Jolene D. Splett

A Liquid Density Standard Over Wide Ranges of Temperature and Pressure Based on Toluene.

The density of liquid toluene has been measured over the temperature range −60 °C to 200 °C with pressures up to 35 MPa. A two-sinker hydrostatic-balance densimeter utilizing a magnetic suspension coupling provided an absolute determination of the density with low uncertainties. These data are the basis of NIST Standard Reference Material® 211d for liquid density over the temperature range −50 °C to 150 °C and pressure range 0.1 MPa to 30 MPa. A thorough uncertainty analysis is presented; this includes effects resulting from the experimental density determination, possible degradation of the sample due to time and exposure to high temperatures, dissolved air, uncertainties in the empirical density model, and the sample-to-sample variations in the SRM vials. Also considered is the effect of uncertainty in the temperature and pressure measurements. This SRM is intended for the calibration of industrial densimeters.

Estimation of Q-factors and resonant frequencies

We estimate the quality factor Q and resonant frequency f/sub 0/ of a microwave cavity based on observations of a resonance curve on an equally spaced frequency grid. The observed resonance curve is the squared magnitude of an observed complex scattering parameter. We characterize the variance of the additive noise in the observed resonance curve parametrically. Based on this noise characterization, we estimate Q and f/sub 0/ and other associated model parameters using the method of weighted least squares (WLS). Based on asymptotic statistical theory, we also estimate the one-sigma uncertainty of Q and f/sub 0/. In a simulation study, the WLS method outperforms the 3-dB method and the Estin method. For the case of measured resonances, we show that the WLS method yields the most precise estimates for the resonant frequency and quality factor, especially for resonances that are undercoupled. Given that the resonance curve is sampled at a fixed number of equally spaced frequencies in the neighborhood of the resonant frequency, we determine the optimal frequency spacing in order to minimize the asymptotic standard deviation of the estimate of either Q or f/sub 0/.

Influence of Ti and Ta doping on the irreversible strain limit of ternary Nb3Sn superconducting wires made by the restacked-rod process

Nb3Sn superconducting wires made by the restacked-rod process (RRP®) were found to have a dramatically improved resilience to axial tensile strain when alloyed with Ti as compared to Ta. Whereas Ta-alloyed Nb3Sn in RRP wires showed permanent damage to its current-carrying capacity (Ic) when tensioned beyond an intrinsic strain as small as 0.04%, Ti-doped Nb3Sn in RRP strands exhibits a remarkable reversibility up to a tensile strain of about 0.25%, conceivably making Ti-doped RRP wires more suitable for the high field magnets used in particle accelerators and nuclear magnetic resonance applications where mechanical forces are intense. A strain cycling experiment at room temperature caused a significant drop of Ic in Ta-alloyed wires, but induced an increase of Ic in the case of Ti-doped strands. Whereas either Ti or Ta doping yield a similar enhancement of the upper critical field of Nb3Sn, the much improved mechanical behavior of Ti-alloyed wires possibly makes Ti a better choice over Ta, at least for the RRP wire processing technique.

Transfer Standard for the Spectral Density of Relative Intensity Noise of Optical Fiber Sources Near 1550 nm

We have developed a transfer standard for the spectral density of relative intensity noise (RIN) of optical fiber sources near 1550 nm. Amplified spontaneous emission (ASE) from an erbium-doped fiber amplifier (EDFA), when it is optically filtered over a narrow band (<5 nm), yields a stable RIN spectrum that is practically constant to several tens of gigahertz. The RIN is calculated from the power spectral density as measured with a calibrated optical spectrum analyzer. For a typical device it is -110 dB/Hz, with uncertainty ⩽0.12 dB/Hz. The invariance of the RIN under attenuation yields a considerable dynamic range with respect to rf noise levels. Results are compared with those from a second method that uses a distributed-feedback laser (DFB) that has a Poisson-limited RIN. Application of each method to the same RIN measurement system yields frequency-dependent calibration functions that, when they are averaged, differ by ⩽0.2 dB.

Method for determining the irreversible strain limit of Nb3Sn wires

We define a rigorous and reliable method for determining the irreversible strain limit of Nb3Sn wires. The critical current (Ic) is measured as a function of applied longitudinal strain (e), Ic(e), at one magnetic field and a temperature of 4.0 K. The sample is loaded and partially unloaded at progressively higher strain levels to determine the irreversible strain limit, eirr, which is defined as the maximum loaded strain where Ic is still reversible. Our method uses a polynomial fit of the loaded Ic(e) to derive the Ic residuals for the loaded and unloaded points that are analyzed to determine the limit of irreversibility. The effect of varying the amount of strain unloading is also studied. The possibility and problems of using the strain dependent n-value (which indicates the steepness of the electric field–current, E–I, curve) to determine eirr are discussed. The method presented here to determine eirr has proven to be repeatable for many types of commercial Nb3Sn wires. This method can also be more generally used to determine eirr for any brittle low-temperature or high-temperature superconducting material.

Strain and Magnetization Properties of High Subelement Count Tube-Type

A tubular technique for economical production of Nb3Sn material with large numbers of subelements is being explored by Supergenics I LLC and Hyper Tech Research Inc. The number of subelements was increased to 919 (744 subelements plus 175 Cu filaments) by increasing the size at which restacking is carried out. The product exhibited no fabrication problems and was drawn down and tested at a wire diameter of 0.42 mm, where the subelements are 10 μm in diameter. Recently we increased the subelement number to 1387 (1248 subelements plus 139 Cu filaments), which gives a subelement size of 12 μm in 0.7 mm diameter wires. Heat treatment (HT) of different subelement restacks has been investigated, and the best results of critical current and stability are presented. The strain tolerance of the strands with 192 and 744 subelements was also tested, and the strand with fine subelement size showed a high intrinsic irreversible strain limit.

{\rm Nb}_{3}{\rm Sn}

Bi₂Sr₂CaCu₂O_8+x round wires are among the most promising high-temperature superconductor candidates for making high-field magnets that operate at fields above 20 T. Owing to the brittle nature of high-temperature superconductors, their electromechanical properties need to be studied and understood prior to magnet design and construction. The irreversible degradation of the critical current at high strains has been previously correlated to damage in the microstructure of the superconductor. The origin of the much smaller reversible strain dependence of the superconducting properties of Bi₂Sr₂CaCu₂ O_8+x wires has not yet been studied in detail. In this paper, we determine the cause of the reversible effect of strain on the pinning force and the critical current at temperatures ranging from 4 K to 65 K. Measurements were made as a function of tensile strain in magnetic fields up to 16 T. The irreversibility fields, determined from the macroscopic pinning force at various temperatures and strains, and the critical temperature of the Bi₂Sr₂CaCu₂O_8+x wire were found to be strain dependent. The reversible change in critical current and pinning force in Bi₂Sr₂CaCu₂O_8+x superconducting wires can be solely attributed to the dependence of the critical temperature on pressure.

Strands

We compare methods for estimating the residual resistivity ratio (RRR) of high-purity niobium and investigate the effects of using different functional models. RRR is typically defined as the ratio of the electrical resistances measured at 273 K (the ice point) and 4.2 K (the boiling point of helium at standard atmospheric pressure). However, pure niobium is superconducting below about 9.3 K, so the low-temperature resistance is defined as the normal-state (i.e., non-superconducting state) resistance extrapolated to 4.2 K and zero magnetic field. Thus, the estimated value of RRR depends significantly on the model used for extrapolation. We examine three models for extrapolation based on temperature versus resistance, two models for extrapolation based on magnetic field versus resistance, and a new model based on the Kohler relationship that can be applied to combined temperature and field data. We also investigate the possibility of re-defining RRR so that the quantity is not dependent on extrapolation.

Correlation Between Pressure Dependence of Critical Temperature and the Reversible Strain Effect on the Critical Current and Pinning Force in

The color error in images taken by digital cameras is evaluated with respect to its sensitivity to the image capture conditions. A parametric study was conducted to investigate the dependence of image color error on camera technology, illumination spectra, and lighting uniformity. The measurement conditions were selected to simulate the variation that might be expected in typical telemedicine situations. Substantial color errors were observed, depending on the measurement conditions. Several image post-processing methods were also investigated for their effectiveness in reducing the color errors. The results of this study quantify the level of color error that may occur in the digital camera image capture process, and provide guidance for improving the color accuracy through appropriate changes in that process and in post-processing.

\hbox{Bi}_{2}\hbox{Sr}_{2} \hbox{CaCu}_{2}\hbox{O}_{8 + x}

We present measurements demonstrating that electronic calibration units are stable enough to be used in place of mechanical vector-network-analyzer verification artifacts. This enables a new fully automated approach to verifying microwave vector-network-analyzer calibrations with a single computer-controlled electronic verification artifact. The new system presents verification results in easy-to-understand performance metrics that, unlike those derived from measurements of mechanical verification artifacts, are independent of the actual artifacts employed.