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NASA NTRS1994

The formation of sharp structures by ambipolar diffusion

The effect of ambipolar diffusion is investigated using simple numerical models. Examples are shown where sharp structures develop around magnetic nulls. In contrast to the case of ordinary diffusion, the magnetic field topology is conserved by ambipolar diffusion. This is demonstrated in an example where differential rotation winds up an initially uniform magnetic field and brings oppositely oriented field lines close together. It is argued that ambipolar diffusion produces structures of scales small enough for reconnection to occur.

Brandenburg, Axel↗

NASA NTRS2011

Effects of Solar Wind Conditions on the Plasma Wake Within a Polar Crater: Preliminary Results

As the solar wind sweeps horizontally past a shadowed lunar crater it simultaneously diffuses toward the surface through an ambipolar process, forming a plasma wake (e.g., Figure 1). Importantly, the resulting electric field structure diverts solar wind protons toward the cold crater floor where they may represent a source of surficial hydrogen. We present a handful of two-dimensional kinetic simulations exploring the range of wake structures and surface particle fluxes possible under various background plasma conditions.

Zimmerman, M. I.↗

NASA NTRS1980

Correlations between neutral meridional winds and ion densities observed on AE-E

The southward directed neutral meridional wind and the nighttime ion density simultaneously measured on AE-E for altitudes from 250 to 290 km are compared and found to correlate when the magnetic field inclination is negative and anticorrelate when the inclination is positive. This effect is attributed to neutral winds transporting the ions up or down a field line depending upon the sign of the field-aligned wind and the inclination. Model calculations for the ion density in the presence of a field-aligned wind and ambipolar diffusion are presented. Six examples of this ionospheric response are analyzed and found to be in qualitative and approximate quantitative agreement with the theoretical results.

Wharton, L. E.↗

NASA NTRS1989

Kinetic modeling of the Saturn ring-ionosphere plasma environment

A time-independent kinetic plasma model was developed on the basis of the Li et al. (1988) semikinetic plasma model and was used to study the interaction of the Saturnian ionosphere and ring plasma. The model includes the gravitational magnetic mirror and centripetal and ambipolar electric forces, and the effect of the mixing of two plasma populations. The results obtained indicate that the density, temperature, and composition of plasma near the rings changing in the direction from the inner C ring to the outer A ring, due to the fact that the predominant source of plasma changes from the ionosphere to the rings. The model results also suggest that the outflow of hydrogen from the ionosphere to the rings may be shut off for field lines passing through the outer B and A ring, due to the ambipolar electric field set up by the warm ring plasma trapped near the ring plane by the centipetal force. In these regions, there will be a net flux of O(+) ions from the rings to the ionosphere.

Wilson, G. R.↗

NASA NTRS1988

The polar wind

The classical polar wind is considered to be an ambipolar outflow of thermal plasma from the terrestrial ionosphere at high latitudes. The paper reviews the characteristic features of this flow, with emphasis placed on recent theoretical predictions. Consideration is given to H(+) and He(+) outflow characteristics both with and without convection electric field effects, the collisionless polar wind characteristics (including the formation of temperature anisotropies), and the time-dependent polar wind behavior in response to localized density perturbations and localized heat sources.

Schunk, R. W.↗

NASA NTRS1997

Photoelectron Effects on the Self-Consistent Potential in the Collisionless Polar Wind

The presence of unthermalized photoelectrons in the sunlit polar cap leads to an enhanced ambipolar potential drop and enhanced upward ion acceleration. Observations in the topside ionosphere have led to the conclusion that large-scale electrostatic potential drops exist above the spacecraft along polar magnetic field lines connected to regions of photoelectron production. A kinetic approach is used for the O(+), H(+), and photoelectron (p) distributions, while a fluid approach is used to describe the thermal electrons (e) and self-consistent electric field (E(sub II)) electrons are allowed to carry a flux that compensates for photoelectron escape, a critical assumption. Collisional processes are excluded, leading to easier escape of polar wind particles and therefore to the formation of the largest potential drop consistent with this general approach. We compute the steady state electric field enhancement and net potential drop expected in the polar wind due to the presence of photoelectrons as a function of the fractional photoelectron content and the thermal plasma characteristics. For a set of low-altitude boundary conditions typical of the polar wind ionosphere, including 0.1% photoelectron content, we found a potential drop from 500 km to 5 R(sub E) of 6.5 V and a maximum thermal electron temperature of 8800 K. The reasonable agreement of our results with the observed polar wind suggests that the assumptions of this approach are valid.

Khazanov, G. V.↗

NASA NTRS1999

Particle Simulations of Plasma Detachment in VASIMR

The particle simulations in a Variable Specific Impulse Magnetoplasma Rocket (VASIMR) currently include self-consistent calculation of. 1) stationary magnetic field in plasma, 2) ion density and velocity, 3) ion-cyclotron radio-frequency heating, 4) ambipolar electric field. The assumptions of quasineutral and collissionless plasma are based on the range of operating VASIMR parameters. The main motivation for the particle simulation in VASIMR is plasma detachment from the magnetic field in the exhaust area. The plasma detachment is caused mainly by the Larmor radius increase. The plasma beta effect on detachment is observed and investigated as well. The results of particle simulations are compared with those from MHD simulations.

Ilin, A. V.↗

NASA NTRS2002

Microscopic Foundation and Simulation of Coupled Carrier-Temperature Diffusions in Semiconductor Lasers

A typical semiconductor-based optoelectronic device, such as a diode laser, consists of three subsystems: an optical field, an electron-hole plasma (EHP), and a host crystal lattice. The physics of such a device involves the interplay of optical, electrical and thermal processes. A proper description of such a device requires that all three processes are treated on equal footing and in a self-consistent fashion. Furthermore, since a semiconductor laser has intrinsic spatial inhomogeneity, such a self-consistency naturally leads to a set of partial differential equations in space and time. There is a significant lacking of research interest and results on the transport aspects of optical devices in the literature with only a few exceptions. Even the most important carrier diffusion coefficient has not been properly derived and studied so far for optically excited plasma, while most of the work adopted results from electronics community where heavily doped semiconductors with mainly one type of carriers are dealt with. The corresponding transport equation for plasma energy or temperature has received even less attention. In this talk we describe our recent results on such a self-consistent derivation of temperature and carrier-density diffusion equations coupled with the lasing process. Starting from the microscopic semiconductor Bloch equations (SBEs) including the Boltzmann transport terms in the distribution function equations for electrons and holes, we derived a closed set of diffusion equations for carrier densities and temperatures with self-consistent coupling to Maxwell's equation and to an effective optical polarization equation. The coherent many-body effects are included within the screened Hartree-Fock approximation, while scatterings are treated within the second Born approximation including both the in- and out-scatterings. Microscopic expressions for electron-hole (e-h) and carrier-LO (c-LO) phonon scatterings are directly used to derive the momentum and energy relaxation rates. These rates expressed as functions of temperatures and densities lead to microscopic expressions for self- and mutual-diffusion coefficients in the coupled density-temperature diffusion equations. Approximations for reducing the general two-component description of the electron-hole plasma (EHP) to a single-component one are discussed. In particular, we show that a special single-component reduction is possible when e-h scattering dominates over c-LO phonon scattering. The ambipolar diffusion approximation is also discussed and we show that the ambipolar diffusion coefficients are independent of e-h scattering, even though the diffusion coefficients of individual components depend sensitively on the e-h scattering rates. Our discussions lead to new perspectives into the roles played in the single-component reduction by the electron-hole correlation in momentum space induced by scatterings and the electron-hole correlation in real space via internal static electrical field. Finally, the theory is completed by coupling the diffusion equations to the lattice temperature equation and to the effective optical polarization which in turn couples to the laser field. The equations derived above are implemented in various limiting cases to a typical diode laser to study the consequences of nonlinear diffusion and the cross diffusion terms on laser behavior, especially the dynamic behavior of a diode laser under modulation. Detailed results will be presented by comparing with the standard rate equation results.

Li, J.↗

NASA NTRS2001

A Hydrodynamic Theory for Spatially Inhomogeneous Semiconductor Lasers: Microscopic Approach

Starting from the microscopic semiconductor Bloch equations (SBEs) including the Boltzmann transport terms in the distribution function equations for electrons and holes, we derived a closed set of diffusion equations for carrier densities and temperatures with self-consistent coupling to Maxwell's equation and to an effective optical polarization equation. The coherent many-body effects are included within the screened Hartree-Fock approximation, while scatterings are treated within the second Born approximation including both the in- and out-scatterings. Microscopic expressions for electron-hole (e-h) and carrier-LO (c-LO) phonon scatterings are directly used to derive the momentum and energy relaxation rates. These rates expressed as functions of temperatures and densities lead to microscopic expressions for self- and mutual-diffusion coefficients in the coupled density-temperature diffusion equations. Approximations for reducing the general two-component description of the electron-hole plasma (EHP) to a single-component one are discussed. In particular, we show that a special single-component reduction is possible when e-h scattering dominates over c-LO phonon scattering. The ambipolar diffusion approximation is also discussed and we show that the ambipolar diffusion coefficients are independent of e-h scattering, even though the diffusion coefficients of individual components depend sensitively on the e-h scattering rates. Our discussions lead to new perspectives into the roles played in the single-component reduction by the electron-hole correlation in momentum space induced by scatterings and the electron-hole correlation in real space via internal static electrical field. Finally, the theory is completed by coupling the diffusion equations to the lattice temperature equation and to the effective optical polarization which in turn couples to the laser field.

Li, Jianzhong↗

NASA NTRS1964

Effect of Initial Velocity on One-Dimensional, Ambipolar, Space-Charge Currents

Oppositely directed flows of positive and negative charges constitute an ambipolar current. A general case is analyzed in which the charges traverse the evacuated space between plane-parallel boundaries and all particles of a given charge species are monoenergetic. It is assumed that the charges may possess non-vanishing initial kinetic energy. This represents an extension of analyses by Langmuir and Muller-Lubeck for vanishing initial kinetic energy. Sample calculations of dimensionless current densities, electric and potential fields, and charge-density distributions are exhibited for cases where the electric field is assumed to vanish at one boundary but the species initial velocities may not. It is shown that resulting ion currents may be several times the Child's law limit if initial kinetic energies are of the order of the potential energy.

SPACE CHARGE↗

NASA NTRS1999

Nonlinear Effects in Single-Pass ICRF Heating

The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) concept employs Ion Cyclotron Resonant Frequency (ICRF) heating as the main power deposition mechanism. Since the ions accelerate to the full energy in a single pass through the cyclotron resonance, their response to the RF-field will be essentially nonlinear - hence the motivation to amend the commonly used linear approach to the problem. In a collisionless plasma, the energy gain of an accelerated ion is limited by the time the particle spends at the resonance. This time is affected by: (1) incident flow velocity, (2) longitudinal grad B force, (3) ambipolar electric field, and (4) ponderomotive force of the RF-field. Our analysis shows that the grad B force is the dominant factor at low to moderate levels of RF-power. We present nonlinear scaling for the energy gain and the absorption efficiency with RF-power and plasma parameters. We also demonstrate that the nonlinear regime exhibits a steep decrease in the plasma density at the resonance.

Arefiev, A. V.↗

NASA NTRS2023

Quantifying the Energy Budget in the Solar Wind from 13.3 to 100 Solar Radii

A variety of energy sources, ranging from dynamic processes, such as magnetic reconnection and waves, to quasi steady terms, such as plasma pressure, may contribute to the acceleration of the solar wind. We utilize a combination of charged particle and magnetic field observations from the Parker Solar Probe (PSP) to attempt to quantify the steady-state contribution of the proton pressure, the electric potential, and the wave energy to the solar wind proton acceleration observed by PSP between 13.3 and ∼100 solar radii (R☉). The proton pressure provides a natural kinematic driver of the outflow. The ambipolar electric potential acts to couple the electron pressure to the protons, providing another definite proton acceleration term. Fluctuations and waves, while inherently dynamic, can act as an additional effective steady-state pressure term. To analyze the contributions of these terms, we utilize radial binning of single-point PSP measurements, as well as repeated crossings of the same stream at different distances on individual PSP orbits (i.e., fast radial scans). In agreement with previous work, we find that the electric potential contains sufficient energy to fully explain the acceleration of the slower wind streams. On the other hand, we find that the wave pressure plays an increasingly important role in the faster wind streams. The combination of these terms can explain the continuing acceleration of both slow and fast wind streams beyond 13.3 R☉.

J S Halekas↗

NASA NTRS1982

A numerical study of the effects of ambipolar diffusion on the collapse of magnetic gas clouds

The gravitational collapse of isothermal, nonrotating magnetic gas clouds have been calculated numerically, including the effects of ambipolar diffusion. The fractional ionization in the clouds is approximated by a power-law function of the gas density, f = K/n to the q-power, where K and q are adjustable parameters. Eleven numerical experiments were run, and the results indicate that the asymptotic character of collapse is determined mainly by the value of q and is largely independent of the other parameters characterizing a cloud (e.g., K, cloud mass). In particular, there is nearly a one-to-one correspondence between q and the slope, x, of the central magnetic field strength-gas density relationship. If q is no more than 0.8, a cloud collapses asymptotically, as though the magnetic field were 'frozen' to the neutral matter. The magnetic field strength at the center of a collapsing cloud is strongly amplified during collapse even for values of q of about 1, despite extremely low values of fractional ionization. A discussion of the theoretical basis for this unexpected behavior is given. Possible implications of our results for the problems of magnetic braking of rotating protostars and star formation in general are also presented.

Black, D. C.↗

NASA NTRS2011

The Plasma Wake Downstream of Lunar Topographic Obstacles: Preliminary Results from 2D Particle Simulations

Anticipating the plasma and electrical environments in permanently shadowed regions (PSRs) of the moon is critical in understanding local processes of space weathering, surface charging, surface chemistry, volatile production and trapping, exo-ion sputtering, and charged dust transport. In the present study, we have employed the open-source XOOPIC code [I] to investigate the effects of solar wind conditions and plasma-surface interactions on the electrical environment in PSRs through fully two-dimensional pattic1e-in-cell simulations. By direct analogy with current understanding of the global lunar wake (e.g., references) deep, near-terminator, shadowed craters are expected to produce plasma "mini-wakes" just leeward of the crater wall. The present results (e.g., Figure I) are in agreement with previous claims that hot electrons rush into the crater void ahead of the heavier ions, fanning a negative cloud of charge. Charge separation along the initial plasma-vacuum interface gives rise to an ambipolar electric field that subsequently accelerates ions into the void. However, the situation is complicated by the presence of the dynamic lunar surface, which develops an electric potential in response to local plasma currents (e.g., Figure Ia). In some regimes, wake structure is clearly affected by the presence of the charged crater floor as it seeks to achieve current balance (i.e. zero net current to the surface).

Zimmerman, Michael I.↗

NASA NTRS1995

Collapse and fragmentation of molecular cloud cores. 2: Collapse induced by stellar shock waves

The standard scenario for low-mass star formation involves 'inside-out' collapse of a dense molecular cloud core following loss of magnetic field support through ambipolar diffusion. However, isotopic anomalies in presolar grains and meteoritical inclusions imply that the collapse of the presolar cloud may have been triggered by a stellar shock wave. This paper explores 'outside-in' collapse, that is, protostellar collapse initiated directly by the compression of quiescent dense cloud cores impacted by relatively slow stellar shock waves. A second-order accurate, gravitational hydrodynamics code has been used to study both the spherically symmetrical and three-dimensional evolution of initially centrally condensed, isothermal, self-gravitating, solar-mass cloud cores that are struck by stellar shock waves with velocities up to 25 km/s and postshock temperatures of 10 to 10,000 K. The models show that such mild shock waves do not completely shred and destroy the cloud, and that the dynamical ram pressure can compress the cloud to the verge of self-gravitational collapse. However, compression caused by a high postshock temperature is a considerably more effective means of inducing collapse. Shock-induced collapse produces high initial mass accretion rates (greater than 10(exp -4) solar mass/yr in a solar-mass cloud) that decline rapidly to much lower values, depending on the presence (approximately 10(exp -6) solar mass/yr) or absence (approximately 10(exp -8) to 10(exp -7) solar mass/yr) of an infinite reservoir of mass. Stellar mass accretion rates approximately 10(exp -7) solar mass/yr have been previously inferred from the luminosities of T Tauri stars; balanced mass accretion (stellar rate = envelope rate) at approximately 10(exp -7) solar mass/yr could then be possible if accretion occurs from a finite mass reservoir. Fluid tracers are used to determine what fraction of the stellar shock material is incorporated into the resulting protostellar object and disk; roughly half the impinging material is injected into the collapsing cloud core when there is a high postshock temperature. The models are consistent with a scenario where an AGB star wind triggered the collapse of the presolar cloud while injecting about 0.01 solar mass of matter derived from the AGB star envelope, as has been separately inferred on the basis of nucleosynthesis calculations.

Boss, Alan P.↗