Resistance Ratio Measurements Research Articles

Image Analysis Capabilities and Methodologies of Nb3Sn Rutherford Cables

The unique Rutherford cabling facility at the Berkeley Center for Magnet Technology, LBNL, has been leading the production of a range of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn cables for a variety of magnet projects domestically and internationally. Cable fabrication is a critical step in accelerator magnet production: to ensure tight dimensional control of the multi-turn coils, fine tolerances must be kept on the cable width, thickness, keystone angle [1] with minimal degradation to the conductor. In addition to these dimensional measurements, we implemented an in-line image acquisition system that can monitor for critical defects such as cross-overs, as well as track the extent of strand deformation at the cable edges. At LBNL, we have collected a large dataset to establish the association between the cable edge facet dimension and the cross-sectional subelement damages. We thus can use image analysis on the facet as an integral and critical quick turn-around-time quality control monitor. Although such measurements are not direct and require a baseline for a given conductor and cable geometry combination, they are non-destructive. They can be measured across the entire length of the cable, which contrasts with critical current or residual resistance ratio measurements, which are only made at the point and tail of the cable and require a lengthy heat-treatment. Moreover, the high frequency of image acquisition coupled with Fast Fourier Transform analysis can give us insights into the key components of the cabling machine as they tend to produce periodic variations within the cable. Such analysis can help find potential faults and inform maintenance planning. This work describes our setup of in-line imaging of Rutherford cables during manufacture, the subsequent image analysis, and data processing. Several case studies illustrate typical usage of the system and lessons learned.

Application of Statistical Tools to Optimize a Dual Source Electrical High Dc Resistance Bridge

At the Istituto Nazionale di Ricerca Metrologica (INRIM), a commercial dual source high resistance bridge has been optimized by means of the application of statistical tools and of the analysis of measurements distributions. These tools help to achieve the best precision for resistance ratio measurements in the range 10 TΩ ÷ 100 TΩ. A measurement procedure consisting of multiple steps, in which at each one the value of the resistor under calibration is updated and has been considered the best one. With this procedure, at the third step, the lowest standard deviation of the mean and the measurements distribution approximately normal are obtained regardless of the settle time, of the resistance ratio, of the measurement voltages and of the resistors under comparison. This comes from the achievement of the white noise regime and from a bridge balance close to the ideal. This measurement procedure therefore allows also to achieve the lowest measurement uncertainty due to the minimization of the type A uncertainty. The Allan variance and the power spectral density were used to identify the white noise analyzing the detector readings. Strict triangulation rules were also established and applied to validate both the measurement process and the chosen model to extrapolate the values of the standard resistors at low voltages.

Application of Copper Thermal Spraying for Electrical Joints between Superconducting Nb3Sn Cables.

This manuscript reports on the application of copper thermal spraying in the manufacturing process of an electrical connection between Nb3Sn cables for superconducting magnets of fusion reactors. The joint is realized through diffusion bonding of the sprayed coating of the two cables. The main requirement for such a connection is its electrical resistance, which must be below 1 nΩ at B = 8 T, I = 63.3 kA and T = 4.5 K. Micrographs of the joint prototype were taken to relate the joint resistance with its microstructure and to provide feedback on the manufacturing process. Optical microscopy (OM) was used to evaluate the grain size of the coating, presence of oxide phases and to analyze the jointed surfaces. Scanning electron microscopy (SEM) and, in particular, energy-dispersive X-ray spectroscopy (EDX) were used to confirm the elemental composition of specimens extracted from the prototype. It is shown that the copper coating has an oxide concentration of 40%. Despite this, the resistance of the prototype is 0.48 nΩ in operating conditions, as the oxides are in globular form. The contact ratio between the jointed surfaces is about 95%. In addition, residual resistivity ratio (RRR) measurements were carried out to quantify the electrical quality of the Cu coating.

RRR measurements and tensile tests of high purity large grain ingot niobium

Residual resistivity ratio (RRR) measurements and tensile tests were performed on samples taken from seven high purity large grain niobium ingots produced by CBMM. Test specimens were taken from the top, middle and bottom regions of each ingot. The RRR measurements were performed with a GM cryocooler system and more than 60% of the samples exceeded RRR values of 250. Room temperature tensile tests were performed on samples from the same regions. 17 samples were machined according to ASTM standard test methods for tension testing of metallic material. Tests were performed with an Instron tensile/compression testing machine, and various mechanical properties such as Young’s modulus, 0.2% proof stress, ultimate tensile strength and strain at fracture were measured. For all the samples, the fracture strain exceeds 35% while the ultimate tensile stress is close to 100 MPa. Contents of impurity (N, O, C, H, Ta) of each region of the seven ingots were also measured. These data provide insights for a possible correlation among RRR values, mechanical properties and impurity contents of niobium ingots.

Effect of bio-cementation on geophysical and cone penetration measurements in sands

Microbially induced calcite precipitation (MICP) is a potentially environmentally conscious ground improvement method that can improve the engineering properties of granular soils through the precipitation of calcite. In this study, an experiment involving two 1.7 m diameter tank specimens was completed to investigate the effect of bio-cementation on cone penetrometer and geophysical measurements in sands. Following nonuniform bio-cementation treatments, specimens achieved calcite contents ranging from 0.5% to 5.3% by mass, shear wave velocity (Vs) values between 131 and 967 m/s, and mid-depth cone penetration resistances (qc) ranging between 3.6 and 32.1 MPa. At calcite contents exceeding 5.0%, qcand Vsimprovements were as high as 527% and 686%, respectively. Although cone penetration resistance, sleeve friction, and friction ratio measurements exhibited limited sensitivity to bio-cementation at calcite contents of less than 3.0%, Vsmeasurements successfully detected bio-cementation at calcite contents near 1.0%. When qcand Vsmeasurements were compared at similar locations, increases in an empirical parameter (KG) enabled improved detection of bio-cementation at calcite contents near 0.5%. Large increases in normalized tip resistances (Qtn) and small decreases in normalized friction ratios (Fr) with increasing bio-cementation resulted in cemented materials plotting near and within the gravelly sand and sand-like dilative soil behavioral type regions using two soil behavior type (SBT) charts.

A PXI-Based Calibration System for Low-Value AC Resistors

This paper presents a sampling measurement system designed to compare ac standard resistors within a range from 1 $\text{m}\Omega $ to 10 $\Omega $ at frequencies ranging from 40 Hz to 10 kHz and for measurement currents up to 10 A. The system allows simultaneous measurement of both components of impedance of the calibrated resistor relative to the reference resistor. Built on the basis of commercial PXI modules, complemented by modules developed by the authors, the system stands out relatively simple structure and fully automatic operation. In order to reduce the impact of parasitic impedances on the comparison accuracy, a completely automated interchange method and the original way of connecting a digitizer to the compared impedances were applied. The consistency of the measured ac resistance ratios from 1:1 to 1:10 is better than 1 $\mu \Omega /\Omega $ and phase difference better than 6 $\mu $ rad, over the entire frequency range. The results of several comparisons and the analysis of their uncertainty show that for the comparisons of resistors with a resistance ratio close to one, the relative uncertainty of the resistance ratio measurement is 2.3 $\mu \Omega /\Omega $ ( $k=2$ ) over the entire frequency range. The uncertainty of the difference between the time constants of the compared resistors is at the level of 0.2 ns ( $k=2$ ) at 1 kHz.

New Generation of Low-Frequency Current Comparators Operated at Room Temperature

This paper details developments in the use of room temperature current comparators operating with low-frequency ( $\Omega $ to quantum Hall resistance (QHR) ratio measurements are demonstrated at frequencies below 0.2 Hz and with relative accuracies of 1 part in $10^{8}$ or better. Resistance bridges based on this technology offer a lower cost, more robust alternative to those using cryogenic current comparators, and the results presented here indicate that the achievable uncertainties should be attractive for many applications. The combination of such a bridge with a compact next-generation QHR reference is our ultimate aim, and we include some first measurements on graphene samples with low-frequency ac.

Niobium superconducting rf cavity fabrication by electrohydraulic forming

Superconducting rf (SRF) cavities are traditionally fabricated from superconducting material sheets or made of copper coated with superconducting material, followed by trim machining and electron-beam welding. An alternative technique to traditional shaping methods, such as deep-drawing and spinning, is electrohydraulic forming (EHF). In EHF, half-cells are obtained through ultrahigh-speed deformation of blank sheets, using shockwaves induced in water by a pulsed electrical discharge. With respect to traditional methods, such a highly dynamic process can yield interesting results in terms of effectiveness, repeatability, final shape precision, higher formability, and reduced springback. In this paper, the first results of EHF on high purity niobium are presented and discussed. The simulations performed in order to master the multiphysics phenomena of EHF and to adjust its process parameters are presented. The microstructures of niobium half-cells produced by EHF and by spinning have been compared in terms of damage created in the material during the forming operation. The damage was assessed through hardness measurements, residual resistivity ratio (RRR) measurements, and electron backscattered diffraction analyses. It was found that EHF does not worsen the damage of the material during forming and instead, some areas of the half-cell have shown lower damage compared to spinning. Moreover, EHF is particularly advantageous to reduce the forming time, preserve roughness, and to meet the final required shape accuracy.

The Verification of the Resistive Voltage Dividers by a Resistance Ratio Measurement Method

Рассмотрена и проанализирована проблематика поверки делителей напряжения. Проанализированы основные метрологические характеристики и методы определения погрешностей резистивных делителей напряжения. Одной из основных проблем в проведении поверки делителей напряжения является необходимость использования высокоточных эталонных средств измерений. Разработана методика автономной поверки делителей напряжения, в которой реализован метод измерения отношения сопротивлений делителя. Методика позволяет определять погрешности резистивных делителей напряжения без использования прецизионных средств измерительной техники, в частности, без эталонных мер электрического сопротивления.

Bulge testing of copper and niobium tubes for hydroformed RF cavities

The heat treatment, tensile testing, and bulge testing of Cu and Nb tubes has been carried out to gain experience for the subsequent hydroforming of Nb tube into seamless superconducting radio frequency (SRF) cavities for high energy particle acceleration. In the experimental part of the study samples removed from representative tubes were prepared for heat treatment, tensile testing, residual resistance ratio measurement, and orientation imaging electron microscopy (OIM). After being optimally heat treated Cu and Nb tubes were subjected to hydraulic bulge testing and the results analyzed. In the final part of the study finite-element models (FEM) incorporating constitutive (stress–strain) relationships analytically derived from the tensile and bulge tests, respectively, were used to replicate the bulge test. As expected, agreement was obtained between the experimental bulge parameters and the FEM model based on the bulge-derived constitutive relationship. Not so for the FEM model based on tensile-test data. It is concluded that a constitutive relationship based on bulge testing is necessary to predict a material's performance under hydraulic deformation.

An HF coaxial bridge for measuring impedance ratios up to 1 MHz

A four-terminal pair coaxial ac bridge developed for calibrating both resistance and capacitance ratios and working in the frequency range from 100 kHz up to 1 MHz is described. A reference inductive voltage divider (IVD) makes it possible to calibrate ratios 1:1 and 10:1 with uncertainty of a few parts in 105. The IVD is calibrated by means of a series–parallel capacitance device (SPCD). Use of the same ac bridge with minimal changes for calibrating the SPCD, IVD and unknown impedances simplifies the whole calibration process. The bridge balance conditions are fulfilled with simple capacitance and resistance decades and by injecting voltage supplied from the auxiliary direct digital synthesizer. Bridge performance was checked on the basis of resistance ratio measurements and also capacitance ratio measurements.

Critical current density and stability of Tube Type Nb3Sn conductors

The critical current densities (Jc) and stabilities of Tube Type Nb3Sn conductors have been measured. The strands had superconducting subelement counts ranging from 192 to 744, and flat-to-flat filament sizes (for 0.7mm OD wire) of from 35μm down to 15μm. These Tube Type conductors had a very simple structure: prior to heat treatment the filaments consist of a Sn core surrounded by a thin Cu tube, itself surrounded by a Nb or Nb alloy tube. Eight different strand types were investigated using various techniques including SEM, residual resistance ratio (RRR), transport Jc, and stability measurement. Most strands were studied at 0.7mm OD, with one representative at 0.42mm. The transport measurements were made at 4.2K in fields up to 14T. Numerous heat treatment schedules were investigated, with reaction temperatures ranging from 615°C to 650°C, and times ranging from 36–500h. The highest Jcs were seen for the lowest reaction temperatures, with 12 T transport Jc values as high as 2450 A/mm2 observed. The RRRs were lower for longer time and higher temperature reactions and ranged from 4 to 180. Strand stability was a strong function of the effective filament diameter, deff, and RRR. The most stable strands showed stability currents, Js, of 8700A/mm2 and 15,300A/mm2 for 0.7mm OD and 0.42mm OD conductors, respectively.

Settling Behavior of the Bridge Voltage in Resistance Ratio Measurements With Cryogenic Current Comparators

A long settling time of the bridge voltage in resistance comparison setups with cryogenic current comparators might become crucial with respect to the type-A uncertainty, because it effectively limits the frequency of current reversals. Using a new bridge voltage detector with considerably increased bandwidth, we were able to trace the transient distortions caused by current reversal. Significant influence of the cabling is observed. Comparing a 12.9-kΩ quantum Hall resistor and a 100-Ω standard resistor, we achieved an expanded type-A uncertainty of about three parts in 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> after 24 min of measurement for current reversal frequencies down to 100 mHz.

Automatic Linearity Calibration in a Resistance Thermometry Bridge

The paper describes a new system that automatically calibrates the linearity of a resistance thermometry bridge or readout. A thermometry bridge, like all measuring and test equipment, must be regularly tested to ensure it is operating properly and accurately. Previously, this could only be done using special equipment and time-consuming procedures. The new technique described in the paper facilitates more frequent calibration of a resistance thermometry bridge. The system is incorporated into the design of the bridge and thus requires no extra equipment. Calibration, once initiated, is performed automatically by the bridge and is relatively fast. Tests were performed to evaluate the effectiveness of the automatic linearity calibration method. The uncertainty analysis and test results presented in the paper indicate that the technique is capable of quantifying resistance ratio measurement error as small as 2 × 10−8 and is effective at identifying a wide variety of failures when they occur.

Improvement of Resistance Ratio Bridge Using Cryogenic Current Comparator by Operating with Low-Noise Voltage Amplifier Using Superconducting Quantum Interference Device for Low Resistance Measurements

We improved a resistance ratio bridge using a cryogenic current comparator (CCC bridge) for low-resistance measurement by operated with a laboratory-developed low-noise voltage amplifier using a superconducting quantum interference device (SQUID voltage amplifier). The combined uncertainty of the resistance ratio measurement of 10 mΩ/1 Ω was evaluated to be about 0.03 ppm.

Residual Resistivity Ratio (RRR) Measurements of LHC Superconducting NbTi Cable Strands

The Rutherford-type superconducting NbTi cables of the LHC accelerator are currently manufactured by six industrial companies. As a part of the acceptance tests, the Residual Resistivity Ratio (RRR) of superconducting strands is systematically measured on virgin strands to qualify the strands before cabling and on extracted strands to qualify the cables and to check the final heat treatment (controlled oxidation to control interstrand resistance). More than 12000 samples of virgin and extracted strands have been measured during last five years. Results show good correlation with the measurements done by the companies and reflect well the technological process of cable production (strand annealing, cabling, cable heat treatment). This paper presents a description of the RRR-test station and the measurement procedure, the summary of the results over all suppliers and finally the correlation between RRR-values of the cables and the magnets

Unexpected effect of field angle in magnetoresistance measurements of high-purity Nb

We report on unexpected field-angle dependence of magnetoresistance measurements of commercial, high-purity Nb discovered during our study of residual resistivity ratio (RRR) measurements. The RRR value indicates the purity and the low-temperature thermal conductivity of the Nb and is used as a specification for superconducting radio-frequency cavities. The RRR is typically defined as the ratio of the electrical resistivities measured at 273 K and 4.2 K. One way to obtain the normal-state resistivity at 4.2 K is to measure the resistivity versus magnetic field at 4.2 K and extrapolate to zero field. The field-angle dependence was measured when a specimen was rotated while the field was transverse to the specimen current. The resistance changed by about 10% as the angle varied at 8 T and the local maxima and minima of the resistance were separated by about 30/spl deg/. This was observed on three polycrystalline Nb bar specimens with nominal dimensions of 64 mm /spl times/ 6.7 mm /spl times/ 3.3 mm and voltage taps separated by 30 mm. A similar field-angle dependence was obtained on a polycrystalline Cu wire specimen. This phenomenon has implications for Nb RRR measurements and interlaboratory comparisons.

Mechanical Properties of the CMS Conductor

CMS (Compact Muon Solenoid) is a general-purpose detector designed to run at the highest luminosity at the CERN Large Hadron Collider (LHC). Its distinctive features include a 4 T superconducting solenoid with 6 m diameter by 12.5 m long free bore, enclosed inside a 10000 ton return yoke. The magnetic field is achieved by a 4-layer superconducting solenoid made of a high purity aluminum (HPA) stabilized Rutherford type superconductor. The magnet is operated at 4.5 K, with a nominal current of 20 kA, for a total stored magnetic energy of 2.7 GJ. Due to the high magnetic forces at nominal field inside the winding pack, the structural component is the conductor itself to get a self-supporting winding structure. The mechanical reinforcement is made from aluminum alloy directly welded to the superconductor by electron beam (EB) welding technology before the winding operation. The external support cylinders also take part to the mechanical integrity. At each step of fabrication of the CMS conductor, the mechanical properties of the components and bonding between them are measured by destructive testing on short samples, in complement of continuous monitoring during production. This paper presents the results of the superconducting cable to pure aluminum shear testing, the tensile testing of the EN AW 6082 aluminum reinforcement, the insert to reinforcement shear testing, and the tensile testing of the full conductor before and after heat treatment induced during coil curing. Possible influence of the EB welding on the mechanical properties of the final conductor is investigated. Residual resistivity ratio (RRR) measurements of the HPA stabilizer are presented. Mechanical properties and equivalent RRR of the CMS conductor are presented for comparison with conductors of other geometry.

Cryogenic current comparator bridge for quantum Hall resistance ratio measurements

A cryogenic current comparator (CCC) bridge for quantum Hall resistance ratio measurement has been constructed. Compared to previous CCC systems at the National Physical Laboratory, UK, the new system differs in two aspects. Firstly, it uses a type-II CCC, i.e. the ratio windings are on the outside and the detection winding is on the inside. This has the advantage that the CCC can be made very compact and fits into a standard storage Dewar. Secondly, the bridge electronics uses a digital servo and the SQUID feedback loop is disabled during current reversals. The overall relative measurement uncertainty of the system is of the order of 10−9.

Optical spectroscopy diagnostics and thin film deposition of laser ablated rare earth–Ni 2B 2C plasma plumes

We describe optical spectroscopy diagnostics of UV laser ablated rare earth–Ni 2–B 2–C superconducting targets. In the light of this characterization, we have optimized the parameters for high-quality thin film deposition of borocarbide compounds. Our measurements have evidenced relevant differences in the flow velocity as well as in the spatial divergence of different plasma components. This is related to the influence of the different masses of the atoms of the multicomponent target during plume expansion. These results have led us to identify the target-to-substrate distance and substrate temperature as the critical parameters for the deposition of high-quality thin films, as clearly evidenced by residual resistivity ratio measurements.