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In the vacuum DC circuit breaker based on the artificial current zero, the change of arc luminosity delays the change of current value significantly in the fast current-zero stage. This phenomenon has impacts on the breaking capacity of the DC vacuum circuit breaker. Therefore, it is necessary to study the dynamic characteristics of the arc plasma under the above condition. In this work, a 3D transient magneto-hydro-dynamic model based on the commercial cup-type axial magnetic field (AMF) contact is established to study this phenomenon. The simulation results show that the fast current-zero stage is too short for the plasma to diffuse. The changes of the physical characteristics of the arc plasma lag the change of current value. Moreover, the magnetic field hysteresis caused by the eddy current on the electrode occurs in this stage, resulting in a continuous stronger AMF. It makes the distributions of the plasma uniform, while the diffusion of the arc plasma reduced at low currents, which is detrimental to the post-arc dielectric recovery stage. In addition, in the fast current-zero stage, the AMF near the contact slot is smaller than in other areas. This indicates that the slotting is effective in suppressing eddy currents and avoiding the negative effect of excessive AMF on plasma diffusion at this stage. The simulation results are consistent with the experimental results. [ABSTRACT FROM AUTHOR]

O2 impurity in the working gas of an He/CF4 atmospheric pressure plasma jet (APPJ) can affect the discharge dynamics and the density of reactive species. Therefore, a two-dimensional (2D) fluid model is built in order to explore the influence of an O2 admixture on the propagation and F-containing species distribution of He/CF4 APPJ. The addition of 0.1% O2 accelerates the ionization rates of APPJ due to the increase of Penning ionization reactions of O2, resulting in the increases of axial speed and F-containing reactive species (CF4+, CF3+, CF2+, CF+, F+, CF3, F, CF3−) when APPJ approaches the dielectric surface. The addition of O2 has the inhibitory effect on the rise of some F-containing reactive species (CF3+ and F). As O2 concentration increases to 2%, the concentration of F-containing reactive species shows a downward trend due to the increase of excitation energy loss and an electron attachment reaction of O2. Different from the axial speed, the radial speed decreases continuously with the increase of O2 because of the high O2 concentration on the dielectric surface when APPJ propagates radially. This also results in a reduced distribution of reactive species fluxes. The excitation energy loss and electronegativity of O2 and CF4 in the case of He + 0.5% CF4 + 0.5% O2 have been presented in this paper. It is discovered that excitation energy loss of O2 is stronger than that of CF4, but the electronegativity of CF4 is stronger than that of O2. [ABSTRACT FROM AUTHOR]

In this paper, a two dimensional (2D) axisymmetric fluid model is built to study the effect of the ratio of CF4 admixture on the plasma dynamics and F-containing species concentration in He atmospheric pressure plasma jet. The steady mole fraction distribution of He and CF4 is first studied, which presents that the mole fractions of CF4 show peaks at 5 mm from the symmetry axis on the dielectric surface due to the dual influences of the boundary layer effect and air mixing. The CF4 admixture enhances the propagation speed of ionization wave, and the axial velocity reaches the peak value in the case of He + 1.5% CF4. The conversion from the ring-shaped plasma–surface interaction to a solid-disk one results from the addition of CF4. The Penning ionization of CF4 accelerates the plasma jet propagation within 1.5% CF4. However, the excitation energy loss and electron attachment caused by the addition of CF4 also quench the propagation of plasma jet, which become obvious in the case of 2% CF4. F-containing groups (CF4+, CF3+, CF3−, F−, CF3, and F), mainly produced by the Penning ionization reaction, electron attachment reaction, and He+ + CF4 → He + CF3+ + F, also show peaks for He + 1.5% CF4. On the dielectric surface, as the radial distance increases, the ratio of F-containing neutral species (CF3 and F) to O atom at 200 ns decreases due to the increase of O atom concentration and then increases at the streamer head because the surface flux of CF3 reaches the maximum value. The maximum surface flux radial distribution distance for ratio of F-containing species to O atom, CF3 and F appear in the case of 1.5% CF4. [ABSTRACT FROM AUTHOR]

This work investigates the arc behaviors with CuCr25 electrodes considering anode vapor using a magneto-hydro-dynamic model. Different kinds of components are considered including ions (Cu and Cr) with different charge numbers, electrons, and atoms (Cu and Cr). The effect of the anode sheath is also considered. The density distributions of these components are analyzed and compared with the experiments during the anode spot mode and the anode plume mode. Simulation results show that the anode vapor can enter the arc column forming a cool and poorly conducting region (i.e., neutral atom vapor area, NAVA) under high anode temperature. Atoms and single-charged ions mainly gather near each electrode. The highest double-charged ion density can be seen in front of the NAVA. Triple-charged ion density is negligibly low and reaches its maximum where the electron temperature is high. Cr is more likely to be ionized to a higher ionization level compared with Cu. Our results agree with experimental measurements of density distributions of different components and plasma temperature. [ABSTRACT FROM AUTHOR]

In this paper, the mechanism of negative streamer formation and branching between planar electrodes is studied using a two dimensional particle-in-cell/Monte Carlo collisional model. Super-particles, each one of which represents many physical particles, are used in our model to reduce the calculation and memory usage. The electric field applied to the gap between electrodes is constant and is above the breakdown value already before the streamer formation. For nitrogen at standard temperature and pressure, the simulation is first performed in the background field of 10 MV/m. The results show that prior to branching, the streamer head is flattened, and the non-uniform distribution of electrons accelerated to energies above 50 eV is obtained at the flat streamer head, which then leads to streamer branching. Then, additional simulations in the background fields of 9 MV/m, 11 MV/m, and 12 MV/m are performed to investigate the effects of background field. The results show that the streamer propagates faster, has larger plasma density, and branches more rapidly in a stronger field. The simulation results of argon are also given at a standard temperature and pressure in the background field of 10 MV/m. The results show that more branches can be obtained in argon compared with nitrogen, which agrees well with the experimental results. [ABSTRACT FROM AUTHOR]

Based on a 2D axisymmetric plasma fluid model, the effect of the levels of O2 admixtures on the plasma dynamics of an atmospheric pressure He plasma jet incident on a dielectric surface situated normal to the jet axis has been studied. The introduction of O2 admixture in the feedstock gas changes the shape of the discharge ionization rate that approaches the target surface from a ring to a solid disk. As the streamer extinguishes radial propagation along the dielectric surface, comparatively larger plasma-surface contact areas were obtained for O2 additions to the He plasma of less than 0.5% relative to that for the discharge in pure He, while in the case of 2% O2, the radial propagation distance dropped off significantly. The propagation of an ionization wave peaked in the range 0.2%–1% for the O2 admixture. The enhanced ionization reactions are considered to predominate for small concentrations of O2 as additive, and the negative role played by the excitation energy loss and the attachment of electrons to O2 is more important with the addition of 2% O2. There was a remarkable incremental gain in the flux intensity of O on the surface center, while the flux of N was mainly concentrated on the streamer head. The magnitude of the O flux on the surface showed a peak at the 0.5% O2 admixture level, and the flux of O3 on the surface was directly proportional to the O2 concentration. [ABSTRACT FROM AUTHOR]

Based on a magnetohydrodynamic model, a numerical simulation of high-current vacuum arc (HCVA) is carried out. In this model, the kinetic energy terms and ion viscosity terms in energy conservation equations are considered. Compared with the supersonic constricted vacuum arc, the ion flow in HCVA is in the subsonic status. Therefore, boundary conditions of cathode and anode sides have to be adjusted for HCVA. According to the simulation results of HCVA, we can find that the maximal plasma pressure appears near the cathode side. The characteristics of HCVA are significantly different from those of supersonic vacuum arc. Then, we make the comparisons between simulation results and experimental results (such as electron number density, electron temperature, ion temperature, high speed charge-coupled device photographs, and so on). The simulation results are in agreement with the experimental results. In addition, we also analyze the influence of different arc models on the distribution of current density in the anode side. [ABSTRACT FROM AUTHOR]

An intense infrared absorption band has been observed in a hydrothermally grown ZnO crystal. At 12 K, the band peaks near 3577.3 cm-1 and has a half width of 0.40 cm-1, and at 300 K, the band peaks at 3547 cm-1 and has a half width of 41.3 cm-1. This absorption band is highly polarized, with its maximum intensity occurring when the electric field of the measuring light is parallel to the c axis of the crystal. Photoinduced electron-paramagnetic-resonance experiments show that the crystal contains lithium acceptors (i.e., lithium ions occupying zinc sites). Lithium and OH- ions are present in the crystal because lithium carbonate, sodium hydroxide, and potassium hydroxide are used as solvents during the hydrothermal growth. In the as-grown crystal, some of the lithium acceptors will have an OH- ion located at an adjacent axial oxygen site (to serve as a passivator), and we assign the 3577.3-cm-1 band observed at 12 K to these neutral complexes. Our results illustrate the role of hydrogen as a charge compensator for singly ionized acceptors in ZnO. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies are used to track the temperature dependence of the A exciton energy (E[SUBXA]) in undoped bulk ZnO crystals grown by the seeded-chemical-vapor-transport method. For T>150 K, the edge emission becomes broad as the A exciton recombination and its longitudinal-optical (LO) phonon replica become superimposed. We use PLE to determine the temperature dependence of E[SUBXA] by monitoring the broad green emission commonly observed in as-grown ZnO crystals, and thus have established the energy difference between the E[SUBXA] and PL emission peak energies. The PL emission at 3.26 eV at room temperature is shown to be offset by about 50 meV to lower energy than the actual E[SUBXA] transition. The temperature dependence of the energy difference between the E[SUBXA] and PL peaks is compared)with predictions based on the lineshape function for the E[[SUBXA]- LO recombination. At 300 K, the PL is predominantly composed of E[SUBXA]- LO recombination. Further, the temperature dependence of the E[SUBXA] transition energy can be described using standard expressions and the Debye and Einstein temperatures are found to be 700±30 and 240±5 K, respectively. The slope of the E[SUBXA] versus T curve for ZnO approaches a constant value of De[SUBXA]/Dt=-0.35 meV/K near room temperature. [ABSTRACT FROM AUTHOR]

Electron paramagnetic resonance (EPR) has been used to investigate molecular nitrogen and isolated nitrogen acceptors in single crystals of ZnO. These samples were grown by the seeded chemical vapor transport method with N[sub 2] added to the gas stream. A five-line EPR spectrum is observed at low temperature in the as-grown bulk crystals and is assigned to N[sup -, sub 2] molecules substituting for oxygen. This structure arises from nearly equal hyperfine interactions with two nitrogen nuclei ([sup 14]N, 99.63% abundant, I = 1). The spin Hamiltonian parameters for the N[sup -, sub 2] center are g[sub ] = 2.0036, g[sub ⊥] = 1.9935, A[sub ⊥] = 9.8 MHz, and A[sub ⊥] = 20.1 MHz, with the unique directions parallel to the c axis. Laser excitation at 9 K, with 325 or 442 nm light, eliminates the N[sup -, sub 2] spectrum (when the N[sup -, sub 2] convert to N[sup 0, sub 2]) and independently introduces the EPR spectrum due to isolated nitrogen acceptors (when N[sup -] acceptors convert to N[sup 0]). Removing the laser light and warming to approximately 100 K restores the crystal to its preilluminated state. In separate experiments, heating between 600 and 800 °C increases the number of N[sup -, sub 2] and N[sup 0] acceptors that can be observed. We suggest that the activation of these nitrogen acceptors occurs when complexes of hydrogen and nitrogen thermally dissociate. Further heating above 800 °C drives the two nitrogen acceptors to inactive forms. [ABSTRACT FROM AUTHOR]

Bulk ZnGeP[sub 2] crystals were diffusion doped with copper and investigated using photoluminescence (PL) spectroscopy, photoluminescence excitation (PLE), and Fourier-transform-infrared spectroscopy. A PL band at 1.3 eV at 5 K was observed. The temperature dependence of the 1.3-eV PL band gives a thermal activation energy of 0.11 ± 0.01 eV. PLE data show a resonance in intensity occurred at 0.1 eV below the conduction band when monitoring the 1.3-eV band. The 1.3-eV emission band is found to be unpolarized in contrast to PL bands observed in nominally undoped ZnGeP[sub 2] crystals. The copper-diffused samples have an absorption band at 10 K peaking at about 0.62 eV. We suggest that the 1.3-eV luminescence band is donor-acceptor-pair recombination with the donor energy level about 0.1 eV below the conduction band and a Cu acceptor energy level about 0.62 eV above the valence band. This transition model is also supported by optical quenching experiments. [ABSTRACT FROM AUTHOR]

The anode activities are critical for high-current vacuum arc characteristics, especially the interruption performance of vacuum interrupters. The serious anode melting and sputter of liquid droplets into arc column often lead to interruption failure. In our previous work, the obvious anode melting and swirl flow of melted anode metal were detected at the center of anode surface when arc current exceeded a critical value under axial magnetic field (AMF). It is found that the AMF has great influence on the anode swirl flow, no swirl flow is found on the butt plate anode without AMF, but obvious swirl flow can be found when a moderate AMF is applied. Meanwhile, the swirl flow direction reversed if the AMF direction also reversed. The electromagnetic forcej×Bin anode melting pool and the impact force of ions coming from cathode plasma jets which are inclined to the arc axis on anode surface were thought to be two main possible reasons. In order to discover the physics behind the anode swirl flow phenomenon, special experiments have been conducted in this paper. Both the theoretical analysis and experimental results indicate that it is not the electromagnetic force but the interaction between ions from cathode plasma jets and anode melting pool that leads to the observed anode swirl flow. The inclination direction of cathode plasma jets, which is consistent with the direction of magnetic field vector, determines the direction of swirl flow. [ABSTRACT FROM AUTHOR]

Quantum cascade laser (QCL) with porous structure is reported in this work. The porous structure is fabricated onto the wafer top surface towards the sides of the QCL waveguide by electro-chemical etching. The pore size of the porous structure is in hundreds of nanometers. Experimental results show that QCL with porous structure exhibits better beam quality. The far field of the typical 60 μm-ridge-width QCL is modulated from double-lobed shape to single-lobed shape due to the utilizing of porous structure. The full width at half maximum is 5.26°, which agrees very well with that of fundamental transverse mode. An interesting spectra behavior is also observed. The QCL with porous structure represents a temperature tuning rate of 0.49 nm/K, which is comparable to distributed feedback QCL. [ABSTRACT FROM AUTHOR]