Surface Impedance Measurements Research Articles

Modeling Geomagnetically Induced Currents in the Alberta Power Network: Comparison and Validation Using Hall Probe Measurements During a Magnetic Storm

AbstractDuring space weather events, geomagnetic disturbances (GMDs) induce geoelectric fields which drive geomagnetically induced currents (GICs) through electrically‐grounded power transmission lines. Alberta, Canada—located near the auroral zone and thus prone to large GMDs—has a dense network of magnetometer stations and surface impedance measurements to better characterize the GMD and ground conductivity, respectively. GIC monitoring devices were recently installed at five substation transformer neutrals, providing a unique opportunity to compare data to modeled GICs. GICs are modeled across the >240 kV provincial power transmission network during a moderate GMD event on 24 April 2023. GIC monitoring devices measured larger neutral‐to‐ground currents than expected up to 117 Amps during peak storm time, providing unequivocal evidence linking network GICs with GMDs. The model performs reasonably well (correlation coefficients >0.5; performance parameter >0.15) at four of five substations, but generally underestimates peak GIC values (sometimes by a factor >2), suggesting that the present model underrepresents overall network risk. The model performs poorly at one of the five substations (correlation = 0.46; performance parameter = 0.10), the reasons for which may be due to simplifications and/or unknowns in network parameters. Despite these underestimates, during this GMD, the model predicts the largest GIC at substations located in the northeastern part of the province (240 kV) or around Edmonton (500 kV)—regions which have significant electrical and industrial infrastructure. Further refinement of the network model with transformer resistances, more line and earthing resistances, and/or including lower voltage levels is necessary to improve data fit.

A neural network-augmented two-microphone impedance tube method to estimate sound propagation characterisics

Two-microphone impedance tubes are a popular method to measure the normal incidence sound absorption coefficient. However, a single measurement with this method does not offer enough information to identify the sound propagation characteristics, i.e. the characteristic impedance and the wave number of absorbent samples. Instead, more elaborate measurement techniques that require more time, more equipment and enhanced user knowledge need to be applied. This contribution presents a technique combining neural networks and the two-microphone impedance tube method to estimate the propagation characteristics of a highly porous sample from a single two-microphone impedance tube measurement. The measured surface impedance serves as input to a U-net architecture, which preserves the high frequency resolution of the data and returns an estimate of the surface impedance mimicking an air cavity attached behind the sample. Then, the two cavity method is used to calculate the propagation characteristics from the measured and the networks predicted surface impedances. The presented approach gives precise estimates of the propagation characteristics of simulated highly porous samples and for two different, physically measured melamine foam samples, while drastically reducing the required resources and expenses. In this way, it provides an efficient way to estimate the propagation characteristics of absorbent samples.

Multi-mode analysis of surface losses in a superconducting microwave resonator in high magnetic fields.

This paper reports on a surface impedance measurement of a bulk metal niobium-titanium superconducting radio frequency (SRF) cavity in a magnetic field (up to 10T). A novel method is employed to decompose the surface resistance contributions of the cylindrical cavity end caps and walls using measurements from multiple TM cavity modes. The results confirm that quality factor degradation of a NbTi SRF cavity in a high magnetic field is primarily from surfaces perpendicular to the field (the cavity end caps), while parallel surface resistances (the walls) remain relatively constant. This result is encouraging for applications needing high Q cavities in strong magnetic fields, such as the Axion Dark Matter eXperiment because it opens the possibility of hybrid SRF cavity construction to replace conventional copper cavities.

Improvement of microwave characteristics for high-T c superconductor (YBCO) films by ion irradiation treatment

Measurements of the microwave surface impedance for high-temperature superconductor YBa2Cu3O7−x (YBCO) films irradiated by 3 MeV Au2+ ions at three irradiation doses (1010 cm−2, 1011 cm−2, and 1012 cm−2) were carried out at different temperatures (below the critical temperature of T с ≈ 90 К) and various rf current amplitudes by the use of the microstrip resonator technique. A noticeable decrease in the surface resistance and an enhancement of the linear response range at low temperatures were observed for moderately irradiated film exposed to an irradiation dose of 1011 cm−2, while at higher irradiation doses (1012 cm−2) an increase in the surface resistance and a decrease in the linear response range were observed for all temperatures below T c. These results are discussed in the framework of phenomenological theory for the microwave response of superconductors with nano-sized structural defects.

Measurements of Surface Impedance in MgB2 in DC Magnetic Fields: Insights in Flux-Flow Resistivity

We present the multifrequency measurements of the surface resistance of spark-plasma-sintered MgB2 performed through a dielectric loaded resonator operating at 16.5 and 26.7 GHz. By normally applying magnetic fields ≤1.2 T to the sample surface, we drove it in the mixed state. By means of data-rooted analysis, we found that the sample vortex dynamics could be fully described within a single-component approach. Pinning phenomena were present and characterized by a depinning frequency smaller than the measurement ones. The multiband nature of the superconductor emerged in the flux-flow resistivity, whose field dependence could be interpreted well within theoretical models. By exploiting them, the upper critical field was extracted in the low-temperature range, which exhibited a consistent temperature trend with the values obtained at the onset of the resistive transition near Tc, and was well in line with literature data on other polycrystalline samples.

Experimental investigation of the variance of ground impedance measurments in coastal environments

The purpose of this work is to understand the results obtained with the ANSI/ASA S1.18 standard for outdoor acoustic surface impedance measurements when applied to surfaces with characteristics at or outside the recommended use case. This study is part of a larger effort working to develop a numerical model for long-range atmospheric acoustic transmission loss over coastal areas. Previous studies were done to characterize the relationship between moisture content and effective flow resistivity of sandy shores by conducting acoustic surface impedance measurements following the procedure specified in the ANSI/ASA S1.18. While the standard specifies surface variation of less than 5 cm and at least four measurements per location, this work seeks to understand the consequences of surface variation in natural terrains on the obtained results. These consequences were evaluated by measuring the effective flow resistivity of multiple outdoor surfaces such as uniform grass, raked sandy shores, and undisturbed sandy shores. At every location, 16 measurements were performed in grid pattern. Subsets of the 16 measurements are used to perform an analysis of the variance of the measured flow resistivities collected at locations with different surface characteristics.

EBG waveguides for contactless surface impedance measurements

A method for the estimation of sheet impedance of thin sample which does not require a direct contact with the sample under test is proposed. The surface impedance is calculated through an inversion procedure exploiting the scattering parameters obtained through a waveguide measurement setup. An inversion procedure based on the representation of the waveguide-air-waveguide section as a π junction is employed. In order to prevent the field leakage from the air gap created for hosting the thin sheet, an EBG surface is introduced on the flange of the waveguide. It is shown that the introduction of the EBG surface remarkably improves the estimation of the surface impedance of the thin sheet with respect to the case without EBG.

Acoustic surface impedance of sandy shores

This work presents an effort to characterize the relationship between moisture content and acoustic surface impedance of sandy soils. This effort is part of a larger project seeking to model long-range acoustic transmission loss over coastal areas. Previous studies were done to measure moisture content along the shore and collect samples for bench-testing with an impedance tube. This work seeks to build on those experiments by performing in situ acoustic surface impedance measurements with an omnidirectional sound source and two microphone receivers. These measurements follow the protocol set forth by the ANSI/ASA S1.18 and are recorded at multiple distances from the water on the shore. At each measurement location, sand samples are collected for gravimetric moisture analysis. Atmospheric conditions and ambient noise level are also recorded in accordance with the standard. These results demonstrate the variation of sand surface impedance from the fully saturated swash zone to nominally dry sand.

Electronic States and Energy Dissipations of Vortex Core in Pure FeSe Single Crystals Investigated by Microwave Surface Impedance Measurements

In order to clarify electronic states and energy dissipations due to a motion of a vortex core in pure FeSe, which is a candidate superconductor possessing a super-clean core, we measured the microwave surface impedance of pure FeSe single crystals under finite magnetic fields. From the magnetic-field dependence of the flux-flow resistivity, we found that a barometer of electronic states inside the vortex core $\omega_{0}\tau_{\rm core}$ is $1\pm0.5$, suggesting that the vortex core of pure FeSe is in the moderately clean regime contrary to the expectation of the super-clean core. We also found that the mean-free path inside the vortex core is suppressed at the distance of the order of the core radius. Based on observed results and previous reports, we discussed possible origins of rather small $\omega_{0}\tau_{\rm core}$ value in terms of the multiple-bands nature of FeSe and additional mechanisms producing extra energy dissipations specific to the vortex core in motion.

A method based on a dual frequency resonator to estimate physical parameters of superconductors from surface impedance measurements in a magnetic field

High frequency applications of superconductors in a dc magnetic field rely on the estimate of the characteristic crossover frequency νc between low and high losses. Customarily, high sensitivity resonant techniques, intrinsically operating at discrete frequencies, are used to estimate νc. We exploit here a method based on a dual frequency resonator. We show that single-frequency evaluations of νc lead to heavy underestimations of the superconductor surface resistance. We describe a combined analytical and experimental approach that gives more accurate estimates for νc, and additionally allows to test the underlying physical model.

Pole identification method to extract the equivalent fluid characteristics of general sound-absorbing materials

A method is presented to characterize general sound-absorbing materials through a pole-based identification of the equivalent fluid. This is accomplished by 1) determining the extended equivalent fluid of the material sample through the transfer function method (TFM), 2) identification of the acoustic response of the material through the poles of the extended effective density and compressibility, and 3) build the effective density and compressibility from the poles associated to the local acoustic response. Real pole pairs describe a dissipative medium (or equivalently an over-damped resonating medium), which is the natural behavior of rigid-frame porous materials, while complex-conjugate pole pairs describe a locally-resonant medium typical of metamaterials. Complex-conjugate poles associated to elastic resonances of the sample are discarded. We test the method for several non-conventional porous materials. In general, a better fit to the measured surface impedance is obtained than with an acoustics-based identification to the Johnson-Champoux-Allard-Pride-Lafarge model (JCAPL), and the method appears also to be robust to errors of the TFM.

CQDs@NiO: An Efficient Tool for CH4 Sensing

A composite material based on carbon quantum dots (CQDs) and NiO was prepared and tested for methane sensing. The synthesis procedure is simple and foresees the preparation of the CQDs by citric acid pyrolysis and NiO by hydrothermal synthesis. A phase sonication and stirring procedure yielded the composite CQDs@NiO at different loads. The composites were characterized by X-ray diffraction, ultraviolet–visible light (UV–Vis) spectroscopy, SEM microscopy, energy-dispersive spectroscopy (EDS) mapping, and surface area, porosity, and impedance measurements. A gas sensor was built in-house and used to probe the response of the synthesized samples to CH4 detection, at constant environmental humidity. The CQDs@NiO at 1% weight load displayed excellent performances in terms of gas response both vs. temperature and vs. concentration, whereas higher loads resulted in CQD aggregation and diminished output. Response/recovery times of the 1%CQDs@NiO sample were good, as well as the selectivity and the stability over time and for variable environmental humidity. The estimated limit of detection was 0.1 ppm.

Multiband acoustic waveguides constructed by two-dimensional phononic crystals

By introducing a lattice translation of the unit cell of a phononic crystal, a general recipe to invert the acoustic Zak phase—i.e. the one-dimensional geometric phase at each phononic band—is provided and verified by surface impedance measurement. Multiband acoustic interface states are thus generated and may help the design of future acoustic waveguides. The propagation properties of these topological interface states are experimentally obtained and we believe that the methods to measure surface impedance and local sound pressure distributions inside phononic crystals developed here can further help experimental studies in acoustic metamaterials.

A correction method for the phase angle error of surface impedance measurements in the 2 kHz–150 kHz frequency range

The first electromagnetic interferences in the frequency range of 2–150 kHz occurred more than a decade ago, but key standards of electromagnetic compatibility still do not cover the whole area of this frequency range. Several important questions arose due to the standardization process. One of them relates to the mechanism of disturbances spreading on the network. To investigate this in depth, we measured the surface impedance of stranded conductors, while we faced the problem that these measurements cannot be done without knowing the frequency dependent phase error of the current sensor used in the setup. This paper presents a method developed here to determine this phase angle error. The biggest advantage of the method is its simplicity and efficiency, because it requires equipment usually available in laboratories. Although the method was developed to aid our surface impedance measurements, it can also be used in other cases, when knowing the frequency dependent phase error of a current sensor is crucially important.

Physics of vortex motion by means of microwave surface impedance measurements (Review article)

In this paper we present a short overview on the results that can be obtained through the study of vortex motion at high frequencies. The phenomenological force balance for isolated-like vortices shaken by microwave currents and subjected to viscous drag, pinning forces and thermal creep is recalled and physically presented. The derived vortex motion resistivity, together with the main vortex parameters (viscosity η and flux flow resistivity ρff, pinning constant kp, creep factor χ), is then commented. Sample measurements are reported to illustrate the main aspects of the involved physical models.

Intrinsic anisotropy and pinning anisotropy in nanostructured YBa2Cu3O7−δ from microwave measurements

Anisotropy is an intrinsic factor that dictates the magnetic properties of YBa2Cu3O7−δ and therefore has a great impact on many applications. Artificial pinning centres are often introduced in an attempt to mitigate its effect, resulting in less anisotropic electrical and magnetic properties. However, the nanoengineering of superconductors makes the quantification of the anisotropy itself uncertain: the intrinsic anisotropy due to the layered structure, quantified by the anisotropy factor γ, mixes with the additional anisotropy due to pinning. As a consequence, there is no consensus on the experimental anisotropy factor γ that can result in YBa2Cu3O7−δ when directional (twin planes, nanorods) or isotropic defects are present. We present measurements of the magnetic field and angular dependent surface impedance in very different nanostructured YBa2Cu3O7−δ films, grown by a chemical route and by pulsed laser deposition with different kind of defects (nanorods, twin planes, nanoparticles). We show that the surface impedance measurements are able to disentangle the intrinsic anisotropy from the directional pinning anisotropy, thanks to the possibility of extracting the true anisotropic flux–flow resistivity and by correctly exploiting the angular scaling. We find in all films that the intrinsic anisotropy γ = 5.3 ± 0.7. By contrast, the pinning anisotropy determines a much more complex, feature–rich and nonuniversal, sample–dependent angular landscape.

Critical fields of Nb3Sn prepared for superconducting cavities

Nb3Sn is currently the most promising material other than niobium for future superconducting radiofrequency cavities. Critical fields above 120 mT in pulsed operation and about 80 mT in CW have been achieved in cavity tests. This is large compared to the lower critical field as derived from the London penetration depth, extracted from low field surface impedance measurements. In this paper direct measurements of the London penetration depth from which the lower critical field and the superheating field are derived are presented. The field of first vortex penetration is measured under DC and RF fields. The combined results confirm that Nb3Sn cavities are indeed operated in a metastable state above the lower critical field but are currently limited to a critical field well below the superheating field.

Challenging microwave resonant measurement techniques for conducting material characterization

Dielectric loaded resonators, by virtue of their high sensitivity, are a de facto standard for the measurement of the microwave surface impedance Z of good conductors, including superconductors. However, new perspective applications of superconductors request measurements in increasingly extreme conditions: cryogenic and magnetic environments, often combined together. Careful evaluations of the dielectric resonator (DR) design are necessary to operate in these conditions. Moreover, the evaluation of the uncertainties and the issues related to an appropriate calibration become problematic. We present here a performance analysis of DRs designed for cryogenic temperature and high magnetic field operation and some sample experimental results on Z measurements on the most technologically interesting superconductors.

Three-dimensional Pulmonary Monitoring Using Focused Electrical Impedance Measurements.

Lung pathologies such as edema, atelectasis or pneumonia are potentially life threatening conditions. Especially in critically ill and mechanically ventilated patients, an early diagnosis and treatment is crucial to prevent an Acute Respiratory Distress Syndrome [1]. Thus, continuous monitoring tool for the lung condition available at the bedside would be highly appreciated. One concept for this is Electrical Impedance Tomography (EIT). In EIT, an electrode belt of typically 16 or 32 electrodes is attached at the body surface and multiple impedance measurements are performed. From this, the conductivity change inside the body is reconstructed in a two-dimensional image. In various studies, EIT proved to be a useful tool for quantifying recruitment maneuvers, the assessment of the ventilation homogeneity, the detection of lung edema or perfusion monitoring [2, 3, 4, 5]. Nevertheless, the main problem of EIT is the low spatial resolution (compared to CT) and the limitation to two dimensional images. In this paper, we try to address the latter issue: Instead of projecting conductivity changes onto a two-dimensional image, we adjust electrode positions to focus single tetrapolar measurements to specific, three-dimensional regions of interest. In earlier work, we defined guidelines to achieve this focusing [6, 7]. In this paper, we demonstrate in simulations and in a water tank experiment that applying these guidelines can help to detect pathologies in specific lung regions.

Heterogeneous oxidation of highly boron-doped diamond electrodes and its influence on the surface distribution of electrochemical activity

The electrochemical active surface area (EASA) of polycrystalline boron-doped diamond (BDD) electrodes is heterogeneous and can be affected by numerous factors. There is a strong need for proper consideration of BDD heterogeneity in order to improve this material's range of application in electrochemistry. Localized changes in surface termination due to the influence of oxidation agent result in increased surface resistance. The observed behavior of this characteristic feature varies among individual grains, depending on their crystallographic orientation. Still, there is not much information about this key factor in terms of its influence on the electrochemical response of BDD. In this study we compared two approaches towards BDD surface oxidation, namely: anodic polarization at potentiostatic and potentiodynamic conditions. The surface impedance measurements via Nanoscale Impedance Microscopy (NIM) allowed the confirmation of diversified propensity for the modification of surface termination in BDD. We showed that the NIM studies provide a deep understanding on the electrical characterization and variation of surface resistance in BDD electrodes. In order to evaluate the actual heterogeneity of electrochemical activity distribution, voltammetry, dynamic electrochemical impedance spectroscopy (DEIS) and scanning electrochemical microscopy (SECM) studies were performed. For each investigated electrode, departure from the Randles-Sevcik equation was observed, with its level depending on the surface heterogeneity and oxidation treatment, justifying the standardization of pre-treatment procedure and development of non-standard model for diffusion transport in proximity of BDD electrode.