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     

Quench properties of high current superconductors

A technique has been developed which allows the simultaneous determination of most of the important parameters of a high current superconductor. The critical current, propagation velocity, normal state resistivity, magnetoresistance, and enthalpy are determined as a function of current and applied field. The measurements are made on non-inductive samples which simulate conditions in full scale magnets. For wide, braided conductors the propagation velocity was found to vary approximately quadratically with current in the 2 to 5 kA region. A number of conductors have been tested including some Nb³Sn braids which have critical currents in excess of 10 kA at 5 T, 4.2K.     

About the propagation velocity in superconducting composites

Various propagation velocities of a normal zone have already been given by several authors. It is shown here how different initial assumptions can lead to discrepancies between the velocity expressions. A new expression is proposed which, in particular, gives the right values of the full recovery and of the cold end recovery currents, together with an exact and finite propagation velocity when the transport current tends towards the critical current.     

Instability of vortex lines in the presence of axial normal fluid flow

We show that a normal mode analysis of the complete Hall two-fluid equations in rotating He II yields a critical velocity for axial normal fluid flow which is identical to the critical velocity derived in an earlier paper from the superfluid equation alone. We also present a simplified derivation of the critical velocity based on consideration of the forces acting on an isolated vortex line. The analysis is extended to ³ He- ⁴ He solutions and it is shown that rather small impurity concentrations can greatly increase the critical heat current in axial thermal counterflow.     

Propagation velocity of normal zones in a SC braid

Propagation velocities of normal zones are obtained experimentally in a superconducting (SC) braid which is wound into a coil to simulate the SC pulsed dipole magnet for a synchrotron from the viewpoint of cooling. A numerical calculation to determine this velocity is carried out by using a modified thermal conduction equation. The equation eliminates a troublesome problem on the boundary condition reported previously by defining a new variable of the square of the heat flux along the conductor. A velocity dependent correction factor is used for cooling to take account of the transient effect. The values calculated are a representation of the experimental dependence of the velocity on both current and magnetic field and this indicates that the correction factor is applicable to the winding of the braid if modified properly. Discussion is also made on the transient cooling in the coil.     

Persistent current states in rotating superfluid helium

Persistent current states in superfluid helium contained in a rotating superleak are studied for various velocities and initial conditions. The change in the superfluid component velocity as the normal component velocity changes is interpreted in terms of a circulation-free potential flow and the entrance of vortices. The appearance of vortices of opposite sign to that of preexisting ones in the superleak is discussed and this is related to the history-dependent nature of persistent currents. An analogy between a superfluid helium system such as ours and a highly irreversible type-II superconductor is made.     

Normal zone propagation in a superconductor carrying time-dependent current

A first consideration of the thermal propagation of a half infinite normal zone in a superconductor with time-dependent current is given. The model implies temperature-independent coefficients of the thermal conductivity equation and the change in the dissiptative conductor properties at a certain critical temperature T_s. The main regularities of the process and qualitative differences from the process of direct current zone propagation have been elucidated. The normal zone oscillations at high frequencies are shown to be mainly influenced by the process of thermal conductivity along the conductor. At low frequencies the zone propagation may be described by quasistationary formulae. Expressions for temperature profiles and normal zone velocity propagation have been obtained by a method of successive approximations.     

Investigation of stability of composite superconductors in typical coil configurations

The stability of various composite conductor designs in realistic coil environments has been examined. We measure the velocity of propagation or contraction of a normal region, the full recovery current, and the minimum propagating current in a coil segment. We examine the dependence of these measurements on background field, transport current, electrical insulation, cooling passage size and orientation, and proximity of other conductors. Comparison of experiment and calculation provides indirect information about local heat transfer to the helium bath and direct information about safe operating current limits for particular coil designs.     

Nucleation of normal phase in the dynamic resistive state in submicron Bi2212 bridges

The resistive state above the Larkin-Ovchinnikov (LO) instability current has been experimentally studied. Analysis of the stepwise I–V curves of thin superconducting bridges ≤1 μm wide, etched from the Bi₂Sr₂Ca₁Cu₂O_y (Bi2212) whiskers, reveals the following scenario of vortex dynamics: first, a pair of current-induced Abrikosov vortices enter the bridge, then they are accelerated by the current above the critical velocity of the LO instability and approach the limiting velocity of the normal phase propagation, of the order of 10⁷ cm/s. Each vortex leaves a tail of normal phase behind itself, which is equivalent to a phase slip line. At low temperatures the tails cover the whole width of the bridge—the I–V curves between the steps become Ohmic.     

Asymmetry of the normal zone propagation velocity in superconductors

The thermoelectric mechanism of the asymmetrical velocity of normal zone propagation in superconductors, relative to the direction of the transport current is suggested. The difference of the normal-superconducting interface velocities parallel and anti-parallel to the current direction due to this mechanism is of the order of 1 – 50 cm s^?1 for different metals. The thermoelectric effect (Thomson heat) has a hysteresis which occurs opon destruction (or restoration) of superconductivity in the sample with respect to the transport current.     

Normal phase propagation along a current carrying superconducting wire and its heat transfer to a cooling He II bath

A theoretical and experimental study of the longitudinal propagation velocity of a normal phase along a superconducting cylindrical wire suspended in a superfluid helium bath is presented. The theoretical model supposes a moving nonplanar separating boundary between the normal and superconducting phases and takes into account the latent heat absorbed during the destruction of the superconducting state. The comparison of the measured and calculated normal zone propagating velocities versus the circulating electrical currents shows an excellent agreement for all the bath temperatures considered. A boundary thermal conductance across the wire wall and superfluid helium bath interface is inferred by adjusting the calculated velocities with the experimental data. The deduced boundary thermal conductance is similar to the Kapitza conductance in the sense that both are proportional to the superfluid helium bath temperature raised to a constant power. Furthermore the deduced boundary conductance seems unaffected by the dynamical aspect of the longitudinal destruction process of the superconducting state and by the heat flux range across the solid and He II separating boundary covered by the normal zone propagation velocity versus the circulating electrical current plots. To the authors knowledge this is the first study of heat transfer using the data obtained from longitudinal normal zone propagation experiments.     

Intrinsic description of 3-dimensional shock waves in nonlinear elastic fluids

The transport equations for the amplitude of 3-dimensional shock waves in nonlinear elastic fluids are examined. It is shown that, with the exception of the term which contains the mean curvature of the shock surface, the transport equations are almost identical to the transport equations for 1-dimensional nonlinear elastic solids if we replace the stress, strain and velocity in the latter by the pressure, specific volume and normal velocity, respectively. Therefore, the results obtained for 1-dimensional shock waves regarding whether the amplitudes of jump in stress, strain and velocity grow or decay simultaneously can be applied here to the jump in pressure, specific volume and normal velocity. New compatibility equations are obtained for the velocity gradients behind the shock wave. We also obtain a universal relation between the variations of amplitudes of jump in pressure, specific volume and normal velocity.     

The segregation of copper and silicon in Al-Si-Cu alloy during electromagnetic centrifugal solidification

The present paper investigates the segregation of copper and silicon in an Al–lwt%Cu–lwt%Si alloy solidified under the co-action of centrifugal and electromagnetic forces. The reasons for the solute segregation and the effect of electromagnetic force on segregation are discussed. Tubular samples cut from the solidified alloy are analyzed, the results showing that the segregation of copper and silicon occurs along the normal direction of the samples and that the electromagnetic field has a remarkable influence on the segregation of both copper and silicon. As the exciting current increases, the segregation of copper decreases, while the segregation of silicon first increases and then decreases. The migration of solute atoms in the melt depends not only on the density difference between the solute and aluminum atoms, but also on the strength of the electromagnetic force. The magnetic force changes the rotation velocity of the melt, reduces the migration velocity of copper and causes the reduction of copper segregation. Because of the difference of the electrical conductivity between the solute and the aluminum melt, the reductions of velocity are not equal.     

环境介质对形状记忆合金弹簧记忆回复效应的影响*

针对SAM电火花加工机器人加工时常用的3种工作介质水、煤油和乳化液，研究了SMA弹簧的记忆恢复变形特性和变形规律。结果表明，对应不同的环境介质，SMA弹簧的记忆回复变形量与驱动电流间存在一一对应关系，通过控制驱动电流可控制记忆回复变形量。在不同环境介质中SMA弹簧的记忆回复变形速度的变化趋势是一致的，都是先随着回复位移的增加而增加，在接近中间段时达到最大，然后随着位移的增加而减小。驱动电流是对SMA弹簧动态变形速度影响最 明显的参数，通过选择合适的驱动电流可有效地控制SMA弹簧的回复变形速度。     

Effect of the local defects induced by bending strain on the quench properties for YBCO tapes

The effect of local defects on the quench properties for YBCO tapes after applying bending strain was investigated at self-field in 77 K. The minimum quench energy (MQE) was related to the position of defects in the tape and the smallest MQE appears where the region of the defects existed in the position of the heater at the same transport current. The normal zone propagation velocity (NZPV) was related to the size and quantity of the regions of defects. The more defects were in the tape, the faster the normal-zone propagation velocity.     

Basic models for thermal stability and zone propagation calculations for a.c. superconductors

A.c. superconductor stability within the framework of four basic models is analysed, with each model illustrating specific features in a.c. superconductor behaviour. Expressions are obtained for the minimum zone existence current, minimum propagation current, the limiting current at high frequencies (large Ω parameter), and also the parameter α corresponding to a fully stabilized a.c. superconductor. A.c. superconductors are shown to have a larger stability factor than d.c. ones. The propagation of normal and resistive zones in a.c. superconductors is studied numerically within a wide frequency range. The current dependences of the zone propagation velocity for each model are obtained. It is shown that taking into account a variable magnetic field gives rise to a system in which the conductor within the cycle becomes fully superconducting.     

Methodic Errors in Laser Measurements of Euler Statistical Moments of a Nonuniform Turbulent Velocity Field

Probability distributions of the velocity obtained by means of laser Doppler spectroscopy with different ratios between the dimensions of the probe volume and the scales of the nonuniformity are studied using a model of a locally normal, spatially nonuniform turbulent field. Sample calculations of the measured probability distributions of the velocity and the statistical moments of these distributions up to the fourth order are presented for model examples that take into account the nonuniformity of the mean flow velocity and the intensity of turbulent fluctuations of the velocity. It is shown that the substantial deviations in the velocity distributions from the normal law which are observed in a number of experiments may be related to noncompliance with the locality condition.     

Normal zone propagation characteristics of the HTS wires by heat energy applied

With the successful commercialization of Bi-2223 powder-in-tube wire, various attempts in the R&D of the high-T_c superconducting (HTS) magnets for high magnetic field applications are being implemented actively. Operating temperature of HTS magnet has to be maintained at the designed level but the magnetic energy and mechanical disturbance can cause unstable operational temperature of HTS magnet. Especially, the generated heat energy of inner HTS winding is apt to be accumulated, so the normal region appears in HTS winding. This paper deals with the quenching characteristics of three kinds of selected Bi-2223 wires: the high current density wire (HC-A) and the high strength wire (HS-A) made by AMSC and HTS wire (HW-I) made by Innost. The Innost wire has the highest minimum quench energy (MQE). The high current density wire has the highest normal zone propagation velocity (NZPV).     

Tunneling of quasiparticles between two weakly coupled systems of superfluid helium

Quasiparticle tunneling in a weakly coupled system of liquid helium is theoretically studied, in analogy with normal electron tunneling of superconductor junctions. Chemical potential difference in this case is to be created across the junction by a superfluid flow parallel to the junction. Calculation shows that a collimated beam of quasiparticles would show several divergences of tunneling current at specific angles of incidence or at specific values of superfluid velocity, while no divergence is expected for the integrated current, which is an important difference from the case of superconductor junctions.