Charge Quasi-neutrality Research Articles

Broadband transistor-injected dual doping quantum cascade laser

A novel design-friendly device called the transistor-injected dual doping quantum cascade laser ( T I - D 2 Q C L ) with two different dopings in each stack of a homogeneous superlattice is proposed. By adjusting the base–emitter bias V be of the bipolar transistor to supply electrons in the dual doping regions, charge quasi-neutrality can be achieved to generate different optical transitions in each cascading superlattice stack. These transitions are then stacked and amplified to contribute to a broad flat gain spectrum. Model calculations of a designed T I - D 2 Q C L show that a broad flat gain spectrum ranging from 9.41 µm to 12.01 µm with a relative bandwidth of 0.24 can be obtained, indicating that the T I - D 2 Q C L with dual doping pattern may open a new pathway to the appealing applications in both mid-infrared and terahertz frequency ranges, from wideband optical generations to advanced frequency comb technologies.

Modeling of the electrically-tunable transistor-injected quantum cascade laser

A detailed quantum mechanical model that assesses the mid-infrared (MIR) and terahertz (THz) wavelength tunability of a three-terminal Transistor-Injected Quantum Cascade Laser (TI-QCL) is presented. It is shown that the device injection efficiency can be considerably enhanced by inserting an i-n layer between the QCL and the base of the transistor to match the quantum impedance between the two regions. Our calculations based on the Schrodinger equation with complex potential boundaries indicate that cascading lasing occurs when charge quasi-neutrality in the superlattice (SL) is achieved with an injection current density of 4.71 kA/cm2, which is comparable to the values obtained in conventional two-terminal QCLs. Our analysis of the transition dipole moments between various quasi-bound states in the QCL SL suggests that the lasing wavelength can vary over a few microns as a function of the electric field at constant current, which indicates that the TI-QCL has potential for tunable MIR and THz sources. Finally, simultaneous multi-color lasing with wide energy separation is anticipated with application in MIR multi-gas detection.

A Free and Fast Three-Dimensional/Two-Dimensional Solar Cell Simulator Featuring Conductive Boundary and Quasi-Neutrality Approximations

Details of Quokka, which is a freely available fast 3-D solar cell simulation tool, are presented. Simplifications to the full set of charge carrier transport equations, i.e., quasi-neutrality and conductive boundaries, result in a model that is computationally inexpensive without a loss of generality. Details on the freely available finite volume implementation in MATLAB are given, which shows computation times on the order of seconds to minutes for a full I-V curve sweep on a conventional personal computer. As an application example, the validity of popular analytical models of partial rear contact cells is verified under varying conditions. Consequently, it is observed that significant errors can occur if these analytical models are used to derive local recombination properties from effective lifetime measurements of test structures.

Charge separation instability in an unmagnetized disk plasma around a Kerr black hole

In almost all of plasma theories for astrophysical objects, we have assumed the charge quasineutrality of unmagnetized plasmas in global scales. This assumption has been justified because if there is a charged plasma, it induces the electric field which attracts the opposite charge, and this opposite charge reduces the charge separation. Here, we report a newly discovered instability which causes a charge separation in a rotating plasma inside of an innermost stable circular orbit (ISCO) around a black hole. The growth rate of the instability is smaller than that of the disk instability even in the unstable disk region and is forbidden in the stable disk region outside of the ISCO. However, this growth rate becomes comparable to that of the disk instability when the plasma density is much lower than a critical density inside of the ISCO. In such case, the charge separation instability would become apparent and cause the charged accretion into the black hole, thus charge the hole.

Physics of Magnetospheric Variability

Many widely used methods for describing and understanding the magnetosphere are based on balance conditions for quasi-static equilibrium (this is particularly true of the classical theory of magnetosphere/ionosphere coupling, which in addition presupposes the equilibrium to be stable); they may therefore be of limited applicability for dealing with time-variable phenomena as well as for determining cause-effect relations. The large-scale variability of the magnetosphere can be produced both by changing external (solar-wind) conditions and by non-equilibrium internal dynamics. Its developments are governed by the basic equations of physics, especially Maxwell’s equations combined with the unique constraints of large-scale plasma; the requirement of charge quasi-neutrality constrains the electric field to be determined by plasma dynamics (generalized Ohm’s law) and the electric current to match the existing curl of the magnetic field. The structure and dynamics of the ionosphere/magnetosphere/solar-wind system can then be described in terms of three interrelated processes: (1) stress equilibrium and disequilibrium, (2) magnetic flux transport, (3) energy conversion and dissipation. This provides a framework for a unified formulation of settled as well as of controversial issues concerning, e.g., magnetospheric substorms and magnetic storms.

Modeling of laser induced plasma expansion in the presence of non-Maxwellian electrons

The one-dimensional expansion into vacuum of ion-electron plasma produced by laser ablation is investigated. The ions considered as an ideal fluid are governed by a fluid model where charge quasineutrality is assumed to prevail, while electron density follows a non-Maxwellian distribution. Showing that the expansion can be described by a self-similar solution, the resulting nonlinear Euler equations are solved numerically. It is found that the deviation of the electrons from Maxwellian distribution gives rise to new asymptotic solutions of physical interest affecting the density and velocity of plasma expansion.

Arrays of nonequilibrium plasmas confined to microcavities: an emerging frontier in plasma science and its applications

A new realm of plasma science has been entered recently with the demonstration of devices in which a nonequilibrium, low temperature plasma is produced within, and essentially confined to, a microcavity having a characteristic dimension (d) between 10 and 100 µm. This development has introduced glow microplasmas exhibiting unique properties with respect to, for example, specific power loading and operation at atmospheric pressure (and beyond). Of equal import is the reduction of d below 100 λD (where λD is the Debye length), and the associated weakening of charge quasineutrality as well as the growing importance of plasma-wall interactions are evident. The general properties of several microcavity plasma devices occupying this region of parameter space are briefly described here, but recent experimental results with arrays (as large as 30 × 30) of Al/Al2O3 devices are emphasized. Several potential applications of single microcavity plasma devices and planar arrays are also discussed.

Effect of electron–phonon energy exchange on thermal wave propagation in semiconductors considering carrier diffusion and recombination

AbstractThe electron, hole, and phonon temperatures are calculated in semiconductors by taking into account the finite carrier diffusion and nonradiative recombination time in the sample. We assume that the energy of the modulated excitation radiation is greater than the energy gap and absorbed at the surface of the semiconductor, therefore, a source of heat and carrier generation at the surface of the sample are time dependent and must be considered in the photothermal theory. Under this situation, the coupled one‐dimensional heat transport (for each quasiparticle system) and carrier diffusion equations in the strong hole–phonon energy interaction approximation and charge quasineutrality condition are solved. Since the nonequilibrium carrier concentration depends sensitively on the electron and phonon fluctuation temperature, the heat power density generated in the sample due to the recombination of the electron–hole pair will be greatly influenced by the inhomogeneous quasiparticle temperature distributions. This latter effect comes through the recombination rate of carriers and it will be considered in the photothermal signal. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ideal magnetohydrodynamic interchanges in low density plasmas

The ideal magnetohydrodynamic equations are usually derived under the assumption VA⪡c, where VA is the Alfvén speed and c is the speed of light. This system of equations is extended to low density plasmas wherein VA can be comparable to or greater than c. This involves relaxation of the usual charge quasineutrality assumption and the inclusion of electromagnetic momentum on par with plasma momentum. The extended system is applied to interchange instabilities in “line-tied” slab geometry as well as to centrifugally confined plasmas. It is found that interchange growth rates are reduced by a factor of 1+VA2∕c2, corresponding to a larger effective mass resulting from the extra electromagnetic momentum. Line tying is unaffected.

Ion Acceleration in a Dipole Vortex in a Laser Plasma Corona

Particle-in-cell simulations show that the inhomogeneity scale of the plasma produced in the interaction of high-power laser radiation with gas targets is of fundamental importance for ion acceleration. In a plasma slab with sharp boundaries, the quasistatic magnetic field and the associated electron vortex structure produced by fast electron beams both expand along the slab boundary in a direction perpendicular to the plasma density gradient, forming an extended region with a quasistatic electric field, in which the ions are accelerated. In a plasma with a smooth density distribution, the dipole magnetic field can propagate toward the lower plasma density in the propagation direction of the laser pulse. In this case, the electron density in an electric current filament at the axis of the magnetic dipole decreases to values at which the charge quasineutrality condition fails to hold. In electric fields generated by this process, the ions are accelerated to energies substantially higher than those characteristic of plasma configurations with sharp boundaries.

Density Structure in a Multicomponent Coronal Loop

We investigate the variation of species densities and electric field along a static model coronal loop consisting of electrons, protons, and heavier ions in a gravitationally stratified stellar atmosphere in an attempt to relate photospheric and coronal loop abundances. The loop plasma is assumed to be confined along a strong magnetic field line, so all forces transverse to the magnetic field are taken to be in balance and the loop can be modeled as a one-dimensional structure. Differential gravitational stratification of the ion species induces a polarization electric field along the loop. By invoking charge quasi-neutrality, we devise an iteration scheme to compute numerical solutions for the species densities and to prescribed accuracy; we also derive approximate analytic solutions. For confined coronal plasma loops that are sufficiently long lived for gravitational settling to occur (e.g., 1 day), severe reduction in coronal ion densities would be expected. Our self-consistent, multicomponent treatment predicts higher loop densities than those predicted by a model that neglects the effect of the heavy ions on the electric field; the density enhancement is an increasing function of the distance along the loop and of the ion charge and ranges from 4% for twice-ionized species to 33% for 14 times-ionized species at the top of an isothermal loop with T = 3 × 106 K and a radius of 109 cm.

Polarkation electric field and Farley-Buneman instability during a precipitation event

An anomalous polarization electric field may be set-up in the lower auroral E-region in response to an electron precipitation event during unstable electrojet conditions. For instance, observations during post-midnight to early morning hours on 06–07 June 1990, using the EISCAT radar facility in Scandinavia, show that the in-situ dynamics of the E-region ionization may be radically affected by the presence of the Farley-Buneman instability. In this case, the measured ion drifts at 105 km height are exceptionally strong and comparable in magnitude with the E × B- drift in the F-region, mapped along the same magnetic fieldline. In this paper we present a model to explain the main features of these observations. We assume a simple relaxation model for the E-region ionization generated by an instantaneous electron precipitation event during diffuse aurora conditions and in the presence of the Farley-Buneman instability. In these conditions and for times smaller than the ionization lifetime (tens of seconds to a few minutes), the induced polarization electric field to restore charge quasi-neutrality may radically increase the ion drift velocity, and effectively decouple the ion motion from the dynamics of the neutral atmosphere.

Ambipolar limit of electron precipitation

In a magnetospheric condition, particles are often precipitated into the ionosphere of the host planet. When a flux tube is not interacting with those neighboring it, charge lost through its open ends to the ionosphere must vanish to ensure charge quasi‐neutrality in the flux tube as a whole. To achieve this, a parallel electric field must be present to impede the faster electron motion. Electric fields so arisen are usually referred to as ambipolar fields. In this paper, we discuss a number of specific aspects of the ambipolar coupling, including the value of the ambipolar potential drop, the effects of loss cone depletion, the outflow of ionospheric electrons extracted by the ambipolar field, and point‐to‐point distribution of the potential. Although the noninteractive assumption for the flux tube restricts the validity of the treatment to cases where field‐aligned currents are very weak, the construction of the model sheds new light on an important problem in magnetospheric physics.

Quasi-exospheric heat flux of solar-wind electrons

Density, bulk-velocity, and heat-flow moments are calculated for truncated maxwellian distributions representing the cool and hot populations of solar-wind electrons, as realized at the base of a hypothetical exosphere. The electrostatic potential is thus calculated by requiring charge quasineutrality and the absence of electrical current. Plasma-kinetic coupling of the cool-electron and proton bulk velocities leads to an increase in the electrostatic potential and a decrease in the heat-flow moment. If the velocities differ by the Alfvén speed along the magnetic field, for example, the potential rises to 72.6 V and the heat flux falls to 2.72×10−2 erg cm−2 s−1. In each case the heat flux is carried mainly by the quasi-exospherichot electrons.