Dynamical identification of a threshold instability in Si-doped heterofullerenes

We rationalize the origins of a threshold instability and the mechanism of finite temperature fragmentation in highly Si-doped C(60-m)Si(m) heterofullerenes via a first-principles approach. Cage disruption is driven by enhanced fluctuations of the most internal Si atoms. These are located within fully segregated Si regions neighboring the C-populated part of the cage. The predominance of inner Si atoms over those involved in Si-C bonds marks the transition from thermally stable to unstable C(60-m)Si(m) systems at m = 20.     

Analysis of the Limiter Material Erosion in T-3M Tokamak

The limiter material erosion dynamics under disruption instability in T-3M tokamak was studied. Erosion mechanisms of graphite (uglesitall) and boron nitride were examined. It was shown that the erosion of the limiter arises not only at the moment of disruption, but also before it, when the MHD perturbations of plasma column emerge. We had evidence of the superthermal electrons playing a significant role in the limiter erosion. The erosion decreases with the plasma density rising above the threshold value.     

The night driving behavior in a car-following model

In this paper, we have studied the night driving behaviors in the car-following model in periodic boundary conditions. The evolution of uniform traffic under both small and large perturbations is investigated. The simulations show that the traffic is always unstable when V^′<0$V^{\prime }<0$ with V the optimal velocity. The traffic clusters, the kink–antikink waves, and the unstable clusters are observed under different sensitivity parameters. Even more interesting phenomenon is observed when the randomness effect is considered. Under large perturbations, it is shown that the traffic will be unstable if its density is smaller than a threshold. The density corresponding to the threshold increases with the decrease of the sensitivity parameter values.     

Instability of Marangoni flow in the presence of an insoluble surfactant. Experiments

The stability and structure of solutocapillary Marangoni flow initiated by a localized concentration source in the presence of an adsorbed layer of insoluble surfactant is investigated experimentally. It has been established that the main axisymmetric flow becomes unstable with respect to azimuthally periodic disturbances which leads to the appearance of the surface flow with a multi-vortex structure. The structure of the secondary flow is investigated depending on the intensity of the main flow and on the surface density of the surfactant. It has been shown that the azimuthal wave number increases with the growth of the Marangoni number and decreases with the surface density of the surfactant. A threshold value of the surface density of the surfactant, at which the Marangoni flow does not occur, has been defined.     

Disruption avoidance in the Frascati Tokamak Upgrade by means of magnetohydrodynamic mode stabilization using electron-cyclotron-resonance heating

Disruption avoidance by stabilization of MHD modes through injection of ECRH at different radial locations is reported. Disruptions have been induced in the FTU (Frascati Tokamak Upgrade) deuterium plasmas by Mo injection or by exceeding the density limit (D gas puffing). ECRH is triggered when the V(loop) exceeds a preset threshold value. Coupling between MHD modes (m/n=3/2, 2/1, 3/1) occurs before disruption. Direct heating of one coupled mode is sufficient to avoid disruptions, while heating close to the mode leads to disruption delay. These results could be relevant for the International Thermonuclear Experimental Reactor tokamak operation.     

Phenomenological model of the unstable stage of a vacuum spark discharge

A model of the unstable stage of a spark discharge in vacuum is proposed, which describes all typical manifestations of this stage, including current spikes in the diode, an increase in the potential at the cathode flame front, collective acceleration of ions in vacuum and plasma diodes, change in the cathode erosion mechanism, and the emergence of electron microbeams with a high current density at the anode. It is shown that these processes are associated with the formation of a charged electron layer of a spatially inhomogeneous plasma at the cathode flame boundary at the unstable stage of the spark discharge in vacuum. The emergence of this layer is associated with a limited emissive ability of the plasma at the cathode flame front during its expansion in vacuum. This leads to disruption of the plasma (field-induced emission of electron from the boundary region of the flame) and the formation of a short-lived charged plasma, viz., high-density ion cluster at the cathode flame boundary. The estimates obtained using this model are in good agreement with the experimental data on physical processes at the unstable stage of a vacuum spark discharge.     

Nonlinear competition between asters and stripes in filament-motor systems

A model for polar filaments interacting via molecular motor complexes is investigated which exhibits bifurcations to spatial patterns. It is shown that the homogeneous distribution of filaments, such as actin or microtubules, may become either unstable with respect to an orientational instability of a finite wave number or with respect to modulations of the filament density, where long-wavelength modes are amplified as well. Above threshold nonlinear interactions select either stripe patterns or periodic asters. The existence and stability ranges of each pattern close to threshold are predicted in terms of a weakly nonlinear perturbation analysis, which is confirmed by numerical simulations of the basic model equations. The two relevant parameters determining the bifurcation scenario of the model can be related to the concentrations of the active molecular motors and of the filaments, respectively, which both could be easily regulated by the cell.     

Dissipative instability of charged aerosol flows in the mesosphere

We consider the possible mechanism of generation of charged-particle density irregularities and electric field in the middle atmosphere based on the development of the dissipative instability of a flow of large charged aerosols. A dispersion equation describing the properties of the spectral component of a quasi-static electric field with allowance for the aerosol charging inertia is obtained. This equation is used to study characteristics of the instability threshold. It is shown that the charging inertia and the presence of photoelectrons lead to an increase and a decrease in the threshold plasma frequency of the aerosols, respectively. It is found that there exist optimal combinations of such parameters as the radius of spherical aerosols and the mass of heavy ion clusters for which the instability threshold is minimum. It is also shown that the instability threshold is lower for the particles stretched along the motion direction. Quantitative estimates are given for medium parameters necessary for the excitation of instability in the region of existence of polar mesospheric summer echo as well as for spatial scales of unstable perturbations. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 11, pp. 942–957, November 2006.     

Sensitisation waves in a bidomain fire-diffuse-fire model of intracellular Ca dynamics

We present a bidomain threshold model of intracellular calcium (Ca²⁺) dynamics in which, as suggested by recent experiments, the cytosolic threshold for Ca²⁺ liberation is modulated by the Ca²⁺ concentration in the releasing compartment. We explicitly construct stationary fronts and determine their stability using an Evans function approach. Our results show that a biologically motivated choice of a dynamic threshold, as opposed to a constant threshold, can pin stationary fronts that would otherwise be unstable. This illustrates a novel mechanism to stabilise pinned interfaces in continuous excitable systems. Our framework also allows us to compute travelling pulse solutions in closed form and systematically probe the wave speed as a function of physiologically important parameters. We find that the existence of travelling wave solutions depends on the time scale of the threshold dynamics, and that facilitating release by lowering the cytosolic threshold increases the wave speed. The construction of the Evans function for a travelling pulse shows that of the co-existing fast and slow solutions the slow one is always unstable.     

A study of the deterministic character of optical damage by femtosecond laser pulses and applications to nanomachining

A remarkable feature of material damage induced by short-pulsed lasers is that the energy threshold becomes deterministic for sub-picosecond pulses. This effect, coupled with the advent of kHz and higher repetition rate chirped pulse amplification systems, has opened the field of femtosecond machining. Yet the mechanism of optical breakdown remains unclear. By examining the damage threshold as a function of polarization, we find that, contrary to established belief, multiphoton ionization plays an insignificant role in optical breakdown. The polarization independence, combined with the observed precise and uniform dielectric breakdown threshold even for nanometer-scale features, leads us to conclude that the fundamental mechanism is self-terminated Zener-impact ionization, and that the deterministic and uniform damage threshold throughout the sample threshold stems from the uniform valence-electron density found in good-quality optical materials. By systematically exploring optical breakdown near threshold, we find that we can consistently machine features as small as 20 nm, demonstrating great promise for applications ranging from Micro ElectroMechanical Systems (MEMS) construction and microelectronics, to targeted disruption of cellular structures and genetic material. PACS 32.80.Rm; 77.22.Jp; 81.16.-c     

Energetic Particle Transport Prediction for CFETR Steady State Scenario Based on Critical Gradient Model

The critical gradient mode(CGM) is employed to predict the energetic particle(EP) transport induced by the Alfven eigenmode(AE).To improve the model,the normalized critical density gradient is set as an inverse proportional function of energetic particle density;consequently,the threshold evolves during EP transport.Moreover,in order to consider the EP orbit loss mechanism in CGM,ORBIT code is employed to calculate the EP loss cone in phase space.With these improvements,the AE enhances EPs radial transport,pushing the particles into the loss cone.The combination of the two mechanisms raises the lost fraction to 6.6%,which is higher than the linear superposition of the two mechanisms.However,the loss is still far lower than that observed in current experiments.Avoiding significant overlap between the AE unstable region and the loss cone is a key factor in minimizing EP loss.     

High-density operation in tokamaks

Conclusion  In this paper we have examined various aspects regarding high-density operation in tokamaks and in particular the density limit, the plasma detachment, the MARFE formation and the fuelling efficiency. As regarding the density limit, both experimental findings and theoretical model indicate that the plasma current and the total input power are relevant in limiting the edge density that can be sustained in a tokamak discharge: radiation losses and SOL momentum and energy conservation are the underlying mechanisms. In the latest divertor experiments, operating in the detached regime, the influence of the input power seems to vanish or even disappear. Edge phenomena such as plasma detachment, occurring beyond a density threshold that can be lowered by means of impurity injection, can lead to the almost complete exhausting of the heating power by radiation which is greatly helpful for the design of the divertor plates. The compatibility of H-mode operation with this regime is still under investigation. The MARFE phenomenon, sometimes precursor of a major disruption, is now understood in terms of a radiation induced thermal instability. Finally, experiments performed in order to investigate the fuelling efficiency of the gas puffing technique have shown that at high density this technique becomes rather inefficient, thus indicating that pellet injection still remains an essential requirement to fuel the reactor plasma. The drop of the fuelling efficiency of gas-puffing at high density can be accounted for by collision phenomena taking place in the SOL.     

Instability of a spherical drop in the field of an electrical dipole

Analytical calculations show that, as a field in which an initially spherical charged conducting incompressible drop is placed becomes more and more nonuniform, coupling between the drop’s oscillation modes grows and the threshold of stability against the electrical field pressure declines. When an electrostatic parameter characterizing the electric field pressure exceeds a value that is critical for a certain mode to be unstable, the amplitude of this mode exponentially grows in an aperiodic manner and the amplitudes of modes coupled with this mode build up in an oscillatory manner, each mode having its own instability growth rate. In all cases, there exists a threshold value of the dimensionless electric parameter above which all oscillation modes are unstable.     

Resonance formation in few (3) body Coulomb systems

The standard physical picture of resonance formation breaks down for three-body Coulomb systems close to the threshold for total fragmentation. Near this threshold the complex as a whole performs collective motions localised essentially along potential energy surface ridges. Above threshold these motions are nonperiodic and describe total fragmentation, below threshold unstable periodic motions lead to new resonances. A semiclassical theory which describes these phenomena for two-electron atoms in excellent agreement with experiments is generalised to the ppp system.     

Gravity driven instabilities in miscible non-Newtonian fluid displacements in porous media

Gravity driven instabilities in model porous packings of 1 mm diameter spheres are studied by comparing the broadening of the displacement front between fluids of slightly different densities in stable and unstable configurations. Water, water–glycerol and water–polymer solutions are used to vary independently viscosity and molecular diffusion and study the influence of shear-thinning properties. Both injected and displaced solutions are identical but for a different concentration of NaNO₃ salt used as an ionic tracer and to introduce the density contrast. Dispersivity in stable configuration increases with polymer concentration – as already reported for double porosity packings of porous grains. Gravity-induced instabilities are shown to develop below a same threshold Péclet number Pe for water and water–glycerol solutions of different viscosities and result in considerable increases of the dispersivity. Measured threshold Pe values decrease markedly on the contrary with polymer concentration. The quantitative analysis demonstrates that the development of the instabilities is controlled by viscosity through a characteristic gravity number G (ratio between hydrostatic and viscous pressure gradients). A single threshold value of G accounts for results obtained on Newtonian and non-Newtonian solutions.     

On the instability of conical protrusions of liquid surface in electric field

Electric field is analyzed at the tip of the conical protrusion of a conductive liquid surface that forms as the field strength approaches certain threshold values. When the field exceeds the threshold, the cone tip generally becomes unstable and emits a slender jet. The field strength at the sharp tip of a conducting liquid cone tends to infinity. Even though the field has a singularity, the cone tip becomes unstable only if the semi-cone angle decreases below the Taylor angle, which is the same for all conducting liquids. It is shown that similar conical protrusions can form if the liquid is a dielectric and its permittivity is sufficiently high. In this case, the equilibrium cone angle depends on the permittivity. When the permittivity is lower than approximately 17.60, cones cannot form, and the ensuing phenomena do not occur.     

Linear and nonlinear operation regimes of a laser with a transverse inhomogeneity of the cavity length and angular selection

Gaussian modes and pumping threshold values are found for a laser with spherical mirrors and angular selector. It is shown that, compared to the case of plane-parallel mirrors, the threshold overshoot is proportional to the square root of the mirror curvature. In the absence of the Kerr nonlinearity of the medium, exceeding the oscillation threshold of a stable cavity causes steady-state regimes to be established, whereas, for unstable cavities, pulsation regimes arise in a wide range of parameters. Calculations confirm the formation of moving localized structures in a laser with misaligned plane mirrors.     

燃料电池阴极相变临界电流模拟

本文根据直接甲醇燃料电池(DMFCs)阴极运行工况特性,考虑一个简单的一维模型,分析水蒸气饱和凝结成水的条件,从而得到电池阴极从单相流动到两相流动的临界电流值。分析结果表明:阴极气体进口速度增大使临界电流密度增大;临界电流随阴极气体进口相对湿度的增加而线性下降;临界电流随流道变长而减小;电池运行温度高有利于提高电池的临界电流。     

Stability analysis of multiple-lattice self-anticipative density integration effect based on lattice hydrodynamic model in V2V environment

Under the environment of vehicle-to-vehicle (V2V) communication, the traffic information on a large scale can be obtained and used to coordinate the operation of road traffic system. In this paper, a new traffic lattice hydrodynamic model is proposed which considers the influence of multiple-lattice self-anticipative density integration on traffic flow in the V2V environment. Through theoretical analysis, the linear stability condition of the new model is derived and the stable condition can be enhanced when more-preceding-lattice self-anticipative density integration effect is taken into account. The property of the unstable traffic density wave in the unstable region is also studied according to the nonlinear analysis. It is shown that the unstable traffic density wave can be described by solving the modified Korteweg-de-Vries (mKdV) equation. Finally, the simulation results demonstrate the validity of the theoretical results. Both theoretical analysis and numerical simulations demonstrate that multiple-lattice self-anticipative density integration effect can enhance the stability of traffic flow system in the V2V environment.