Experimental identification of the kink instability as a poloidal flux amplification mechanism for coaxial gun spheromak formation

The magnetohydrodynamic kink instability is observed and identified experimentally as a poloidal flux amplification mechanism for coaxial gun spheromak formation. Plasmas in this experiment fall into three distinct regimes which depend on the peak gun current to magnetic flux ratio, with (I) low values resulting in a straight plasma column with helical magnetic field, (II) intermediate values leading to kinking of the column axis, and (III) high values leading immediately to a detached plasma. Onset of column kinking agrees quantitatively with the Kruskal-Shafranov limit, and the kink acts as a dynamo which converts toroidal to poloidal flux. Regime II clearly leads to both poloidal flux amplification and the development of a spheromak configuration.     

Filling of a Plane Slit Volume with a Glow Discharge in a Transverse Magnetic Field and Its Effect on the Discharge Contraction

We consider a dc glow discharge in a plane slit volume with electrodes in the slit plane in a magnetic field transverse to the current, which has been studied experimentally. As in the experiment, the discharge is artificially confined at one of the dielectric boundaries of the volume and propagates to the opposite dielectric boundary until it is stabilized. It is shown using a 2D calculation of the nonstationary process that the discharge in a magnetic field occupies a noticeably larger volume (with a lower current density at the electrodes) than in zero magnetic field. The effect of the magnetic field is also manifested in that it hampers the contraction of the discharge, substantially elevating the threshold current of the diffuse discharge. The discharge contraction is calculated in the approximation of a homogeneous positive column along the current right to the attainment of the stationary state. In calculations with a magnetic field, hysteresis appears in transitions from the diffuse to the contracted state and back.     

Do columnar defects produce bulk pinning?

From magneto-optical imaging performed on heavy-ion-irradiated YBa(2)Cu(3)O(7-delta) single crystals, it is found that at fields and temperatures where strong single vortex pinning by individual irradiation-induced amorphous columnar defects is to be expected, vortex motion is limited by the nucleation of vortex kinks at the specimen surface. In the material bulk, vortex motion occurs through (easy) kink sliding. Depinning in the bulk determines the screening current only at fields comparable to or larger than the matching field, at which the majority of vortices is not trapped by an ion track.     

Kink-induced transport and segregation in oscillated granular layers

We use experiments and molecular dynamics simulations of vertically oscillated granular layers to study horizontal particle segregation induced by a kink (a boundary between domains oscillating out of phase). Counterrotating convection rolls carry the larger particles in a bidisperse layer along the granular surface to a kink, where they become trapped. The convection originates from avalanches that occur inside the layer, along the interface between solidified and fluidized grains. The position of a kink can be controlled by modulation of the container frequency, making possible systematic harvesting of the larger particles.     

Near integrability of kink lattice with higher order interactions

We make use of Manton's analytical method to investigate the force between kinks and anti-kinks at large distances in 1+1 dimensional field theory.The related potential has infinite order corrections of exponential pattern,and the coefficients for each order are determined.These coefficients can also be obtained by solving the equation of the fluctuations around the vacuum.At the lowest order,the kink lattice represents the Toda lattice.With higher order correction terms,the kink lattice can represent one kind of generic Toda lattice.With only two sites,the kink lattice is classically integrable.If the number of sites of the lattice is larger than two,the kink lattice is not integrable but is a near integrable system.We make use of Flaschka's variables to study the Lax pair of the kink lattice.These Flaschka's variables have interesting algebraic relations and non-integrability can be manifested.We also discuss the higher Hamiltonians for the deformed open Toda lattice,which has a similar result to the ordinary deformed Toda.     

Kink instability in applied-field magneto-plasma-dynamic thrusters

Measurements of magnetic and electrostatic fluctuations in an applied field magneto-plasma-dynamic thruster have shown that a m/n=1/1 kink mode becomes unstable whenever the Kruskal-Shafranov limit is violated. A positive correlation is established between the kink and performance degradation at high current, which has until now prevented the use of this kind of thruster in space missions.     

Dynamical behavior of the kink motion in carbon nanotubes

By performing molecular dynamics calculations, we studied the motion of the kink between two carbon nanotubes. Based on the sequential evaporation of the most energetic carbon atom, our calculations show that the kink has complex longitudinal and spiral motions, in good agreement with the experiments. The kink moves towards the nanotube of larger diameter, resulting in an overall diameter shrinking of the nanotubes without inducing any disorder or damage. The kink motions are found to be dependent on the chirality of the nanotubes. The kink connecting two zigzag nanotubes can have either a pseudoclimb or a spiral motion, while the kink between the armchair nanotubes has an interesting spiral motion with periodic split and recombination of the topological defects.     

Application of stain-etched porous silicon in light emitting diodes and solar cells

Porous silicon/c-Si heterostructures have been formed by the method of stain etching.The properties of light emitting diodes (LED) and solar cells have been studied. The transport mechanism of the diode has been investigated from the current–voltage characteristics measured at different temperatures (296–380 K). A model based on multi-step tunneling of carriers at reverse and low forward bias (<1 V) and on field tunneling across a narrow barrier at higher forward bias (>1.5 V) is proposed for the LED. In the case of the solar cells the porous silicon is formed in between the fingers of the front grid contact. Application of porous silicon in solar cells results in an increase of the short-circuit current and efficiency of the cells by about 30%.     

Topological Kinks in φ2n Field Theories and Low Energy Excited States

The static kinks and their low energy excited states in scalar field theories with V[φ]= -(1/2)m²φ² + gφ²ⁿ/2n are studied in a function-series method. We give a universal formal solution, and a universal approach to find the low energy excited states as well. Excellent agreement between the FS solution and the exact answer is found. This confirms our predicted results of the energies of the excited quantum states by considering small oscillations about the kink motion.     

平行面多金属钨丝Z箍缩实验研究

 利用“阳”加速器对平行面多金属钨丝（直径17 μm，丝间距1 mm，丝数3根或5根）负载进行了Z箍缩聚爆实验研究。在峰值为350 kA,上升沿约80 ns的电流作用下，使用针孔成像技术和X射线诊断技术，获得多钨丝等离子体融合图像及相应的软X射线辐射信号。对金属丝聚爆过程中负载电流变化对实验影响进行了分析，并探讨了引起聚爆过程中的等离子体柱腊肠型不稳定性、扭曲型不稳定性和“热点”及其周围弥散斑等现象的原因。     

振动弯结与点缺陷交互作用时间平均场的对称破缺及自组织现象

计算了各向同性弹性介质中螺型位错上几何弯结与替代溶质原子弹性交互力场和刚度场。计算了简谐振动弯结的弹性交互作用的按时间平均场沿螺型位错芯管道的分布。当振幅大于近似为弯结宽度的临界振幅后,平均交互作场发生对称破缺、由单峰变成双峰,弯结中心由溶质原子的低能稳定位置变成不稳定位置。分析了替代溶质原子沿螺型位错管道扩散自组织形成双峰气团并使弯结振动由简谐型过渡到多谐振动的参数范围。 关键词：     

Hydrogen promoted corrosion of tungsten by oxygen in an electric field: A field ion microscope study

The extensive changes in surface topography observed to occur on tungsten field ion emitters as a result of exposure to oxygen in presence of hydrogen at 78°K and at fields of 2 V/Å have been studied in detail. Field promoted diffusion of gas from the low field region of the emitter shank over the imaged area of the surface removes kink site metal atoms and subsequently deposits them on either side of the well defined diffusion paths along close packed zones; field evaporation of such atoms may also occur when the change in local surface contour causes sufficient field enhancement. Although oxygen is primarily responsible for the corrosion effects, its diffusion at such temperatures is promoted by the presence of hydrogen, and the rate of reaction is dependent, inter alia, on hydrogen pressure.     

Insights into the Discrepancy between Single Molecule Experiments

Sharp bending as one of the mechanical properties of double-stranded DNA(dsDNA) on the nanoscale is essential for biological functions and processes. Force sensors with optical readout have been designed to measure the forces inside short, strained loops composed of both dsDNA and single-stranded DNA(ssDNA). Recent FRET singlemolecule experiments were carried out based on the same force sensor design, but provided totally contrary results. In the current work, Monte Carlo simulations were performed under three conditions to clarify the discrepancy between the two experiments. The criterion that the work done by the force exerted on dsDNA by ssDNA should be larger than the nearest-neighbor(NN) stacking interaction energy is used to identify the generation of the fork at the junction of dsDNA and ssDNA. When the contour length of dsDNA in the sensor is larger than its critical length, the fork begins to generate at the junction of dsDNA and ssDNA, even with a kink in dsDNA. The forces inferred from simulations under three conditions are consistent with the ones inferred from experiments, including extra large force and can be grouped into two different states, namely, fork states and kink states. The phase diagrams constructed in the phase space of the NN stacking interaction energy and excited energy indicate that the transition between the fork state and kink state is difficult to identify in the phase space with an ultra small or large number of forks, but it can be detected in the phase space with a medium number of forks and kinks.     

Spatial Dependence of the Strong Optogalvanic Effects Due to Metastable Quenching in a DC Helium Discharge

The optogalvanic effects of the He 2.058-?m line (21P-21S) from He lamps illuminating a weak dc He discharge (current density ?10 ?A/cm2, field/pressure ~25 V/cm - torr) are reported. For illumination at the positive column, we have made quantitative measurements of the decreases in the discharge current, electron density, and metastable densities, as weli as the increase in the electric field in the positive column, as the intensity of illumination increases. We have also observed that for sufficiently strong illumination (using He lamps only), the optogalvanic effect is catastrophic, i.e., the discharge is switched off; this clearly shows that a sufficiently large metastable density (which is reduced by the illumination) is necessary to maintain a weak He discharge. For illumination at the cathode regions, the optogalvanic effects are "anomalous": the discharge current is strongly suppressed by illumination at the cathode dark space next to the cathode, but is enhanced by iumination at the adjacent negtive glow region.     

Three-dimensional dynamics of collisionless magnetic reconnection in large-scale pair plasmas

Using the largest three-dimensional particle-in-cell simulations to date, collisionless magnetic reconnection in large-scale electron-positron plasmas without a guide field is shown to involve complex interaction of tearing and kink modes. The reconnection onset is patchy and occurs at multiple sites which self-organize to form a single, large diffusion region. The diffusion region tends to elongate in the outflow direction and become unstable to secondary kinking and formation of "plasmoid-rope" structures with finite extent in the current direction. The secondary kink folds the reconnection current layer, while plasmoid ropes at times follow the folding of the current layer. The interplay between these secondary instabilities plays a key role in controlling the time-dependent reconnection rate in large-scale systems.     

Kink instability of the semiconductor plasma in silicon parallelepipeds

Experimental results on the onset of the kink instability of the semiconductor plasma in silicon p⁺-p-n⁺ structures are analyzed. The structures were parallelepipeds. The experiments were carried out over the temperature range from 77 to 300 K. The shape of the current-voltage characteristics and that of the threshold curves of the test samples are discussed. The frequency and amplitude of the alternating current which arises as a result of the kink instability are described as a function of the electric field and the magnetic induction at levels substantially above the excitation threshold.V. D. Kuznetsov Siberian Physicotechnical Institute, Tomsk State University. Scientific-Research Institute of Semiconductor Devices. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 103–110, May, 1992.     

基于石墨烯电极的Co-Salophene分子器件的自旋输运

采用基于非平衡格林函数结合第一性原理的密度泛函理论的计算方法,研究了基于锯齿型石墨纳米带电极的Co-Salophene分子器件的自旋极化输运性质.计算结果表明,当左右电极为平行自旋结构时,自旋向上的电流明显大于自旋向下的电流,自旋向下的电流在[-1V,1V]偏压下接近零,分子器件表现出优异的自旋过滤效应.与此同时,在自旋向上电流中发现负微分电阻效应.当左右电极为反平行自旋结构时,器件表现出双自旋过滤和双自旋分子整流效应.除此之外,整个分子器件还表现出较高的巨磁阻效应.通过分析器件的自旋极化透射谱、局域态密度、电极的能带结构和分子自洽投影哈密顿量,详细解释该分子器件表现出众多特性的内在机理.研究结果对设计多功能分子器件具有重要的借鉴意义.     

Kink effect in AlGaN/GaN high electron mobility transistors by electrical stress

The kink effect is studied in an AlGaN/GaN high electron mobility transistor by measuring DC performance during fresh, short-term stress and recovery cycle with negligible degradation. Vdg plays an assistant role in detrapping electrons and short-term stress results in no creation of new category traps but an increase in number of active traps. A possible mechanism is proposed that electrical stress supplies traps with the electric field for activation and when device is under test field-assisted hot-electrons result in electrons detrapping from traps, thus deteriorating the kink effect. In addition, experiments show that the impact ionization is at a relatively low level, which is not the dominant mechanism compared with trapping effect. We analyse the complicated link between the kink effect and stress bias through groups of electrical stress states: Vds = 0-state, off-state, on-state (on-state with low voltage, high-power state, high field state). Finlly, a conclusion is drawn that electric field brings about more severe kink effect than hot electrons. With the assistance of electric field, hot electrons tend to be possible to modulate the charges in deep-level trap.     

Kink propagation induced by multiplicative noise

The influence of spatio-temporal external multiplicative fluctuations on a single kink in a bistable distributed system is studied. For this purpose we derive a stochastic dynamic equation for the position of the shifted kink. An analytical estimate for spatio-temporally uncorrelated fluctuations is represented and discussed. We draw the conclusion that multiplicative noise induces a propagation of the most probable kink into the region of larger noise. This effect is demonstrated in numerical simulations.     

Multi-phonon capture of charge carriers by dislocation kinks in semiconductors

In view of the problem of recombination-enhanced motion of dislocations in semiconductors, we studied the thermal capture of an electron by a smooth dislocation kink. Multi-phonon capture becomes possible due to localization of the carrier on the kink. The localized state on the smooth kink is studied in the deformation potential approximation. In this case the potential created by the kink is described by Poschl-Teller function, which enables to find the analytical expressions for the eigenstates and the corresponding wave functions. With the use of the ground state wave function we find the multi-phonon capture cross-section for two limiting temperature cases, corresponding to the thermally activated and quantum transitions between vibronic terms.