress expanding in several dimensions the accessible and useful parameter space. While state-of-the-art performance sometimes outpaces thorough understanding, the improving scientific understanding from active SRF research is clarifying routes to obtain optimum performance from present materials and opening avenues beyond the standard bulk niobium. The improving technical basis understanding is enabling process engineering to both improve performance confidence and reliability and also unit implementation costs. Increasing confidence in the technology enables the engineering of new creative application designs. We attempt to survey this landscape to highlight the potential for future accelerator applications.

physics.acc-ph↗