Quasi Neutrality Research Articles

Analysis of the Ambipolar Diffusion Approximation in Hypersonic Ionizing Reentry Flows

Simulations of the flow surrounding the Stardust Return Capsule at Mach 40 at various altitudes were used to test the validity of the ambipolar diffusion approximation. Results of direct simulation Monte Carlo (DSMC) simulations at 71 km altitude show that atomic N and O are dominant in the shock layer. The highest concentration of ions produced in the shock corresponds to , followed by , , , and . The only charged species present in the wake are , , and electrons, as , , and are formed and then depleted in the bow shock. At 100 km altitude, the influence of charged species is negligible. Simulations were also conducted in which the ambipolar diffusion approximation was not applied, and, instead, electrons were transported at the DSMC velocity. There was a net negative charge upstream from the bow shock and net positive charge in the plasma sheath adjacent to the vehicle. The magnitude of the electric field needed to maintain quasi neutrality is discussed. Results indicate that, for practical reentry flow conditions, the Debye length in the bow shock and wake is orders of magnitude smaller than the mean free path, and that invoking the ambipolar diffusion approximation is valid.

Anomalous Sorption Kinetics of Self-Interacting Particles by a Spherical Trap

In this paper we propose a computational framework for the investigation of the correlated motion between positive and negative ions exposed to the attraction of a bubble surface that mimics the (oscillating) cell membrane. The correlated diffusion of surfactants is described by a Poisson-Nernst-Planck (PNP) system, in which the drift term is given by the gradient of a potential which includes both the effect of the bubble and the Coulomb interaction between the carriers. The latter term is obtained from the solution of a self-consistent Poisson equation. For very short Debye lengths one can adopt the so called Quasi-Neutral limit which drastically simplifies the system, thus allowing for much faster numerical simulations. The paper has four main objectives. The first one is to present a PNP model that describes ion charges in presence of a trap. The second one is to provide benchmark tests for the validation of simplified multiscale models under current development [1]. The third one is to explore the relevance of the term describing the interaction among the apolar tails of the anions. The last one is to quantitatively explore the validity of the Quasi-Neutral limit by comparison with detailed numerical simulation for smaller and smaller Debye lengths. In order to reach these goals, we propose a simple and efficient Alternate Direction Implicit method for the numerical solution of the non-linear PNP system, which guarantees second order accuracy both in space and time, without requiring solution of nonlinear equation at each time step. New semi-implicit scheme for a simplified PNP system near quasi neutrality is also proposed.

Weakly nonlinear ion sound waves in gravitational systems.

Ion sound waves are studied in a plasma subject to gravitational field giving rise to vertically inhomogeneous steady-state plasma conditions. Such systems are interesting by exhibiting a wave growth that is a result of energy flux conservation for pulses propagating in an inhomogeneous system. The increase of the amplitude of a pulse as it propagates along the density gradient in the direction of decreasing density gives rise to an enhanced interaction between waves and plasma particles that can be modeled by a modified Korteweg-de Vries equation. Analytical results are compared with numerical particle-in-cell simulations of the problem. Our code assumes isothermally Boltzmann distributed electrons resulting in a nonlinear Poisson equation. The ion component is treated as a collection of individual particles interacting through collective electric fields. Deviations from quasineutrality are allowed for.

Stochastic field theory for the ionospheric fluctuations in Equatorial Spread F

Abstract In order to construct an analytical model of evolution for the plasma structures in the ionospheric Equatorial Spread F, an attempt is presented to include the plasma perturbations “seeding” the instabilities that cause the Equatorial Spread F in a theoretically consistent way. This is done assuming that the local discrepancies from quasi neutrality and from zero ionization-loss-and-production to be noise terms in the ionospheric density balance equation: doing so, the balance equation becomes a stochastic equation. The program is then to obain the probability that a certain evolution of the ionospheric density takes place from the interplay between plasma dynamics and noise statistics, i.e. to produce a stochastic field theory for the ionospheric density. The expression of this realization probability is obtained in the mathematically tractable cases of noises with Gaussian or exponential distribution. Then, practical limits and possible developments of these achievements are discussed.

Analytical finite-Lamor-radius and finite-orbit-width model for the LIGKA code and its application to KGAM and shear Alfvén physics

In this paper, a finite Larmor radius (FLR) and finite orbit width (FOW) model within the LIGKA [1] framework is derived and its implementation in LIGKA is verified against analytical theory. The model is based on an expansion in k ⊥ ϱi and thus valid for low and intermediate mode numbers. Assuming Maxwellian distribution functions and considering circulating ions only, analytical expressions for the non-adiabatic response in the quasi-neutrality (QN) and the gyrokinetic moment equation (GKM) are derived. It is shown how these expressions, also valid for n ≠ 0 can be connected to results in the literature for n = 0. Verification tests with analytical literature [2] are carried out for mode frequency, damping and radial mode propagation. A set of parameters is chosen that is based on recent experiments at ASDEX Upgrade [3] featuring strong EGAM (energetic particle induced geodesic acoustic mode) and BAE (beta-induced Alfvén eigenmode) activity. For finite n, the model can be used to determine the local radiative damping consistently, which is needed for the fast evaluation of linear AE (Alfvén eigenmode) stability boundaries that are typically required by energetic particle transport models.

Plasma Wall Transition and Effects of Geometry in Presheath

When plasma interacts with the wall of a conductor, electrons due to high mobility reach the wall first and develop negative potential on the wall and very near to the wall plasma is divided into sheath and presheath regions. The quasi-neutral plasma is shielded from the wall by a space charge sheath of the positive ions of the order of few electrons Debye’s lengths (λD) . At the sheath edge quasi neutrality breaks down from presheath side. In asymptotic limit ε=λD/L → 0 varying area of geometry affects the structure of the presheath scale. In addition to geometry, collisions and ionization also affects the presheath structure. But the sheath region is universal and is independent of either of geometry, ionization rate and collision frequency. The region which play the role of a link between these two regions has characteristics of both regions and is known as intermediate region. Even in the absence of ionization source and collision expanding area of geometry can accelerates the ions towards the wall. The characteristic length of the geometric presheath depends on radius of curvature c R = A/A′ , where “A” is the area of geometry and “A′= dA/dz”. If either of ionization or collisions is present along with the expanding area of geometry then dominant factor for the acceleration of ions in the presheath region is not the expanding area of geometry.

Perturbed soliton excitations of Rao-dust Alfvén waves in magnetized dusty plasmas

We investigate the propagation dynamics of the perturbed soliton excitations in a three component fully ionized dusty magnetoplasma consisting of electrons, ions, and heavy charged dust particulates. We derive the governing equation of motion for the two dimensional Rao-dust magnetohydrodynamic (R-D-MHD) wave by employing the inertialess electron equation of motion, inertial ion equation of motion, the continuity equations in a plasma with immobile charged dust grains, together with the Maxwell's equations, by assuming quasi neutrality and neglecting the displacement current in Ampere's law. Furthermore, we assume the massive dust particles are practically immobile since we are interested in timescales much shorter than the dusty plasma period, thereby neglecting any damping of the modes due to the grain charge fluctuations. We invoke the reductive perturbation method to represent the governing dynamics by a perturbed cubic nonlinear Schrödinger (pCNLS) equation. We solve the pCNLS, along the lines of Kodama-Ablowitz multiple scale nonlinear perturbation technique and explored the R-D-MHD waves as solitary wave excitations in a magnetized dusty plasma. Since Alfvén waves play an important role in energy transport in driving field-aligned currents, particle acceleration and heating, solar flares, and the solar wind, this representation of R-D-MHD waves as soliton excitations may have extensive applications to study the lower part of the earth's ionosphere.

Three-wave coupling in electron-positron-ion plasmas

The three-wave coupling processes in electron-positron-ion plasmas are investigated. The non-linear dispersion relation is derived along with the non-linear growth rate in both resonant and non resonant processes. It is shown that the inclusion of positron affects the dielectric properties of the plasma as well as the nonlinear growth rates of parametric processes. As one increases the positron density to electron density ratio from 0 to 1, maintaining quasi neutrality of the plasma, the growth rates of stimulated Raman, Brillouin, and Compton scattering processes in an isothermal plasma tend to zero due to the ponderomotive forces acting on electrons and positrons due the pump and scattered waves being equal.

Egy szétszakított térség újraintegrálódásának kezdete

In 1988-1989 a stream of East Germans sought refugee in Hungary at the Embassy of West Germany in Budapest. The three countries taking part in the issue made different solutions to deal with the increasing problem. The East German leadership tried to call back the refugees and calm down the emigration process, but it had lost its confidence. West Germany tried to solve the problem not only on the level of consulship but exten-sively, in part with recognizing the status those waiting for West German citizenship and those for refugee status - without any success. In the interest of the resolution and bring the Hungarian leadership to their point of view, they tried to make use of the international organizations. However, the Hungarian leadership made efforts to stay out of the issue to get the two German states make an agreement. But there was no chance for that so Buda-pest gave up its quasi neutrality and tried to solve the problem opening the border by avoiding Hungary to turn into a refugee camp of the region. Besides making this ad hoc arrangement, the whole issue needed more radical crisis management, not only in Hungary.

HOME BIAS IN GOVERNMENT SPENDING AND QUASI NEUTRALITY OF FISCAL SHOCKS

We show how introducing home bias in government spending in the redux model generates quasi neutrality of fiscal policy shocks. We offer an intuitive explanation for this result and we stress its policy implications.

Plasma Expansion in Vacuum: Modeling the Breakdown of Quasi Neutrality

We consider a system consisting of a vacuum gap delimited by two electrodes. A quasi-neutral plasma (ions and electrons) is injected from the cathode and expands. Electrons are emitted from the plasma-vacuum interface to the anode forming a beam. In this paper, from a two-fluid isentropic system coupled with the Poisson equation, we perform a formal asymptotic analysis. This leads to a quasi-neutral model for the plasma region and a Child--Langmuir model for the beam region. The main point of the analysis is the connection between these two models. This is done by studying a transmission layer problem. Finally, we numerically show the accuracy of the asymptotic model, comparing it to the original two-fluid model.

Hybrid (Particle-Fluid) Modeling of Pulsed Plasma Thruster Plumes

Integration of a pulsed plasma thruster (PPT) onboard spacecraft requires the evaluation of potential plume/spacecraftinteractionsthatcanbedetermined through plumemodelingand characterization.APPTplume model, numerical results, and comparisons with experiments are presented. The physical characteristics of PPT plumesare reviewed e rst, and theoutstanding modeling issuesrelated to theunsteady,partially ionized, collisional PPT plume plasma are presented. The PPT plume model is axisymmetric and based on a hybrid particle ‐euid approach. Neutrals and ions are modeled with a combination of the direct simulation Monte Carlo and a hybridparticle-in-cell method. Electronsaremodeled asa masslesse uid with a momentum equation that includeselectric e elds, pressure gradients, and collisional contributions from ions and neutrals. The nontime-counter methodology isusedforneutral ‐neutral,elasticion ‐neutral,and charge-exchangecollisions. Ion ‐electron collisionsaremodeled with the use of a collision force e eld. Electric e elds are obtained from a charge conservation equation under the assumption of quasi neutrality. Thecodeincorporates subcycling forthetimeintegration and unsteady particle injection.SimulationsareperformedusingPPTconditionsrepresentativeofa NASAJohn H.GlennResearch Center atLewisFieldlaboratory-modelPPToperatingatdischargeenergiesof 5,20,and40J.Theresultsdemonstratethe expansion of the neutral and ion components of the plasmoid during a pulse, thegeneration of low-energy ions and high-energy neutrals due to charge-exchange reactions, and the generation of neutral and ion backe ow. Numerical predictions are compared with unsteady plume electron density data and show good quantitative agreement. Backe ow predictions are presented for the three discharge energy levels considered.

Computation of Hall Thruster Performance

A quasi-neutral one-dimensional hybrid model of a stationary plasma thruster has been used to predict the performance of a laboratory model thruster tested in the Propulsion Ionique pour Vols Orbitaux, Interpretation et Nouvelles Experiences, facility. In this model ions are treated as particles and electrons as a  uid and quasi neutrality is assumed.We describe the calculated efŽ ciencies, thrust, and speciŽ c impulse using xenon and krypton as propellant and their variations as a function of applied voltage and mass  ow rate. Typically, for a voltage of 350 V and an anode mass  ow rate of 5 mg/s, the thrust is on the order of 75 mN for xenon and krypton, and the global efŽ ciency is 40 and 30%, respectively, for xenon and krypton. Although the model is rather simple, the predicted trends are in reasonable agreement with the experimental measurements. The question of doubly charged ions is also examined, and results show that they do not strongly contribute to performance in a stationary plasma thruster-100 Hall thruster.

A new approach to low‐frequency “MHD‐like” waves in magnetospheric plasmas

This paper explores the consequences of enforcing the quasi‐neutrality (QN) condition on a magnetotail and auroral plasma. We show that this condition has never been handled correctly in the literature when the bounce motion of particles is involved, for either adiabatic or stochastic ion motion. Correct treatment yields a global electrostatic potential which is constant along the magnetic field line. We describe how the stability results of Hurricane et al. (1995) are modified. We show that the QN condition modifies the classical MHD analysis of symmetric interchange/ballooning modes with respect to low‐frequency MHD waves. An additional consequence of the QN condition on MHD‐like waves is a modification of the plasma equation of state. More precisely, we show that the plasma remains isothermal during such wave propagation. For antisymmetric modes there is no associated electrostatic potential, but we give the necessary corrections to the standard analysis upon entering a stochastic ion regime. Last, we show how the electrostatic potential couples to the ionosphere, revealing a new understanding of auroral arcs.

Adiabatic and Viscous Simulations of Symmetric Instability: Structure, Evolution, and Energetics

This paper reports on simulations of dry-adiabatic and viscous symmetric instability circulations with a two-dimensional research version of the French Weather Service Limited Area Model PERIDOT. The simulations of an idealized two-dimensional case of a dry symmetric instability presented here constitute a preliminary work to future real case studies. As a first step, the capacity of the model to simulate circulations of the dry inviscid symmetric instability that are in agreement with the linear hydrostatic theory has been verified. This has allowed the design and verification of diagnostic tools. In order to use these diagnostics on simulations of real data cases, a spectral method has been developed to separate the basic state from the symmetric instability circulations. For all of our simulations, the energetics of the circulation show that the circulations draw their kinetic energy mainly from the term linked to the basic vertical shear and the correlation of the vertical and longitudinal components of the motion of the perturbation. Adequate horizontal and vertical resolution and diffusion formulation are found to be necessary to simulate a reasonable numerical evolution of the symmetric instability circulations. In particular, the nonlinear evolution of the dry viscous symmetric instability circulations has been studied by the inclusion of a realistic turbulent diffusion: a complete life cycle is obtained, and a quasi neutrality is reached at the end of the integrations. These simulations provide a first approach to the adjustment problem (i.e., how can the neutrality with respect to symmetric instability, which is frequently observed in real bands, be reached?) and of its representation in a numerical model.

Kinetic models of the solar and polar winds

In this paper the application of the kinetic theory to the collisionless regions of the polar and solar winds is discussed. A brief historical review is given to illustrate the evolution of the theoretical models proposed to explain the main phenomenon and observations. The parallelism between the development of the solar wind models and the evolution of the polar wind theory is stressed especially. The kinetic approaches were in both cases preceded by the hydrodynamic models, and their publication gave rise to animated controversies; later on, semikinetic and hydromagnetic approximations were introduced. A kinetic method, based on the quasi neutrality and the zero current condition in a stationary plasma with open magnetic field lines, is described. The applicability of this approach on the solar and polar winds is illustrated by comparison of the predicted results with the observations. The kinetic models are also compared with hydrodynamic ones. The validity of the criticism and remarks uttered during the Chamberlain‐Parker controversy (for the solar wind), and the dispute between Banks and Holzer on the one hand, and Dessler and Cloutier on the other (for the polar wind), are carefully analyzed. The main result of this study is that both approaches are in fact not contradictory but complementary. The classical hydrodynamic descriptions are only appropriate in the collision‐dominated region, whereas the kinetic theory can be applied only in the collision‐free domain.

Collisionless solar wind, 2, Variable electron temperature

We consider a two-component ‘model’ for the solar wind, in which the protons become collisionless beyond r0≥10 RS, where they are already highly supersonic. The proton temperatures are found from the double adiabatic equation of state. The electrons are highly subsonic, and their temperature profile is prescribed ad hoc. Solar rotation is considered in a semi-self-consistent fashion. The momentum equations for the electrons and protons are solved subject to the conditions of quasi neutrality and zero charge efflux from the sun. The principal results are the following. (1) The proton thermal anisotropy is substantially reduced when solar rotation is considered. We find Tp∥/Tp⊥ < 3 if r0 = 40 RS, while Tp∥/Tp⊥ < 2 if r0 = 55 RS. Wave-particle interactions may therefore play a less significant role in destroying proton anisotropy than has been heretofore thought. (2) The observed dependence of Tp∥/Tp⊥ on solar-wind speed is consistent with collisionless flow, and the double adiabatic equation of state, beyond r0=55 RS, if r0 and υ0 = υ(r0) do not change. (3) Solar rotation leads to significantly lower mean proton temperatures, Tp = ( Tp∥ + 2Tp⊥)/3, than are obtained when the magnetic field is radial. Thus the apparent advantage of collisionless models in reproducing the observed solar-wind conditions does not persist when solar rotation is included. (4) Even slight electron anisotropy, Te∥/Te⊥ ≌ 1.2, in the region r>10 RS reduces the solar-wind speed at the earth by 10–15%, thus worsening the disagreement between observations and the two-fluid model. (5) The electron temperature profile in the supersonic region is the primary parameter determining flow acceleration there; we urge that the details of the electron energy balance, and the possibility of electron heating, be carefully examined in future work.