Slow Instability Research Articles

Tuning anisotropic ion transport in mesocrystalline lithium orthosilicate nanostructures with preferentially exposed facets

Li2TMSiO4 (TM = Mn, Fe, Co, etc.) is regarded as a new class of cathode materials for next generation Li-ion batteries because of the theoretical possibility of reversible deintercalation of two Li ions from the structure (ca. 330 mA hg−1). Nevertheless, the silicate cathode still suffers from low electronic conductivity, slow Li ion diffusion and structural instability upon deep cycling. To solve these problems, for the first time, we propose a rational design of mesocrystalline Li2FeSiO4 hollow discoids with an ordered single-crystal-like structure and highly exposed (001) facets. The Li2FeSiO4 mesocrystals display a near theoretical discharge capacity, superior rate capability and good cycling stability. The enhanced Li storage performance is ascribed to the unique structural features with a large surface area generated from the hollow mesocrystal structure and a shortened Li+ diffusion path along (001) exposed facets. This new facile, elegant synthesis method that enables the manipulation of crystal growth and subsequent improvements in the electronic and ionic kinetics and structural integrity should have a positive impact on the research and development of silicate materials as promising cathodes for next generation Li-ion batteries.

The role of zonal flows in disc gravito-turbulence

The work presented here focuses on the role of zonal flows in the self-sustenance of gravito-turbulence in accretion discs. The numerical analysis is conducted using a bespoke pseudo-spectral code in fully compressible, non-linear conditions. The disc in question, which is modelled using the shearing sheet approximation, is assumed to be self-gravitating, viscous, and thermally diffusive; a constant cooling timescale is also considered. Zonal flows are found to emerge at the onset of gravito-turbulence and they remain closely linked to the turbulent state. A cycle of zonal flow formation and destruction is established, mediated by a slow mode instability (which allows zonal flows to grow) and a non-axisymmetric instability (which disrupts the zonal flow), which is found to repeat numerous times. It is in fact the disruptive action of the non-axisymmetric instability to form new leading and trailing shearing waves, allowing energy to be extracted from the background flow and ensuring the self-sustenance of the gravito-turbulent regime.

Fabricating 3D ordered marcoporous Na2MnSiO4/C with hierarchical pores for fast sodium storage

Despite being considered as a promising candidate for cathode of next-generation batteries, sodium manganese orthosilicate suffers from sluggish kinetics, while no effective enhancement through structural manipulation has been reported yet. Herein a three-dimensionally ordered macroporous Na2MnSiO4/C is fabricated via a templated sol-gel method and investigated as a cathode material for sodium ion batteries. It delivers a high initial reversible capacity of 207 mAh g−1 (∼1.5 mol of Na+ insertion/extraction) at 0.1 C (1C = 139 mAh g−1), excellent high-rate capability (76 mAh g−1 at 5 C) and long-term cyclability (76.0% after 345 cycles at 2 C). The superior electrochemical properties are ascribed to short Na-ion diffusion path of honeycomb-like 3D ordered macroporous structure, fast electron transportation of interconnected carbon frameworks and effective constraint of volumetric changes upon electrochemical cycling. The rational design of 3D ordered porous structure may facilitate the development of silicate-based cathode materials that have problems of low electronic conductivity, slow ion diffusion and structural instability for new high-energy sodium ion batteries.

Analysis of transverse beam stabilization with radio frequency quadrupoles

A radio frequency (rf) quadrupole has been considered as a potential alternative device for Landau damping in circular hadron colliders. The objective of this study is to benchmark and confirm its stabilizing effect predicted by stability diagram theory by means of numerical tracking simulations. To that end, two complementary models of the device are implemented in pyheadtail, a 6D macroparticle tracking code designed to study the formation and mitigation of collective instabilities. The rf quadrupole model is applied to a slow head-tail instability observed experimentally in the Large Hadron Collider to show that such a device can in principle provide beam stability similarly to magnetic octupoles. Thereafter, alternative usage schemes of rf quadrupoles also in combination with magnetic octupoles are proposed, discussed, and benchmarked with simulations.

Sovereign Democratic Transformation in Ecuador (2007-2016)

Between 1981 and 2007, Ecuador experienced neoliberal reforms coupled with an elite form of democracy. These led to slow growth, upwards redistribution, and political and economic instability. Since then, Ecuador adopted a Citizens’ Revolution to counteract these economic and political forms of domination. This paper advances a few elements of the theoretical understanding of democratic justice inspired by the constitutional vision of Buen Vivir, and their policy applications in reconfiguring the political institutions that shape the state and its international relations. The application in the Citizens’ Revolution movement and historical phase has helped advance the democratization of the Constitution, the Executive, the Legislative, the Judicial system, political rights, the right to life, freedom of expression, national natural resources and investments, external public debt, as well as social rights and the welfare state.

Suppression of Hysteresis Effects in Organohalide Perovskite Solar Cells

Thin‐film solar cell based on hybrid perovskites shows excellent light‐to‐power conversion efficiencies exceeding 22%. However, the mixed ionic‐electronic semiconductor hybrid perovskite exhibits many unusual properties such as slow photocurrent instabilities, hysteresis behavior, and low‐frequency giant capacitance, which still question us so far. This study presents a direct surface functionalization of transparent conductive oxide electrode with an ultrathin ≈2 nm thick phosphonic acid based mixed C60/organic self‐assembled monolayer (SAM) that significantly reduces hysteresis. Moreover, due to the strong phosphonates bonds with indium tin oxide (ITO) substrates, the SAM/ITO substrates also exhibit an excellent recyclability merit from the perspective of cost effectiveness. Impedance studies find the fingerprint of an ion‐based diffusion process in the millisecond to second regime for TiO2‐based devices, which, however, is not observed for SAM‐based devices at these low frequencies. It is experimentally demonstrated that ion migration can be considerably suppressed by carefully engineering SAM interfaces, which allows effectively suppressing hysteresis and unstable diode behavior in the frequency regime between ≈1 and 100 Hz. It is suggested that a reduced density of ionic defects in combination with the absence of charge carrier accumulation at the interface is the main physical origin for the reduced hysteresis.

Design and Implementation of Self-Adaptive PD Controller Based on Fuzzy Logic Algorithm for Omni-Directional Fast Robots in Presence of Model Uncertainties

In this paper, a self-adaptive PD (SAPD) is employed for motion control of omni-directional robots. The method contains a PD controller that can be tuned online using a fuzzy logic system (FLS). Fast and accurate positioning is one of significant challenges in robot platforms. In addition, some uncertainties have adverse effects on traditional control system's performance during the robot's motion. Slow responses, low accuracy and instability are the most important drawbacks of widespread controllers in presence of uncertain dynamics. Since the fuzzy algorithm can deal with uncertainties and nonlinearities, the proposed method can tackle the mentioned problems. The controller is designed based on an uncertain model and implemented on a four wheeled omni-directional fast robot. The novelty of this article is proposing an enhanced version of well-known gain scheduling PD controller to improve positioning performance of the robot in different circumstances. Experimental results show that the method can provide a desirable performance in the presence of uncertainties.

Effect of Route Reflection on IBGP Convergence and an approach to reduce convergence time

Request for Comments (RFCs) for Border gateway Protocol (BGP) suggest that the network topology using BGP must have full mesh of IBGP sessions to avoid routing loops. Routing loop is a condition where packets are routed between two or more routers resulting in slow convergence and routing instability. Route reflection is one of the strategies to avoid full mesh of internal BGP sessions between BGP speaking routers in an autonomous system. This paper focuses on demonstrating an effect of using route reflection on convergence time in a small network. A mixed network scenario of external and internal BGP sessions is considered for demonstration. Results of the simulation have shown that, route reflection can significantly increase the convergence time avoiding full mesh. Also an approach to reduce the convergence time has been proposed. Proposed algorithm is based on the principle of creating logical group of router reflectors. The same concept can be scaled for larger networks for different scenarios.

Source localization using TDOA and FDOA measurements based on modified cuckoo search algorithm

This paper introduces a new algorithm for solving the localization problem of moving multiple disjoint sources using time difference of arrival and frequency difference of arrival. The localization of moving sources can be considered as a least-square problem. There are many algorithms used to solve this problem such as, two-step weighted least squares, constrained total least-square and practical constrained least-square. However, most of these algorithms suffer from either slow convergence or numerical instability and don't attain Cramer---Rao lower bound. We introduce a free-gradient algorithm called cuckoo search which avoids the slow convergence problem. The cuckoo search provides a combined global and local search method. Simulation results show that the proposed algorithm achieves better performance than other algorithms and attains Cramer---Rao lower bound.

Rapid integrated parametric CAE modeling method of Linear Variable Differential Transformer based on a script template

Generally, Computing Aided Engineering (CAE) modeling and simulation of Linear Variable Differential Transformer (LVDT) has drawbacks such as the slow modeling speed and instability in simulation results. In this study, a rapid integrated parametric CAE modeling method is proposed for electromagnetic simulation based on a script template implemented in Maxwell electromagnetic simulation software with Visual Basic, so that the proposed method only requires the establishment of a simplified electromagnetic simulation model of a LVDT. The standard and parametric modeling process of LVDT is initially recorded as a script and then identifiers are added into the script to form a script template and transform Computing Aided Design (CAD) parameters to CAE modeling parameters according to parameter constraints of the model. Finally, parameter values are automatically assigned to identifiers in the script template in order to generate the script file through which the accurate CAE model can be created conveniently. The parametric LVDT simulation can not only be rapidly established but also significantly improves the quality of LVDT design in industry. In addition, the proposed method can improve CAD information availability in realizing seamless integration of CAD/CAE modeling in similar sensor product.

Hydrogen Storage with Spectroscopic Identification of Chemisorption Sites in Cu-TDPAT via Spillover from a Pt/Activated Carbon Catalyst

Hydrogen spillover to the Cu-TDPAT (TDPAT = 2,4,6-tris(3,5-dicarboxylphenylamino)-1,3,5-triazine) metal–organic framework is probed with adsorption measurements, ex situ characterization techniques, and density functional theory (DFT) calculations. At 1 bar and 300 K, hydrogen chemisorption to Pt/AC/Cu-TDPAT exceeds that expected for physisorption by 8-fold, which is attributable to both catalyst insertion and the creation of structural defects. Hydrogenation of (a) the Cu–O–C bond of the Cu paddlewheel, (b) the sp2 N heterocycle, and (c) the secondary amine is demonstrated with ex situ spectroscopy. Exothermic (with respect to H2) hydrogenation at the Cu–O–C bond of the paddlewheel is substantiated by DFT. However, hydrogenated Cu–O–C is metastable, as evidence for dissociation is found at higher temperature (i.e., 473 K H2). DFT calculations demonstrate hydrogenation of the N groups may occur exothermically only for a charged ligand, suggestive that defects may contribute to hydrogen chemisorption. At high pressure, slow adsorption rates and material instability render the material unsuitable for practical hydrogen storage applications.

Nonnegative matrix factorization based on projected hybrid conjugate gradient algorithm

Nonnegative matrix factorization (NMF) is a popular matrix decomposition technique that has proven to be useful across a diverse variety of fields. Over the years, several algorithms have been proposed to improve the convergence of iterative algorithms in NMF, such as the multiplicative, the projected gradient, the second-order algorithms and recently the projected conjugate gradient algorithms. However, most of these procedures suffer from either slow convergence or numerical instability. In this paper, we propose a projected hybrid conjugate gradient algorithm which avoids the slow convergence problem by using orthogonal searching direction at each step, which ensures that is a descent direction, also avoids jamming that occurs in other conjugate gradient methods such as Fletcher–Reeves. We presented experimental results on both synthetic and real-world datasets that demonstrate the superiority of our algorithm, both in terms of better approximations as well as computational efficiency.

The Fluid Dynamics of Solid Mechanical Shear Zones

Shear zones in outcrops and core drillings on active faults commonly reveal two scales of localization, with centimeter to tens of meters thick deformation zones embedding much narrower zones of mm-scale to cm-scale. The narrow zones are often attributed to some form of fast instability such as earthquakes or slow slip events. Surprisingly, the double localisation phenomenon seem to be independent of the mode of failure, as it is observed in brittle cataclastic fault zones as well as ductile mylonitic shear zones. In both, a very thin layer of chemically altered, ultra fine grained ultracataclasite or ultramylonite is noted. We present an extension to the classical solid mechanical theory where both length scales emerge as part of the same evolutionary process of shearing the host rock. We highlight the important role of any type of solid-fluid phase transitions that govern the second degree localisation process in the core of the shear zone. In both brittle and ductile shear zones, chemistry stops the localisation process caused by a multiphysics feedback loop leading to an unstable slip. The microstructural evolutionary processes govern the time-scale of the transition between slow background shear and fast, intermittent instabilities in the fault zone core. The fast cataclastic fragmentation processes are limiting the rates of forming the ultracataclasites in the brittle domain, while the slow dynamic recrystallisation prolongs the transition to ultramylonites into a slow slip instability in the ductile realm.

Edge states for the turbulence transition in the asymptotic suction boundary layer

AbstractWe demonstrate the existence of an exact invariant solution to the Navier–Stokes equations for the asymptotic suction boundary layer. The identified periodic orbit with a very long period of several thousand advective time units is found as a local dynamical attractor embedded in the stability boundary between laminar and turbulent dynamics. Its dynamics captures both the interplay of downstream-oriented vortex pairs and streaks observed in numerous shear flows as well as the energetic bursting that is characteristic for boundary layers. By embedding the flow into a family of flows that interpolates between plane Couette flow and the boundary layer, we demonstrate that the periodic orbit emerges in a saddle–node infinite-period (SNIPER) bifurcation of two symmetry-related travelling-wave solutions of plane Couette flow. Physically, the long period is due to a slow streak instability, which leads to a violent breakup of a streak associated with the bursting and the reformation of the streak at a different spanwise location. We show that the orbit is structurally stable when varying both the Reynolds number and the domain size.

Ion acceleration from thin foil and extended plasma targets by slow electromagnetic wave and related ion-ion beam instability

When ions are accelerated by the radiation pressure of a laser pulse, their velocity cannot exceed the pulse group velocity which can be considerably smaller than the speed of light in vacuum. This is demonstrated in two cases corresponding to a thin foil target irradiated by high intensity laser light and to the hole boring produced in an extended plasma by the laser pulse. It is found that the beams of accelerated ions are unstable against Buneman-like and Weibel-like instabilities which results in the broadening of the ion energy spectrum.

NeNMF: An Optimal Gradient Method for Nonnegative Matrix Factorization

Nonnegative matrix factorization (NMF) is a powerful matrix decomposition technique that approximates a nonnegative matrix by the product of two low-rank nonnegative matrix factors. It has been widely applied to signal processing, computer vision, and data mining. Traditional NMF solvers include the multiplicative update rule (MUR), the projected gradient method (PG), the projected nonnegative least squares (PNLS), and the active set method (AS). However, they suffer from one or some of the following three problems: slow convergence rate, numerical instability and nonconvergence. In this paper, we present a new efficient NeNMF solver to simultaneously overcome the aforementioned problems. It applies Nesterov's optimal gradient method to alternatively optimize one factor with another fixed. In particular, at each iteration round, the matrix factor is updated by using the PG method performed on a smartly chosen search point, where the step size is determined by the Lipschitz constant. Since NeNMF does not use the time consuming line search and converges optimally at rate in optimizing each matrix factor, it is superior to MUR and PG in terms of efficiency as well as approximation accuracy. Compared to PNLS and AS that suffer from numerical instability problem in the worst case, NeNMF overcomes this deficiency. In addition, NeNMF can be used to solve -norm, -norm and manifold regularized NMF with the optimal convergence rate. Numerical experiments on both synthetic and real-world datasets show the efficiency of NeNMF for NMF and its variants comparing to representative NMF solvers. Extensive experiments on document clustering suggest the effectiveness of NeNMF.

Mathematical properties of plasma vertical instability in Tokamak

The model of plasma vertical instability in tokamak has been investigated. Fast, slow and hybrid magnetohydrodynamical instabilities have been studied. The models of passive, active, linear and nonlinear feedback have been studied. Several theorems, which are interesting from the physical point of view, have been proven.

Wave Instabilities of a Collisionless Plasma in Fluid Approximation

AbstractWave properties and instabilities in a magnetized, anisotropic, collisionless, rarefied hot plasma in fluid approx‐imation are studied, using the 16‐moments set of the transport equations obtained from the Vlasov equations. These equations differ from the CGL‐MHD fluid model (single fluid equations by Chew, Goldberger, and Low [5,9]) by including two anisotropic heat flux evolution equations, where the fluxes invalidate the double polytropic CGL laws. We derived the general dispersion relation for linear compressible wave modes. Besides the classic incompressible fire hose modes there appear four types of compressible wave modes: two fast and slow mirror modes – strongly modified compared to the CGL model – and two thermal modes. In the presence of initial heat fluxes along the magnetic field the wave properties become different for the waves running forward and backward with respect to the magnetic field. The well known discrepancies between the results of the CGL‐MHD fluid model and the kinetic theory are now removed: i) The mirror slow mode instability criterion is now the same as that in the kinetic theory. ii) Similarly, in kinetic studies there appear two kinds of fire hose instabilities ‐ incompressible and compressible ones. These two instabilities can arise for the same plasma parameters, and the instability of the new compressible oblique fire hose modes can become dominant. The compressible fire hose instability is the result of the resonance coupling of three retrograde modes ‐ two thermal modes and a fast mirror mode. The results can be applied to the theory of solar and stellar coronal and wind models (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Waves in expanding electronegative plasmas containing double layers

Expanding electronegative (EN) plasmas have been previously observed, experimentally, to generate wave activity. Using a particle-in-cell (PIC) code we have investigated these waves in expanding EN plasmas containing a double layer (DL) between an upstream source region and an expanded downstream region. Oxygen reaction rates were used but modified to correspond more closely to experimental conditions. Under a subset of pressures, for which a DL existed, waves were observed traveling upstream in the expanded region, and growing in amplitude in the direction of travel. Both slow and fast waves were observed. The fast wave existed only over part of the slow wave pressure range. The PIC results were compared to both fluid and kinetic theory, both of which assumed axial uniformity. The results of a somewhat simplified fluid theory, ignoring fast wave coupling and collisions with the background gas, gave a remarkable result: if the theory predicted a slow wave instability for any axial parameters in the downstream region, the instability was observed in the simulation. Conversely, if no instability was predicted at any axial position, no instability was observed. More accurate kinetic calculations, including electron and ion Landau damping, and also collisional damping against the background gas, gave wavelengths and growth rates that were consistent with the PIC simulations, and with the fluid results. The kinetic theory also indicated that the fast waves were always stable but became weakly damped for conditions of unstable slow waves. We postulate that nonlinear and nonuniformity effects excite the fast waves.

Stability of an ion-ring distribution in a multi-ion component plasma

The stability of a cold ion-ring velocity distribution in a thermal plasma is analyzed. In particular, the effect of plasma temperature and density on the instability is considered. A high ring density (compared to the background plasma) neutralizes the stabilizing effect of the warm background plasma and the ring is unstable to the generation of waves below the lower-hybrid frequency even for a very high temperature plasma. For ring densities lower than the background plasma density, there is a slow instability where the growth rate is less than the background-ion cyclotron frequency and, consequently, the background-ion response is magnetized. This is in addition to the widely discussed fast instability where the wave growth rate exceeds the background-ion cyclotron frequency and hence the background ions are effectively unmagnetized. Thus, even a low density ring is unstable to waves around the lower-hybrid frequency range for any ring speed. This implies that effectively there is no velocity threshold for a sufficiently cold ring.