Losses in a high-temperature superconductor exposed to AC and DCtransport currents and magnetic fields

Summary form only given, as follows. The development of high-temperature superconductors (HTSs) is for the moment intensive and it is likely that HTSs will be used in different electric devices in a few years. In some of these devices that conductor will be exposed to both AC and DC transport currents and magnetic fields. In the design of electric devices, the power loss is indeed one of the most important parameters. Therefore there is a need of models that predict the power loss in an HTS under application-like conditions. In this paper, the authors present a semi-empirical model including a transport current and an applied magnetic field with both AC and DC components. The semi-empirical model is mainly based on the critical state theory. The results of the model are compared and found to be in good agreement with experimental results obtained with a calorimetric experimental set-up. The power loss was measured on a multi-filamentary Bi-2223 tape as function of transport current and applied magnetic field at a fixed temperature and frequency     

Losses in a BSCCO/Ag tape carrying AC transport currents in ACmagnetic fields applied in different orientations

In most prospective electric power applications of high-temperature superconductors (HTSs), the conductor is wound in a coil configuration. For a coil of finite length, the magnetic field orientation is axial in the middle of the coil, while the field contains a substantial radial component at the coil ends. In a superconducting BSCCO/Ag tape, the AC losses depend strongly on the orientation of the magnetic field. In this study, we present experimental results of the AC losses in a multifilamentary silver-sheathed Bi-2223 HTSs tape, carrying alternating transport currents in externally applied alternating magnetic fields at different orientations, with respect to the face of the tape and perpendicular to the transport current. The AC losses were measured calorimetrically, at fixed temperature and frequency. The results are compared to semi-empirical models of the AC losses in HTSs tape. We present a more general model of the angular dependence of the AC losses     

Impact of DC components in transport current and external magnetic field on hysteresis losses in an HTS tape

The high-temperature superconductor (HTS) development is presently intensive, and one can expect that HTSs will be used in different electric devices in the future. In some of these devices, e.g., a magnetostrictive actuator, the conductor will be exposed to both ac and dc transport currents and external magnetic fields. One of the most important parameters in the design of electric devices is the power loss. It is, therefore, of importance to develop models that are able to predict the power loss in devices based on HTSs. In this paper, we present an experimental characterization of the hysteresis losses in an HTS tape exposed to ac or dc transport currents in combination with external magnetic fields. The experimental results are compared to some existing models based on the critical state model. The power loss was measured on a multifilamentary Bi-2223 tape at constant temperature (70 K) and frequency (50 Hz).     

Magneto-optical visualization of magnetic processes in superconducting strip with the application of magnetic fields and currents

The magneto-optical imaging (MOI) technique is widely employed to characterize the magnetic properties of superconductors and other magnetic materials because it can easily show the variation of flux density over a plane with relatively high resolution. In this paper, flux profiles visualized by the MOI technique will be presented for different experimental conditions. The sample investigated is a high-temperature superconducting single strip. Three cases are investigated: 1) application of external currents; 2) application of external magnetic fields; and 3) simultaneous application of external magnetic field and current. The behavior of complicated vortex penetration into the samples for different conditions will be discussed in detail. The magnetic flux profiles are calculated taking into account the sample thickness, as well as the interval between the sample and the MOI indicator film. The calculated and the experimental results are compared and discussed.     

Detection of DC and low-frequency AC magnetic fields using an all single-mode fibre magnetometer

A technique for the determination of weak DC and low-frequency AC magnetic fields using a single-mode-fibre magnetometer is described. The technique utilises the dependence between the AC responsitivity and bias field to detect small changes in the bias field. Detection sensitivities in the milli-gauss/metre region have been obtained using a bulk nickel magnetostrictive element with microgauss sensitivities predicted for certain types of metallic glass.     

A review of analytically determined electric fields and currents induced in the human body when exposed to 50-60-Hz electromagnetic fields

In this review paper, analytical methods are used to determine the electric field and current induced in the conducting human body when this is exposed to an electromagnetic field at extremely low frequencies (ELFs) or very low frequencies (VLFs). This is done by treating it as a parasitic antenna when this is modeled successively as a sphere, an ellipsoid, and a cylinder. Because the body is electrically very short at ELF and VLF, the axial current depends primarily on the length of the body. Comparison with the ellipsoidal shape shows that the induced current is virtually independent of the cross-sectional shape. It is concluded that the axial current induced in the cylinder is a good approximation of the current induced in an actual body with the same length and mean cross sectional area. References to persons standing on the earth and with the arms raised are given. The significance of the accurate knowledge of induced currents and fields for biomedical purposes is discussed.     

Major noise reduction in the eddy-current sensor for DC and low-frequency magnetic fields

A major improvement in the sensitivity of an eddy-current sensor is described. This was achieved as a result of internal noise reduction by a supplementary heat treatment of the ferromagnetic core. Its present sensitivity of 0.1 nT and its simple construction make this eddy-current magnetometer a real competitor with other similar devices, e.g., the flux-gate magnetometer.     

Monte Carlo calculation of noise transport in electric and magnetic fields

In computing noise transport by the Monte Carlo technique, electrons, their two velocity components and emission times, are generated in a random manner. The electron equations of motion are integrated numerically to determine subsequent position and velocity coordinates of the electrons. In the first phase of the computer program initial conditions are specified in order to fill the diode. In the second phase the electrons drift according to the self-consistent space-charge fields. Under steady-state conditions data for the number of electrons and the velocity components are recorded for several sections of the beam and for several unit time intervals. Fluctuations are then calculated from the average values to compute the various noise parameters for particular sections of the beam. The current fluctuations are smoothed due to the presence of the potential minimum, although the velocity fluctuations are increased as the beam is accelerated which accounts for large values of noise figure, commonly characteristic of crossed-field devices. The over-all growth for the slow space-charge wave is larger than that for the fast space-charge wave in traversing one complete cycloid.     

Analysis and modeling of a high-temperature superconducting floating resonant strip in waveguide

A two-gap floating resonant strip is used for characterization of the high-temperature superconductor, YBa₂Cu₃O_7?δ. The method has the advantages of simplicity, no electrical contact, operation at various resonant frequencies, and of requiring only a small sample. An analysis was devised that allows for the accurate design of the strip dimensions to produce a desired resonant frequency. Experimental measurements in X and Kuband (8–18 GHz) agree well with the calculations. The sensitivities of the circuit to positional errors and size variations of the resonant strip were investigated. The surface resistance was measured and compared favorably with the theory.     

Numerical study of currents and fields in a photoconductive detector

A numerical study of the current, field, and carrier density distributions within a photoconductive detector is presented. The photodetector, an interdigitated Schottky barrier diode, is made with metallic fingers of alternating voltage bias on a thin semiconductor layer grown on a transparent dielectric substrate. The Poisson and continuity equations for electrons and holes are treated in two dimensions. A modified successive line overrelaxation method, faster than the capacitance matrix method, is developed as the Poisson solver. A simple alternative to the Scharfetter-Gummel treatment of current density is also introduced. We investigate steady-state cases with and without optical illumination, and transient responses to picosecond optical pulses. The steady-state current Shows near saturation with increasing voltage, as observed experimentally. The calculated typical response of a silicon detector to a picosecond optical pulse is a current pulse lasting on the order of 10 ps.     

Experimental observation and modified driving method to improve the high-temperature misfiring in AC PDP

Misfiring is often observed during the high-temperature quality assurance test of plasma display panels. This limits the productivity of the plasma display panel industry. In this paper, experimental observations on the misfiring at high-panel temperatures have been performed through time-dependent discharge light output and static margin measurement. For the high-temperature condition, the firing voltage increment is found in both surface and facing discharges. This in turn increases time lag in address discharge, and results in increment of misfiring probability. In order to reduce this kind of misfiring, a modified method that applies automatically to a different slope of ramp erase pulse on the sustain electrode according to temperature variation is proposed. The experimental results show that controlling the slope of the ramp erase pulse is quite effective for compensating the temperature-dependent variation of reset and address discharge.     

Simple approximations of the DC flux influence on the core losspower electronic ferrites and their use in design of magnetic components

The effect of DC flux on the core loss is examined for the practical range of power and frequency. Relevant core loss equations are derived and applied to an optimization algorithm to determine the minimum core loss at a given ratio of s (DC flux density to AC peak flux density). It has been found that the curves of hysteresis loss density versus the ratio of s exhibit a peak at a critical ratio. Below or above this critical ratio, the loss density decreases drastically. On the other hand, the curves of eddy-current loss density versus the ratio of s exhibits a minimum point at a critical ratio. Below or above this critical ratio, the loss density increases gradually     

Motion of a charged particle in electric and magnetic fields

The motion of a charged particle in time-varying uniform electric and magnetic fields has been determined exactly by writing the Lorentz force equation in a matrix form. The general solution is obtained by solving the ordinary first-order linear differential equation.     

Compact modeling of the noise of a bipolar transistor under DC and AC current crowding conditions

The effect of current crowding on dc, on ac, and in particular on the noise characteristic of bipolar transistors, is studied. An equivalent circuit able to model these effects is presented. General formulations to calculate current crowding in arbitrary geometries are derived. Both rectangular and circular geometries are discussed in detail.     

On using a sense wire to quantitate the magnetic flux leakagethrough an aperture in an electromagnetic shield

A formulation is developed for determining the sensitivity of positioning a sense wire behind an aperture in an electromagnetic shield to quantitate the flux leakage through the aperture. Both hardened and unhardened circular apertures are considered. Comparisons with measured data are made to verify the analysis     

The analysis of the fault currents according to core saturation andfault angles in an inductive high-Tc superconducting fault currentlimiter

In this paper, we investigate the fault currents in an inductive high-Tc superconducting fault current limiter (SFCL). The currents can cause serious damage to the reliability and stability of the power system. To analyze the transient fault characteristics of the SFCL, we fabricated an inductive high-Tc SFCL and tested it under different fault conditions. To simulate a fault condition, a fault angle controller was connected to a load. As the firing angle of the triac in the fault angle controller was controlled, various angles of the fault instant can be selected. An important parameter for the design and the fabrication of the SFCL is the reduction of the fault currents. If abnormally high currents due to low impedance of SFCL do not occur in the network with SFCL, the currents flowing under fault conditions can be limited to a desired value within one cycle. The fault current reduction depends on saturation, normal zone propagation velocity, turns ratio, and the fault angle (the instant of the fault occurrence within a cycle)     

The temperature dependence of the DC base and collector currents in silicon bipolar transistors

Detailed measurements have been made of the base and collector-current characteristics of both n-p-n and p-n-p silicon transistors as a function of temperature. The collector current shows ideal behavior over the temperature range -60 to 150°C in thatI_{C} infin exp (eV_{BE}/kT). On the other hand, the base current is nonideal:I_{B} infin exp (eV_{BE}/nkT), wheren > 1.0. The nonideality of IBis the main source of the temperature dependence ofh_{FE} = I_{C}/I_{B}. There is no evidence for bandgap narrowing in the devices we have investigated. The temperature dependence of the collector current is given byI_{C} infin T_{m} exp (-e E_{g0}/kT) exp (eV_{BE}/kT), wherem = 1.4or 1.7 for n-p-n or p-n-p devices, respectively.E_{g0} = 1.19 pm 0.01eV. This result is consistent with the findings of others. The base current is a complicated function of temperature due to the presence of nonideal components.     

Modeling of the internal fields distribution in human inner hearing system exposed to 900 and 1800 MHz

This paper investigates the internal electric and magnetic field distribution and the specific absorption rate (SAR) values in a magnetic resonance imaging-based model of the inner hearing system exposed to 900 and 1800 MHz. The internal fields distributions were calculated using the Finite Integration Technique. The estimation of the field values was evaluated along lines passing through that target organ, specifically from the vestibular to the cochlear region and from the apex to the base of the cochlea. The specific findings are: 1) higher internal fields strength and SAR value in the vestibular region rather than in the auditory region, especially for the inner ear closer to the external source; 2) higher internal fields strength in the basal and apical region of the cochlea than in the middle one; 3) local differences in the internal fields distribution and SAR value, comparing the head models including or not the inner auditory system model; 4) results' variability evaluated by changing the head-source mutual position and the dielectric properties of the inner hearing system.     

Phase states and magnetic structure of superconducting lead inclusions in a narrow-gap PbTe semiconducting host

Samples of the narrow-gap semiconductor PbTe with n-and p-type conductivities are investigated using a SQUID magnetometer in the temperature range 1.7–20 K and in magnetic fields up to 1 kOe. It is shown that there are microscopic inclusions of superconducting lead with minimum dimensions ～1300 Å containing ～(1?5)×10¹⁸ lead atoms/cm³ in the lead telluride matrix and that the samples undergo a phase transition that is characteristic of type-II superconductors.