Reignition phenomena after high-frequency current interruption withshort vacuum gaps

Based on a large number of measurements of high-frequency (HF) current interruptions in vacuum at small contact gaps (⩽600 μm), the statistical reignition behavior of vacuum switching devices after a HF current zero is investigated. Three types of reignitions can be classified. Statistical evaluation of post-arc current measurements for different parameters at current zero and different HF current ignition processes gives information about the stress of the gap as a result of the transient recovery voltage after a HF current zero (HF-TRV) and the accumulated post-arc charge. Comparing post-arc current values at the beginning of the HF-TRV and at the moment of reignition reveals a production of charge carriers during the recovery interval. Possible reasons for the different types of reignitions are discussed     

Plasma density decay of vacuum discharges after current zero

In vacuum circuit breakers the post-arc current caused by the remaining ions and electrons in the contact gap is an indication of the residual ionization and its decay. It coincides with the formation of a positive space charge sheath in front of the new cathode, which grows toward the new anode. In a vacuum test chamber an arc (1.5-15 kA RMS) is drawn between high current electrodes of the spiral type. At different times after current zero a transient recovery voltage is applied across a separate pair of high voltage electrodes. In contrast to real circuit breakers, where the transient recovery voltage reappears between the arcing contacts, this separation allows the study of residual plasma free from the thermal stress and melting on the contact surfaces. From the post-arc current across these electrodes, in comparison with a mathematical model of sheath growth, the density of the charge carriers can be evaluated. Such values and their temporal decay are presented     

Influence of interrupted current amplitude on the post-arc currentand gap recovery after current zero-experiment and simulation

For commercial vacuum circuit interrupter valves with radial field contact, the post-arc current (PAC) waveform was measured after short circuit current interruption applying a transient recovery voltage (TRV) of about 15 kV/s (RRRV). Keeping the current decline di/dt at current zero constant, the power frequency (PF) current amplitude Iˆ_PF was varied from 0.5 up to two times of the rated short circuit current. Significant influence of Iˆ_PF on the gaps memory is shown in particular effecting the post-arc current duration which varies in a range between 2 and 8 μs. Based on the existing physical models an interpretation of the results is given. It is shown that the memory effect influences the gap recovery time. The experimental results were further used to determine the parameters of the sheath growth model described by Andrews and Varey (1971). The initial ion density is fitted according to the current ramp di/dt before current zero (CZ) and according to the measured Q_PF=∫i _PF×dt during the arcing phase. The results of two different test procedures were applied in order to separate the effect of current ramp di/dt and Q_PF=∫i_PF×dt. The influence of both parameters and of the rate of rise of the recovery voltage du/dt (RRRV) on the sheath edge velocity could be demonstrated by simulation     

A Model for DC Interruption in Diffuse Vacuum Arcs

A theoretical model for current interruption in a diffuse vacuum arc with dc commutation is described. Before current zero the interelectrode plasma is modeled as an ion-neutral fluid through which electrons are flowing. After current zero a positive ion sheath grows into the plasma from the former anode, driven by the transient recovery voltage. Using the basic laws of conservation, the decay of the plasma during commutation is evaluated numerically, enabling the post-arc current, the electric field at the former anode, and the power input to this electrode after current zero to be calculated. For copper electrodes, with a commutation time of 30 ?s, the ion density and velocity at current zero are 23 percent and 35 percent of their respective steady state values. The calculated post-arc currents of tens of amps are in good agreement with experimental data. The post-arc data generated with this model can be used to study reignition mechanisms and the interrupting capability of different contact materials.     

Observation of zero current density in the core of jet discharges with lower hybrid heating and current drive

Simultaneous current ramping and application of lower hybrid heating and current drive (LHCD) have produced a region with zero current density within measurement errors in the core ( r/a< or =0.2) of JET tokamak optimized shear discharges. The reduction of core current density is consistent with a simple physical explanation and numerical simulations of radial current diffusion including the effects of LHCD. However, the core current density is clamped at zero, indicating the existence of a physical mechanism which prevents it from becoming negative.     

Upon the influence of the post-arc current on the low voltageswitching process in vacuum

The paper presents one stage of the scientific research, performed for the investigation of the post-arc (p.a.) current within low voltage switching process in vacuum, when interrupting short-circuit currents of high values. A large power equipment, supplied by the mains and providing an accurate reproduction of the processes, concerning the short circuit phenomenon (U=1.5 kV; I=100 kA) was used. An experimental model, which can reproduce the switching conditions of the power circuit-breaker in vacuum, was available as well. The influence of an axial magnetic field on the p.a. current and the switching process was also explored     

Study of a circuit-breaker arc with self-generated flow. III. Thepost-arc phase

For pt.II see ibid., vol.16, p.615-22, 1988. The third part of this study of a circuit-breaker arc with self-generated flow concerns the model of the post-arc phase. The greater interruption capability of SF₆ compared to N₂, already suggested in pt.II, is confirmed by the following results: amplitude and duration of the postarc current are much lower in SF₆ and the maximum value of the rate of rise of recovery voltage is about three times higher in SF₆ than in N₂. The model also predicts the existence of a plateau in post-arc current variation and the original behavior of this type of circuit-breaker: interrupting efficiency increases with the current peak in the initial phase     

Capillary discharge soft X-ray lasing in Ne-like Ar pumped by low main current pulses

Under the condition of the low main current pulse, the capillary discharge soft X-ray laser is obtained. The laser energy of the same discharge current amplitude at different Ar pressures and of different current amplitudes at the same Ar pressure are compared, respectively. At different Ar pressures, the amplitude of the main current pulse evidently impacts the soft X ray, especially at a pressure higher than 30 Pa. In order to obtain the laser output at a high Ar pressure, the amplitude should be increased. However, at the same Ar pressure, it might have an optimum current, in which case the laser spike is the highest and the most stationary. The laser achieving time at different amplitudes is also discussed. As far as we know, this is the first time the influence of the amplitude of the main current pulse has been analyzed.     

Excitation of spin dynamics by spin-polarized current in vortex state magnetic disks

A spin-polarized current with the polarization direction perpendicular to a disk in the vortex ground state will result in renormalization of the effective damping of excitations on this state. As the current is increased to a threshold current Ic the effective damping will be zero and the lowest threshold current corresponds to the vortex gyrotropic mode. For larger values of the current the excitation is a nonlinear gyrotropic mode having nonsmall amplitudes and larger frequency than the linear mode. This effect occurs for any mode of the vortex-state disk, and the value of Ic is proportional to the mode frequency.     

Josephson junction with two superconducting current components

The properties of a Josephson junction with the 2π- and 4π-periodic superconducting current component have been analyzed. In the range of low voltages, such a junction exhibits the 4π periodicity of the phase difference for the Majorana current amplitude much smaller than the Josephson current, which makes it possible to observe Josephson current oscillations with a fractional period for small dissipation β < 1 in the hysteresis region. The effect the 4π-periodic Majorana current component is also manifested in a change in the sequence of steps in the ladder structure emerging on the current–voltage (I–V) characteristic of the junction. We have determined the interval of external electromagnetic radiation amplitudes, in which the manifestation of the fractional Josephson effect on the I–V characteristic is most significant.     

Tokamak equilibria with reversed current density

Observations of nearly zero toroidal current in the central region of tokamaks (the "current hole") raises the question of the existence of toroidal equilibria with very low or reversed current in the core. The solutions of the Grad-Shafranov equilibrium equation with hollow toroidal current density profile including negative current density in the plasma center are investigated. Solutions of the corresponding eigenvalue problem provide simple examples of such equilibrium configurations. More realistic equilibria with toroidal current density reversal are computed using a new equilibrium problem formulation and computational algorithm which do not assume nested magnetic surfaces.     

On the nonlinear theory of current instability of short-wave drift oscillations

The paper deals with the studies of current instability in the inhomogeneous plasma resulting in excitation of short-wave drift oscillations with a frequency near the low-hybrid resonance.It is shown that the saturation of such an instability is associated with the spectral pumping of oscillations into the short-wave region that occurs due to the modulation instability; and maximum amplitudes of the electrical fields of oscillations are determined. The effective frequency of electron collisions due to current instability is calculated. It is indicated that the diffusion model of the Parker-Sweet magnetic field reconnection modified taking into account the anomalous resistance mechanism studied here leads to the estimate of the magnetopause width being in satisfactory agreement with the experiment.     

The anomalous acoustoelectric current in single-electron transport devices with three pairs of shallow-etched gates

In order to study the influence of the modulation effect in systems of multiple etched gates, we design single-electron transport devices in an AlGaAs/GaAs heterostructure with three pairs of shallow-etched gates in series. In the experiment, the voltage applied to V_g1 is set in the pinched-off region; V_g2 is set in open region and V_g3 is swept from the pinched-off region to the open. Interestingly, the results indicate that the acoustoelectric current increases from zero to a maximum and then decreases to the zero again, a sharp peak is observed. Change of the electron reservoir's chemical potential is considered as the physical origin of this experimental phenomenon.     

Dielectric Recovery of Vacuum Arcs after Strong Anode Spot Activity

Recovery of dielectric strength and post-arc currents after diffuse and constricted vacuum arcs were measured for filat OFHC-Cu contacts (D = 25 mm, d = 7.5 mm) enclosed in a bakable UHV chamber. The arc current pulse had a trapezoidal shape of 5.5-ms duration with peak values up to 11 kA. In comparison with the fast recovery of diffuse arcs, the recovery of constricted arcs with gross melting is considerably retarded. Post-arc currents are simulated using the Andrews-Varey model extended to include the effects of secondary electron emission due to ion bombardment of the cathode and loss of the plasma due to thermal motion. The flow of charge carriers to the anode and the shield, which is at the anode's potential, are registered separately. The amount and decay of the residual plasma is evaluated from the measurements of post-arc current. The decay times of a few tens of a microsecond give evidence of ions with energies below 1 eV. The origin and effect of slow ions on recovery is discussed.     

Critical current measurement on Y-Ba-Cu-O

We report critical current measurements on sintered Y_0.35Ba_0.65CuO_y. The sample, in the perovskite phase, shows zero resistance at 87 K. The critical current transition is seen, in zero field and at 77 K, at a current densityJ _c of 50 A/cm².     

High density current drive by high frequency radiation

A self-consistent theory is developed for current drive by intense radiation in the presence of the ion-acoustic instability. The spectrum of ion-acoustic turbulent noise generated by the driven current and concentrated in a limited cone of angles along the propagation direction of the wave is found. Excitation of the instability is accompanied by the establishment of an electron drift that is excited by the electromagnetic wave and has a velocity on the order of the ion acoustic speed. This current drive regime is realized over a wide range of intensities, as long as the region of turbulence in the angles of the acoustic wave vector is expanding. At yet higher intensities, the driven current increases in proportion to the intensity of the fundamental wave. Similar behavior is found for driven heat fluxes. Zh. éksp. Teor. Fiz. 111, 107–119 (January 1997)     

The immutability of the critical current under any sample cross-sectional form modification

The temperature and size dependences of the critical currents in bismuth-based high-temperature superconducting ceramic samples consisting of randomly oriented grains have been studied in zero magnetic field. It is shown that the critical current is a function of the sample cross-sectional area only. At constant sample cross-sectional area the value of the critical current is independent of any variation of a sample cross-sectional shape. At the same time the distributions of the critical current density and induced magnetic field in ceramics are functions of the sample cross-sectional shape. These values very in accordance with a change of the sample cross-sectional shape (at constant cross-sectional area i.e. at constant value of the critical current). The field and current distributions in the rectangular sample are reported.     

Zero-point fluctuations and the quenching of the persistent current in normal metal rings

The ground state of a phase-coherent mesoscopic system is sensitive to its environment. We investigate the persistent current of a ring with a quantum dot which is capacitively coupled to an external circuit with a dissipative impedance. At zero temperature, zero-point quantum fluctuations lead to a strong suppression of the persistent current with decreasing external impedance. We emphasize the role of displacement currents in the dynamical fluctuations of the persistent current and show that with decreasing external impedance the fluctuations exceed the average persistent current.     

Current clamp at zero level in JT-60U current hole plasmas

It is found that no current is driven in a central region of a tokamak plasma once the central current density becomes nearly zero ("current hole"), in spite of high electric conductivity, at the current drive by a toroidal electric field and a radio-frequency wave in experiments on the JT-60U tokamak. This is a new, stiff, self-organized structure of a magnetic field in an axisymmetric toroidal plasma.     

无碰撞电流片低频电磁模不稳定性：MHD模型

利用含无电阻广义Ohm定律的可压缩磁流体力学(MHD)理论，研究了在具有剪切磁场的无碰撞电流片中低频电磁模不稳定性，假定等离子体压力各向同性，推导出了三维扰动传播波模的色散关系.色散关系的数值求解集中在电流片中间平面(z=0)和半厚度边缘(z=1)上，并分别考虑了二维传播和三维传播，以及不同的离子惯性长度情况.主要结果如下：1)对 于二维扰动传播(k_z=0)的波，在z=0平面上，Alfven波增长率最大，不稳定的波 频率 和波数范围也更宽.离中间平面越远，增长率越小，波数区域越小.同时，随着离子惯性长度 的增大，Alfven波不稳定性的增长率变大.2)对于三维扰动传播(k_z≠0)的波， 哨声是 不稳定的.在电流片中间平面上，哨声有明显的增长率；而在离子惯性区外边，哨声的增长 率还变大.3)在电流片中间(z=0)平面上，低频波主要是电流不稳定性激发的.在离中间 平面较远处，电流、密度和压力的梯度不稳定性变得更重要. 关键词： 无碰撞电流片 磁流体力学 色散关系 不稳定性