Influence of the measuring-circuit configuration on the time-dependent flux-flow voltage

The instantaneous voltage generated by multiquantum flux tubes moving through a superconducting specimen depends upon both the instantaneous flux-flow velocity and the spatial configuration of the measuring circuit. For the case of two different voltage lead arrangements, for the first time the different temporal voltage profiles along constricted type I superconducting films have been recorded, using a highly sensitive signal-averaging technique. All experimental results were in good agreement with theoretical model considerations.     

Time-dependent voltage generated by flux motion across a superconducting strip

A brief review is given of the theory predicting that the instantaneous voltage generated by single-quantum vortices or multiquanta flux tubes moving through a superconducting specimen depends upon both the specimen shape and the spatial configuration of the measuring circuit. This theory is then applied to obtain the voltage pulses expected from two different measuring circuits attached to a superconducting strip. The time-dependent voltage is predicted to be proportional not only to the vortex or flux-tube velocity but also to a geometry-dependent function of the position of the vortex or flux tube relative to the measuring circuit.Operated for the U.S. Department of Energy by Iowa State University under contract No. W-7405-Eng-82. This research was supported by the Director for Energy Research, Office of Basic Energy Sciences, WPAS-KC-02-02-03, and by the NATO Research Grants Program.     

Combined time-resolved magnetooptical and electrical flux detection in thin-film superconductors

The dynamic behavior of the current-induced dissipative flux-flow state in a thin-film type I superconductor was studied by simultaneous stroboscopic magnetooptical flux detection and direct recording of the time-resolved flux-flow voltage. Employing high-resolution magnetooptical flux detection with a high-speed stroboscope, it was possible to visualize globally individual multiquantum flux tubes during their rapid motion across the superconducting Pb film, yielding spatial and temporal resolution of better than 1 μm and 0.1 μsec, respectively. Simultaneously, the temporal structure of the flux-flow voltage was recorded using a highly sensitive signal-averaging procedure, thereby yielding a voltage resolution of about 30 nV at a time resolution of 10 nsec (corresponding to a recording bandwidth of 25 MHz). The recorded temporal voltage structures agreed well with the voltages expected from the velocity profiles of all flux tubes existing simultaneously obtained from the magnetooptical data. The experiments are the first to demonstrate full agreement between both independent flux-detection measurements, clearly confirming the existing theory.     

Peculiarities of transport current penetration in a composite superconductor taking into account different models of flux creep

The flux-creep diffusion of transport current in superconducting composite slab and cylinder based on a hard superconductor is investigated theoretically. Power and exponential current-voltage characteristics were used to define the electric field in the flux-creep regime. Using scaling solutions, the Maxwell equations describing transport current penetration in a one-dimensional superconducting composite are solved analytically. A proposed analysis indicates the existence of special macroscopic distribution of electromagnetic field in superconductor during the flux creep. It is shown that the flux-creep electromagnetic field cannot promptly propagate into the composite and penetrates at the finite rate like in the flux-flow regime. Therefore, a moving current boundary exists in the superconducting composite. It separates the region where the current flows from free current region. Moreover, in the flux creep specific conditions are fulfilled at moving boundary in contrast to the flux-flow regime. According to these conditions the electric and magnetic field induced by current charging smoothly approach to its undisturbed values. They are independent of any parameters of current-voltage characteristics and appear due to zero value of the differential resistivity of the composite at moving boundary. To confirm these results, the numerical calculation based on more general model is executed. The equations describing the violation of stable distribution of fully penetrated current are defined.     

Correlation studies on flux flow noise of polycrystalline niobium and vanadium foils

Using two movable contact pairs, cross-correlation functions of flux flow noise from polycrystalline niobium and vanadium foils have been measured. The results are compared with theoretical predictions derived from Clem's theory for the voltage associated with vortex motion across superconductors. Two cases are considered: (a) shot noise of randomly distributed flux entities moving with constant velocity and (b) noise due to locally generated random fluctuations of vortex velocity. There is strong evidence that the observed flux flow noise is produced by local flux flow fluctuations at pinning sites.Supported by Deutsche Forschungsgemeinschaft.     

Flux flow velocity in superconducting films by measurement of passage time

The velocity of flux tubes in thin superconducting films has been determined by measuring the time for their passing across the film. A legion of flux tubes has been pulsed in on the left side of a film carrying a current large enough to break the pinning. The arrival of the flux tubes on the right side is observed with a pick-up coil. Flow velocities of 3 to 50×10⁴ cm/sec have been observed with current densities up to 10⁵ A/cm².Supported in part by the National Science Foundation, in part by the Cryogenics Center through a grant from the DOD Themis program.     

Measurement of rotating flux in silicon iron laminations

A method of calculating the magnitude and direction of the instantaneous flux density in a thin lamination of magnetic material is described. The technique was demonstrated by measuring the rotating flux in samples of mild steel, grain-oriented silicon iron, and at a point in the T-joint of a three-phase, three-limbed transformer core. The flux in the mild steel sample was found to rotate at a constant angular velocity and to be constant in magnitude. In the silicon iron sample, and at a point in the T-joint, the flux density varied both in angular velocity and magnitude. A large 150 Hz component of rotational flux was found to cause high localized power loss in the T-joint. The power loss was measured using the "initial rate of rise of temperature method" and was compared with the predicted rotational hysteresis loss.     

基于引信炮口磁探测的单线圈被动测速原理

现有的炮口实时测速都是通过改造发射系统和火控系统完成的,存在成本高,改造技术复杂和存在交联电路等诸多弊端.针对这些问题,提出了一种新的弹上被动测速方法.在分析弹丸运动、磁感应强度变化和单线圈传感器输出信号关系的基础上,提出通过捕获弹丸开始加速和出炮口时线圈输出的两个信号,来实现被动测速的原理,阐述了单线圈被动测速需要考虑的线圈设计、测试误差分析、盲区问题、信号处理以及装定对测速的影响等工程技术问题,列举了单线圈测速的若干优点,指出了需要进一步探讨和研究的问题.     

On the orientation of the moving flux line lattice in a superconducting film

The experimental counter-evidence is presented for Schmid and Hauger's theory which predicts the orientation of the moving flux line lattice relative to the direction of its flow. The pinning current and the current-versus-voltage characteristics of the flux flow state, together with the interference effects of the moving flux line lattice of indium films, are measured under two different experimental conditions, where either externally regulated dc voltage or current is supplied to the specimen. No difference is found between the two conditions. The theoretical arguments which predict a different orientation according to the conditions are no longer valid.     

Flux-flow instability in a type II superconductor

The stability of the flux-flow state in a superconducting slab is studied. The flux-flow kinematics is based on the Boltzmann equation. To investigate the interaction between the flux lines, the relaxation function is derived. The instability of the flux-flow state is associated with the growth of the autocorrelation function at the characteristic frequency of the flux-line collective mode. Thus the upper critical current value is obtained.     

Analysis of the current-induced breakdown of superconductivity

Recently, the breakdown of superconductivity has been investigated in thin-film samples with extreme temporal resolution. As a result, the voltage drop across the flux-flow path shows a temporal pattern of striking structure caused by migrating domains of magnetic flux. This reveals the dynamic character of the so-called current-induced resistive state. This voltage structure is analyzed on the basis of the extended Gibbs free-energy-barrier model, and quantitative agreement is achieved for the single-domain and the multidomain cases as well. In the single-domain case, the influence of fluctuations of the transport current on the time-resolved voltage is demonstrated. In the multidomain case, where more than one domain exists within the sample, a transient oscillation of the number of flux quanta per domain is revealed occurring immediately after the onset of flux flow. This size oscillation dies out after a small number of cycles and approaches the stationary value observed in previous experiments. Thus, a successful explanation of the temporal aspects of the breakdown of superconductivity is established, which holds from its very onset to the stationary regime of the current-induced resistive state.     

Flux flow and resonant modes in multi-junction Josephson stacks

Magnetic field dependent current-voltage characteristics of stackled Nb/(Al−AlO_x/Nb)_n long Josephson junctions are investigated experimentally. The thickness of their common superconducting electrodes provides the magnetic coupling between the junctions. For stacks of n=7 Josephson junctions the current-voltage characteristics display collective flux-flow behaviour of Josephson vortices. In the interior layers Josephson vortices move simultaneously under the influence of the bias current. The flux-flow behaviour is modulated by a complicated structure of cavity-like resonances which show broad range of characteristic frequencies. The measurements can be qualitatively explained by the Kleiner model for the resonances in stacks. Mutual locking of junctions in the stack is indicated by pronounced cavity resonances with large voltage spacing.     

An estimate of the errors in measuring electric power by the digital method of multiplying current and voltage

Estimates on the systematic digitization and quantization errors, and also the dynamic dating errors (or the aperture error) of a method of measuring power are obtained. The method is based on the digital processing of pulse-time analog-to-digital converters of the codes of instantaneous values of the voltage and current in the circuit.     

Characterization and Experimental Verification of the Nonlinear Distortion in a Technique for Measuring the Relative Velocity Between Micromachined Structures in Normal Translational Motion

Applications exist for microelectromechanical systems (MEMS) devices where the measurement or estimation of the relative velocity, or at least the direction of instantaneous relative velocity, between two microstructures in normal translational motion is required. A technique for directly measuring the relative velocity has not been available. This paper presents a technique for directly measuring the relative velocity between two microstructures in normal translational motion. The technique consists of measuring the current flowing into the capacitance formed between the two microstructures when a constant voltage is applied across them. The technique and the resulting nonlinear distortion in the velocity measurement are characterized. A prototype relative velocity sensor is fabricated and evaluated to verify the measurement technique     

A magnetic feedback modulator for improved FM carrier recording

Magnetic multivibrators are often used as voltage-controlled oscillators in FM carrier recording systems. These oscillators generally consist of two switching transistors connected to a center tapped coil on a square loop magnetic core. The transistors operate alternately in saturation, driving the core between its positive and negative saturation limits. Frequency then depends upon saturation flux and applied voltage. In the Magnetic Feedback Modulator, stability and linearity improvement of an order of magnitude is possible with a new method of applying negative feedback to a square-loop core magnetic multivibrator. In addition, circuit complexity is reduced considerably over previous methods using conventional frequency detectors to derive feedback voltage. Based on Faraday's Law, a voltage which is a function of frequency and flux change is derived from the magnetic circuit. This voltage is then used as negative feedback to the dc differential amplifier to drive the magnetic multivibrator. Since the magnetic circuit is included in the negative feedback loop, frequency errors occurring in the modulator cause corresponding changes in the magnetic feedback voltage. The errors are then reduced in magnitude by the negative feedback.     

ac Response and losses of differently pinned type-II superconductors

Measurements of the penetration of flux and losses in ac fields (20–2000 cps) at 4.2 K together with quasistatic magnetization curves on three samples of Ta ₉₂ Nb ₈ having different degrees of pinning are presented. The results are explained in terms of surface shielding currents taking into account both the pinning strength and the flux-flow resistivity exhibited by the bulk. A quantitative approach is used in which the concept of flux-flow resistivity proves applicable when the sample is subjected to an ac magnetic field.     

An experimental study of flux presetting-resetting characteristics of magnetic cores with rectangular hysteresis loop

When magnetic cores with a square hysteresis loop are used for a time delay Crcuit or a pulse counter circuit, and if these circuits are designed by the use of the static hysteresis loop, the ac magnetization curve, or the control magnetization curve (CMC) which is the basic characteristic of a magnetic amplifier, there is apt to be a marked discrepancy between their design values and measured values. Since, in the flux control of magnetic cores used for a time delay circuit or a pulse counter circuit, the flux, contrary to the case with a magnetic amplifier, arrives at the negative saturation from the positive saturation by many intermittent signal voltages, a new characteristic quite different from CMC and other characteristics must be adopted as the standard of the design in this case. It was for this reason that instantaneous relation between the rate of flux change and the exciting current was examined in the case in which real flux resetting behaviors of a time delay circuit or a pulse counter circuit were taken into consideration, by using grain-oriented 50 per cent permalloy as the sample, with a view to obtaining core characteristics serviceable enough for exact design methods of a time delay circuit or a pulse counter circuit. Various useful results have been obtained as to: 1) how the correlation between rate of flux changedPhi/dtand instantaneous values of exciting current icchanges in proportion to the flux values, 2) how the amount of reset fluxDeltaPhi_{r}will change according to the flux valuePhiand the rate of flux changedPhi/dt, and 3) how the above-mentioned relations are connected to the tape thickness of the magnetic core. This paper reports the results of this investigation.     

Measurement of Specific Heat Capacity and Electrical Resistivity of Industrial Alloys Using Pulse Heating Techniques

The determination of the specific heat capacity and electrical resistivity of Inconel 718, Ti-6Al-4V, and CF8M stainless steel, from room temperature to near the melting temperatures of the alloys, is described. The method is based on rapid resistive self-heating of a solid cylindrical specimen by the passage of a short-duration electric current pulse through it while simultaneously measuring the pertinent experimental quantities (i.e., voltage drop, current, and specimen temperature). From room temperature to about 1300 K, the properties are measured using an intermediate-temperature pulse-heating system by supplying a constant current from a programmable power supply and measuring the temperature using a Pt-Pt:13% Rh thermocouple welded to the surface of the specimen. From 1350 K to near the melting temperatures of the alloys, the properties are measured using a millisecond-resolution high-temperature pulse-heating system by supplying the current from a set of batteries controlled by a fast-response switching system and measuring the temperature using a high-speed pyrometer in conjunction with an ellipsometer, which is used to measure the corresponding spectral emissivity. The present study extends the application of these techniques, previously applied only to pure metals, to industrial alloys.     

FRACTURE MECHANICS ANALYSIS BASED ON A LOCAL SIMULATION PRINCIPLE

Abstract— A local simulation principle is presented which allows the transfer of geometry-dependent fracture parameters from a specimen to a component if the local constraint conditions of the crack tip in the component are simulated by the specimen. This can be achieved by adapting the specimen to the component with respect to the ligament length, the ligament width, the crack length and the loading geometry. If these requirements are met, then the relationship between the load and the driving force in the component can be estimated from the specimen by a simple scaling procedure. The application of the local simulation principle is illustrated by an example.