Dynamics studies of cathode spots in a vacuum-arc discharge with ring electrodes

The effect of an external pulse magnetic field with axial and radial components on electric characteristics of the discharge and the configuration of cathode spots of a vacuum arc discharge with ring electrodes is studied experimentally. For arc currents within the range of 0.05–2 kA, shots of the cathode spots at different instants of time are obtained. The dependences of the number of the spots on the discharge current and the mean current per spot are determined. It was found that the expansion rate of the cathode spots area depends significantly on the instant value of the discharge current. It is shown that, when the pulse magnetic field is applied, the discharge voltage increases and the discharge current and number of the cathode spots decreases. It was found that the current interruption is a probability process. The probabilities of the current interruption depending on the maximal value of the external pulse magnetic field induction are determined.     

Minimum voltage principle in the theory of a high-current vacuum-arc discharge

The effect of an inhomogeneous magnetic field with axial and radial components on the structure of a high-current vacuum-arc discharge (VAD) has been theoretically studied. The characteristic features of current passage in a short VAD are considered using analytical expressions for the slope of the total current lines (TCLs) relative to the discharge axis, the axial components of the electric and magnetic fields, and the effective conductivity of discharge plasma. A two-dimensional mathematical model has been used to calculate the TCL shapes and the discharge voltage for various dimensions of the region of cathode spots. Calculations showed that the voltage drop on the discharge gap as a function of the arc discharge radius on the cathode has a minimum. The shape of TCLs and the arc radius on the cathode at this minimum agree with the available experimental data.     

A theoretical study of the impact of magnetic field of Hall current on the parameters of a high-current vacuum-arc discharge

It is demonstrated that the magnetic field of Hall current in a short high-current vacuum-arc discharge significantly distorts the external axial magnetic field and affects the distribution of current density in the discharge gap. This effect decreases with increasing external magnetic field, with decreasing arc current, and with decreasing ratio of the length of discharge gap to its transverse dimension. A 2D magnetohydrodynamic mathematical model is used to calculate the discharge parameters for different values of induction of external magnetic field. The calculation results are compared with similar results obtained using a 1.5D model in which the impact made by magnetic field of Hall current is ignored. It is inferred that the simpler 1.5D model may be employed in calculations of parameters of vacuum arc in a wide range of variation of arc current and of induction of external magnetic field.     

Distribution of heat flux transported by a magnetically driven arc

An alternating magnetic field is applied to a transferred arc to create a plasma arc for heating a large metal surface. The arc root oscillates on the metal surface at the frequency of alternating electromagnetic force (I × B force). The magnitude of the applied magnetic field controls the amplitude of the oscillatory motion. By changing the waveform of the applied field, the distribution of the heat flux density is adjustable. An overly strong magnetic field renders the oscillatory arc motion unstable. Experimental observations reveal that the heat flux is affected considerably by the neutral gas flow surrounding the arc.     

Current distribution measurement in YBCO thin film for a superconducting fault current limiter

Current distribution in the superconducting film for a resistive fault current limiter is important, because it influences AC loss and a uniformity of S/N transition. The lateral current distribution of the film was reconstructed from the magnetic field distribution which is measured by multiple Hall probes. The following results were obtained. (1) Non-uniform current distribution in the superconducting film was observed when the current was less than 1.3 times of critical current (I_c). (2) The current in a superconducting film was uniform when the current was much higher than I_c. The current can be considered uniform when the film works as a fault current limiter, because the S/N transition starts about twice of I_c. (3) The validity of the measurement was verified by the comparison with the electric circuit simulation.     

Critical flow of high-speed ions in a high-current vacuum-arc discharge

A physical model of vacuum arc is developed, which is used to construct a quasi-one-dimensional model and a two-dimensional two-fluid mathematical model. The two-dimensional magnetohydrodynamic model is based on the method of trajectories, according to which a set of partial equations is reduced to a set of ordinary differential equations written for derivatives along the lines of current. The quasi-one-dimensional model is based on the principle of compensation of radial magnetic forces, which is valid for a short high-current vacuum arc in axial magnetic field. A criterion is obtained, which defines the range of validity of the quasi-one-dimensional model in external axial magnetic field B ₀. Calculations are performed of the dependence of critical current on B ₀ for discharge gaps of different geometries. The calculation results agree with experiment.     

A two-dimensional mathematical model of a short vacuum arc in external magnetic field: results of numerical calculations

The paper deals with the investigation of the impact made by two-dimensional effects on the process of passage of current in a short vacuum arc in an axial magnetic field. A two-fluid mathematical model is used, which is based on hydrodynamic and electrodynamic equations. The axial magnetic field B _z affects significantly the magnitude of two-dimensional effects: the two-dimensional effects increase with decreasing B _z . The simulation results demonstrate that the contraction of plasma density exceeds that of current density. The distribution of anode drop of potential on the anode surface is nonuniform; in the case of certain (critical) values of current, the anode drop goes to zero on the external boundary of plasma. The dependence of the critical current on B _z is determined. The distribution of current density on the starting plane is nonuniform with a maximum on the axis, and the ion trajectories are inclined to the discharge axis. The possibility is discussed of matching the solution in the plasma region of vacuum arc with that for cathode flames.     

The fault current limiting characteristics of a flux-lock type high-Tc superconducting fault current limiter using series resonance

We analyzed the fault current limiting characteristics of a flux-lock type high-T_c superconducting fault current limiter (HTSC-FCL) using series resonance between capacitor for series resonance and magnetic field coil which was installed in coil 3. The capacitor for the series resonance in the flux-lock type HTSC-FCL was inserted in series with the magnetic field coil to apply enough magnetic field into HTSC element, which resulted in higher resistance of HTSC element.However, the impedance of the flux-lock type HTSC-FCL has started to decrease since the current of coil 3 exceeded one of coil 2 after a fault accident. The decrease in the impedance of the FCL causes the line current to increase and, if continues, the capacitor for the series resonance to be destructed. To avoid this operation, the flux-lock type HTSC-FCL requires an additional device such as fault current interrupter or control circuit for magnetic field.This paper investigated the parameter range where the operation as mentioned above for the designed flux-lock type HTSC-FCL using series resonance occurred from the experimental results. In the design of the flux-lock type HTSC-FCL, the some methods to avoid the continuous increase of the line current were suggested and confirmed by the experiments that the suggested methods were available to prevent the continuous increase of the line current after a fault happened.     

Effects of ferroelectric/metal interface on the electric properties of PMN-PT thin films epitaxially grown on Si substrates

Lead magnesium niobate-lead titanate 0.72Pb(Mg_1/3Nb_2/3)O₃-0.28PbTiO₃ (PMN-PT) ferroelectric thin films were deposited on SrRuO₃ (SRO), SrTiO₃, and TiN-buffered Si substrates by pulsed laser deposition. X-ray diffraction θ–2θ and Phi scans reveal that PMN-PT films were epitaxially grown on Si substrates. Pt, Al, and Gd metals were employed as top electrodes to investigate the ferroelectric and dielectric properties of these metal/PMN-PT/SRO capacitors. It was found that the coercive field (E _C) of the Gd(or Al)/PMN-PT/SRO capacitor is 4.5 times larger than that of the Pt/PMN-PT/SRO capacitor while the permittivity for the former is only ~27 % of that for the latter, which is analyzed using the model for metal-ferroelectric-metal heterostructures with Schottky contacts. Compared with the Pt/PMN-PT/SRO capacitor, the higher E _C and lower permittivity of the Gd(or Al)/PMN-PT/SRO capacitor are attributed to the stronger space charge field at the Gd(or Al)/PMN-PT interface. The capacitance–voltage characteristics of the metal/PMN-PT/SRO capacitors were also discussed.     

The Effect of External Magnetic Field on a Vacuum Arc

The effect of an external longitudinal magnetic field on the time required for anode spot formation in a high-current (5 to 12 kA) vacuum arc is experimentally studied, and the minimal value of a magnetic field inhibiting the anode spot formation is determined. The experimental results are compared with theory. The phenomenon of break of current in a low-current (100 to 300 A) vacuum discharge upon superposition of a magnetic field with transverse component on the discharge is investigated. The probability of break of current increases with the magnetic field. The possible mechanism of break of current is discussed.     

Flux Dynamics and Time Dependent Effects in Superconducting MgB2

We have investigated the time dependent flux motion in a polycrystalline superconducting MgB₂ sample by means of current-voltage (I?CV curves) and transport relaxation (V?Ct curves) measurements. The transport measurements were carried out as functions of temperature (T), transport current (I), and applied magnetic field (H). The time effects in the I?CV curves were investigated with the help of the sweep rate of transport current (dI/dt). The I?CV curves exhibit nearly reversible behavior for both slow and high current sweep rates (dI/dt) upon cycling the transport current. It was observed that the evolution of the I?CV curves is nearly independent of dI/dt. The small instabilities, voltage jump, and drops appearing at low dissipation levels were interpreted as a kind of plastic flow of the vortices evolving in the form of stripe or cluster in different sizes and local fluctuations in the superconducting order parameter. It was shown that the experimental I?CV curves are in good agreement with a power law behavior, V(I)??I ⁿ . The pinning potential U ₀ extracted from the fitting procedure is found to be approximately independent of dI/dt. In addition to the standard procedure, a reverse procedure was employed to study how the transport current (or associated vortices) penetrates from the surface into the sample. It was found that there is no marked difference between the I?CV curves obtained in the standard and reverse procedures. These results suggest that, in polycrystalline MgB₂, the weak links between grains (standard procedure) and surface weak links (reverse procedure) do not have a considerable effect on the evolution of the I?CV curves and also on the other transport measurements. In order to understand better the flux dynamics, the experimental results for polycrystalline MgB₂ sample were compared to previous similar studies on superconducting Y₁Ba₂Cu₃O_7?x (YBCO) and Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O _x (BSCCO) polycrystalline samples.     

The ion charge-current strength relationship in stationary and pulsed vacuum discharges

It is shown that the electron temperature in the region where the vacuum-discharge plasma is in equilibrium with the intrinsic magnetic field is proportional to the discharge current I and inversely proportional to the coefficient of erosion of the cathode material. For I≥300 A, this region features additional electron-impact ionization of ions coming from the near-cathode layer. As a result, the average ion charge Z increases with the current strength, the Z(I) dependence being more pronounced at a lower discharge pulse duration.     

Energy supply to the region of colliding plasma flows with opposite polarization fields

The interaction of two colliding deuterium plasma flows propagating in opposite directions across the magnetic field and having opposite directions of the polarization fields has been experimentally studied. The plasma flows with densities up to 10¹⁶ cm^?3 and a velocity of up to 2 × 10⁷ cm/s were formed using discharges initiated in crossed E × H fields. Each discharge operated at an electric power of up to 300 mW and a discharge current up to 100 kA. The results of measurements of the equivalent capacitance of polarized plasma flows were used to estimate the transverse permittivity of plasma. The rate of depolarization in colliding flows was determined. The frequency and energy characteristics of a plasma LC circuit formed by colliding flows were estimated. The temporal modulation of plasma density in the flows was measured.     

Power Law Dependence of I?CV Characteristics of Bi-2212 Whiskers at Low Magnetic Fields

In this study, BiSrCaCuO superconducting whiskers were fabricated by using a glass-ceramics process. The T _c value of the whiskers fabricated was found to be 90.2?K. I?CV characteristics of the whiskers were studied in the temperature range of 10?C70?K under low magnetic fields. The transport critical current density of whiskers was calculated between 19 and 0.63??10⁴ A/cm² at 10?C60?K. I?CV data were fitted using the Levenberg?CMarquardt technique. Results were discussed and so was their dependence on magnetic and electrical properties of the superconductors.     

A study into statistical regularities of extinction of a DC electric arc in an axisymmetric magnetic field

A statistical investigation is performed of the extinction of a 30–350 A electric arc in a vacuum gap, in which permanent magnets are used to generate an axisymmetric magnetic field with different ratios between the radial and axial components of magnetic field induction. Dependences are given of the duration of stable arcing and limiting interruption currents on the magnetic field induction and on the size of contact gap.     

Peculiarities of the current-voltage characteristics of a Josephson medium in a YBCO high-temperature superconductor

The influence of a weak magnetic field (H < 150 Oe) on the current-voltage (I-U) characteristic of a YBa₂Cu₃O_{7 ? x} (YBCO) high-temperature superconductor (HTSC) near the superconducting transition temperature has been studied. It is established that there exist narrow (<0.2 K) temperature regions where the I-U curve exhibits sharp bending for H < 30 Oe and the ohmic behavior changes to a quadratic dependence of the voltage on current in a region of several milliamperes. At higher temperatures, the I-U curve bending exhibits smearing. This behavior is observed at a temperature below that corresponding to a zero critical current. Above a certain current, the temperature and magnetic field exhibit equivalent effects on the I-U curve of YBCO. Experimental results are explained by a sharp decrease in the critical currents of intergranular Josephson junctions under the action of magnetic field and by the current-induced formation of uncoupled (with respect to the order parameter) superconducting grains. Characteristic currents for the transition of the intergranular Josephson medium into an incoherent state are determined and the first critical fields in YBCO are evaluated.     

Analysis of Current and Magnetic Distributions in REBCO Superconducting Coated Conductors in Self- and External Fields

The influence of a self-field on the critical current density J _c for a REBCO superconducting tape is presented in this paper. The distributions of the current density and magnetic field are analyzed in the tape under three kinds of conditions, i.e., applying an external magnetic field only, applying a transport current only, and applying a transport current together with an external magnetic field. In the analysis, the two-dimensional Poisson equation for the vector potential is employed. For the convenience of calculation, that the dependence of critical current versus the perpendicular and parallel fields tested from experiment is substituted for the traditional Kim-type or Bean model. The results show that the distributions of the current density and magnetic field in the REBCO tape change for the different frequencies and amplitudes of the transport current I _a and applied magnetic field B _a.     

Stabilization Time of Josephson Tunnel Junctions

The stabilization time is from the initial state to the steady state. The resistive-capacitive-inductive shunted junction (RCLSJ) model was used to study the nonlinear dynamical behavior of Josephson tunnel junction (JJ). The stabilization time of JJs is calculated by the information of voltage waveform. With the certain irradiation, four-dimensional image of stabilization time of JJs is calculated in the three-dimensional parameter space composed of the junction capacitance C, shunt inductance L, and the bias current I. The average of stabilization time under different conditions was applied to thr RCLSJ model and the superconducting quantum interference device (SQUID) model. In this paper, the average of stabilization time at different initial conditions, irradiation frequencies, and intensity was simulated. The average of stabilization time was nearly unvaried with initial value of normalized voltage and increased with the intensity of irradiation. The average of stabilization time fluctuates with the frequency of irradiation. Similar results are obtained in SQUID model. The average of stabilization time in SQUID model is less than that in JJ.     

基于图像处理技术的真空电弧参数化研究

真空电弧的演变规律对于真空断路器的设计具有重要意义。本文结合图像处理技术,利用拆除屏蔽罩的玻璃真空灭弧室,对交流电弧和直流开断电弧进行了拍摄及数字化分析。并采用改进的双阈值灰度图像分割算法,将电弧图像分割为三个特征区域,并计算出电弧直径、高灰度值区面积、高灰度值区面积率三个特征参数。结果表明,在纵向磁场下的作用下,交流电弧在2 ms左右的初始扩散过程后,会继续稳定燃烧直至电流自然过零熄灭;而直流开断电弧在被引燃后,存在1 ms左右的高灰度值区面积率较高的过程,随后经历短暂的扩散过程后,由于换流电流的投入而快速熄灭。