Maxwellian Electron Distribution Research Articles

Evolution of nanohillocks by fullerene ion-induced localized plasma

Surface nanostructures etch without chemicals; owing to this, their development is a crucial technical process. Surface nanohillocks may be created by irradiating yttrium iron garnet (YIG) with 30-MeV C60 cluster ions. The nanohillock creation mechanism is disputed. In this study, we propose that the formation mechanism is a plasma collective effect of charged particles that depends on localized rogue waves. Rogue waves will explain YIG surface nanohillock creation using a traditional hydrodynamic plasma model. Analytically solving hydrodynamic ion fluid equations and Maxwellian electron distributions yields a non-linear Schrödinger equation. Solving the latter gives us plausible rogue wave domains. Rogue waves concentrate charged ions from the surroundings into a small, confined zone, generating surface nanohillocks. The relevance of different plasma parameters is highlighted in the rogue wave profile.

Evaluation of the effectiveness of energetic thermal electrons on the excitation of the ionospheric red 6300 Å airglow

In this paper we use previously calculated ionosphere electron temperatures Te in order to estimate the 6300 Å (red) airglow intensity enhancements attributed to thermal electrons from the tail of a Maxwellian electron distribution of sufficiently increased electron temperature. Then, we compare our results with the relevant observed measurements and we comment on the quality of the comparison and the possible causes of the differences. We also calculate the airglow intensity characteristic rise and decay times and compare them to the experimental ones, and we investigate the origin of the electrons which excite the red airglow. In the end, we assess the departure of the ionosphere electron energy distribution from a purely Maxwellian one, and the role of the electron elastic and inelastic collisions with the other components of the ionospheric F-region.

Radiative Recombination Plasma Rate Coefficients for Multiply Charged Ions

Radiative recombination (RR) plasma rate coefficients are often applied to estimate electron densities and temperatures under quite different plasma conditions. Despite their frequent use, however, these rate coefficients are available only for selected (few-electron) ions and isoelectronic sequences, mainly because of the computational efforts required. To overcome this limitation, we report here a (relativistic) cascade model which helps compute fine-structure and shell-resolved as well as total RR plasma rate coefficients for many, if not most, elements of the periodic table. This model is based on Jac, the Jena Atomic Calculator, and supports studies on how the electron is captured in selected levels of the recombined ion, a relativistic (Maxwellian) electron distribution, or how the multipoles beyond the electric-dipole field in the electron-photon interaction affect the RR rate coefficients and, hence, the ionization and recombination dynamics of hot plasma. As a demonstration of this model, we compute, compare, and discuss different RR plasma rate coefficients for initially helium-like ions, with an emphasis especially on Fe24+ ions.

Plasma sheath in the presence of warm ions

AbstractThe numerical solution of a warm ion sheath for an electrode/wall in contact with a low‐density isotropic plasma is analysed for sonic and supersonic flow of ions at the sheath entrance. For this, we consider half Maxwellian electron and water bag ion velocity distribution functions at the sheath edge. The generalized kinetic Bohm criterion is used to calculate the average ion velocity at the sheath edge. A relation between electrode/wall floating potential, sheath edge potential, and ion temperature is derived from the floating electrode/wall condition. The numerical solution of potential profiles for sonic and supersonic ions is calculated with varying ion temperature at different values of sheath edge potential. The results show that for sonic ions, the sheath solution merges smoothly into presehath solution, but for supersonic ions, we have density gradients, which increase with increasing ion temperature.

Kinetic simulations of ignited mode cesium vapor thermionic converters

Cesium vapor thermionic converters are an attractive method of converting high-temperature heat directly to electricity, but theoretical descriptions of the systems have been difficult due to the multi-step ionization of Cs through inelastic electron–neutral collisions. This work presents particle-in-cell simulations of these converters, using a direct simulation Monte Carlo collision model to track 52 excited states of Cs. These simulations show the dominant role of multi-step ionization, which also varies significantly based on both the applied voltage bias and pressure. The electron energy distribution functions are shown to be highly non-Maxwellian in the cases analyzed here. A comparison with previous approaches is presented, and large differences are found in ionization rates due especially to the fact that previous approaches have assumed Maxwellian electron distributions. Finally, an open question regarding the nature of the plasma sheaths in the obstructed regime is discussed. The one-dimensional simulations did not produce stable obstructed regime operation and thereby do not support the double-sheath hypothesis.

Oblique Alfvén waves in a stellar wind environment with dust particles charged by inelastic collisions and by photoionization

ABSTRACT The characteristics of Alfvén waves propagating in a direction oblique to the ambient magnetic field in a stellar wind environment are discussed. A kinetic formulation for a magnetized dusty plasma is adopted considering Maxwellian distributions of electrons and ions, and immobile dust particles electrically charged by absorption of plasma particles and by photoionization. The dispersion relation is numerically solved and the results are compared with situations previously studied where dust particles were not charged by photoionization, which is an important process in a stellar wind of a relatively hot star. We show that the presence of dust causes the shear Alfvén waves to present a region of wavenumber values with zero frequency and that the minimum wavelength for which the mode becomes dispersive again is roughly proportional to the radiation intensity to which the dust grains are exposed. The damping rates of both shear and compressional Alfvén waves are observed to decrease with increasing radiation flux, for the parameters considered. For the particular case where both modes present a region with null real frequency when the radiation flux is absent or weak, it is shown that when the radiation flux is sufficiently strong, the photoionization mechanism may cause this region to get smaller or even to vanish, for compressional Alfvén waves. In that case, the compressional Alfvén waves present non-zero frequency for all wavenumber values, while the shear Alfvén waves still present null frequency in a certain interval of wavenumber values, which gets smaller with the presence of radiation.

Effects of dust particles charged by inelastic collisions and by photoionization on Alfvén waves in a stellar wind

ABSTRACT Using a kinetic description of a homogeneous magnetized dusty plasma with Maxwellian distribution of electrons and protons and dust particles charged by inelastic collisions and by photoionization, we analyse the dispersion relation considering the case where waves and radiation propagate exactly parallel to the ambient magnetic field. The investigation emphasizes the changes that the photoionization process brings to the propagation and damping of the waves in a stellar wind environment, since Alfvén waves are believed to play a significant role in the heating and acceleration processes that take place in the wind. The results show that, in the presence of dust with negative equilibrium electrical charge, the Alfvén mode decouples into the whistler and ion cyclotron modes for all values of wavenumber, but when dust particles acquire neutral or positive values of electrical charge, these modes may couple for certain values of wavenumber. It is also seen that the whistler and ion cyclotron modes present null group velocity in an interval of small wavenumber, and that the maximum value of wavenumber for which the waves are non-propagating is reduced in the presence of the photoionization process. For very small values of wavenumber, the damping rates of the modes could change significantly from very small to very high values if the sign of the dust electrical charge is changed.

Propagation of Large-Amplitude Ion-Acoustic Double Layers in Warm Negative Ion Plasmas

Employing the Sagdeev pseudopotential technique, we have investigated ion-acoustic double layers (IADLs) in multicomponent plasma comprising adiabatic ions (positive and negative) with Maxwellian electron distribution. We have numerically calculated the occurrence range of IADLs in negative ion plasmas and studied the influence of various parameters on Mach number ( $M$ ). The existence of large-amplitude IADLs and corresponding phase portrait is investigated analytically. Our system also supports the coexistence of compressive and rarefactive DLs for certain set of parameters. This present model is applied to study the large-amplitude IADLs in the plasmas containing (Xe $^{+}$ , $\text{F}^{-}$ ), (CS $^{+}$ , Cl $^{-}$ ), (Ar $^{+}$ , $\text{F}^{-}$ ), and ( $\text{H}^{+}\,\,\text{H}^{-}$ ) ion species. Our theoretical predications are found to be a good agreement with the experimental observations of large-amplitude IADLs in (Ar $^{+}$ , SF $_{6}^{-}$ ) negative ion plasma. Present investigations may be helpful to understand the DLs in laboratory and space plasma, where negative ions are present with Maxwellian electrons.

Creation of surface nanometer-scale plasma region by irradiation with slow highly charged ions

Slow highly charged ions (HCIs) were used to create a plasma region of nanometer scale on the surface of different materials. The creation of the plasma in a localized region is accompanied by the formation of surface nanostructures. The plasma expansion approach is used to explain the creation mechanism of HCI-induced nanostructures in lithium niobate single crystals, considering the Maxwellian and non-Maxwellian electron distributions. The numerical solutions of the applied hydrodynamic equations show that the profile of the early stage expanded plasma is independent of the ratio of ion-to-electron temperatures. However, an increase in the temperature ratio leads to a wider expansion domain. In addition, the effect of material constituents ions are studied in terms of the modifications induced in the plasma-expanded region.

Stochastic Turbulent Acceleration in a Fractal Environment

Abstract We analyze the stochastic acceleration of particles inside a fully developed turbulent plasma. It is well known that large-amplitude magnetic fluctuations and coherent structures in such an environment obey a fractal scaling, and our specific aim is to study for the first time the effects of the fractality of these environments on stochastic acceleration. We have shown that an injected Maxwellian energy distribution is heated and forms a high-energy tail in a very short time. Using standard parameters for the low solar corona, the injected Maxwellian distribution of electrons gets heated from the initial 100 eV to 10 KeV, and the power-law index of the high-energy tail is about −2.3. The high-energy tail starts around 100 keV, and reaches 10 MeV. The index of the power-law tail depends on the system size, and it is in good agreement with observed values for realistic system sizes. The heating and acceleration process is very fast (∼2 s). The reason why the acceleration time is so short is that the particles are trapped within small-scale parts of the fractal environment, and their scattering mean free path reduces drastically. The presence of small-scale activity also easily pulls particles from the thermal pool, so there is no need for a seed population. The mean square displacement in space and energy is superdiffusive for the high-energy particles.

Kinetic theory of the electron strahl in the solar wind

ABSTRACT We develop a kinetic theory for the electron strahl, a beam of energetic electrons which propagate from the sun along the Parker-spiral-shaped magnetic field lines. Assuming a Maxwellian electron distribution function in the near-sun region where the plasma is collisional, we derive the strahl distribution function at larger heliospheric distances. We consider the two most important mechanisms that broaden the strahl: Coulomb collisions and interactions with oblique ambient whistler turbulence (anomalous diffusion). We propose that the energy regimes where these mechanisms are important are separated by an approximate threshold, ${\cal E}_\mathrm{ c}$; for the electron kinetic energies ${\cal E}\,\lt\, {\cal E}_\mathrm{ c}$ the strahl width is mostly governed by Coulomb collisions, while for ${\cal E}\,\gt\, {\cal E}_\mathrm{ c}$ by interactions with the whistlers. The Coulomb broadening decreases as the electron energy increases; the whistler-dominated broadening, on the contrary, increases with energy and it can lead to efficient isotropization of energetic electrons and to the formation of the electron halo. The threshold energy ${\cal E}_\mathrm{ c}$ is relatively high in the regions closer to the sun, and it gradually decreases with the distance, implying that the anomalous diffusion becomes progressively more important at large heliospheric distances. At 1 au, we estimate the energy threshold to be about ${\cal E}_\mathrm{ c}\,\sim\, 200\, {\rm eV}$.

Kinetic trajectory simulation method for the multi-component magnetized plasma sheath

Using kinetic theory, the magnetized plasma sheath characteristics with two species of positive ions has been investigated. This work is based on the assumption that the electrons and both ions have cut-off Maxwellian distribution at the sheath entrance. In order to couple the presheath-sheath interface parameters, the 1d3v kinetic coupling equations are used. The most fundamental kinetic Bohm’s condition has been derived for the plasma consisting of two singly charged positive ions. It is found that the presence of a second ion species affects the amplitude of velocity distribution functions of both ion species at the sheath entrance as well as at the wall. The magnitude of cut-off and Maxwellian maximum velocities has same value at the sheath entrance whereas its value increases at the wall by increasing the concentration of the second ion species. Although ion and electron fluxes decrease towards the sheath region, both fluxes attain equal at the wall. In addition, it is shown that the self-consistent potential profile monotonically decreases towards the wall. The cut-off Maxwellian velocity distribution of electrons cause to deviate our simulation results from the theoretical results is less than 0.2% in magnitude.

Electron radiative recombination with a hydrogen-like ion

We survey the results of a long-term study of the process of radiative recombination (RR). A rigorous theory of non-relativistic electron RR with a hydrogen-like ion is used to calculate the total cross section of the process, the effective radiation, the recombination rate coefficient, and the emission coefficient in a plasma with a Maxwellian electron distribution. The exact results are compared with the numerous known asymptotic and interpolation formulas. We propose interpolation formulas which ensure a uniform approximation of all mentioned quantities in a wide range of plasma temperatures.

Impact of non-Maxwellian electron velocity distribution functions on inferred plasma parameters in collective Thomson scattering

Optical collective Thomson scattering provides precise density and temperature measurements in numerous plasma-physics experiments. The accuracy of such measurements depends on the core assumption that the underlying electron distribution functions in under-dense laser-produced plasmas are Maxwellian. A statistically based, quantitative analysis of the errors in the measured electron density and temperature is presented when synthetic data calculated using a non-Maxwellian electron distribution function is fit assuming a Maxwellian electron distribution. Such analysis can lead to errors of up to 50% in temperature and 30% in density, in the specific case of super-Gaussian distributions characteristic of inverse bremsstrahlung heating. Including the proper family of non-Maxwellian electron distribution functions, as a fitting parameter, in Thomson-scattering analysis removes the model-dependent errors in the inferred parameters at a minimal cost to the statistical uncertainty.

Temperature-averaged and total free-free Gaunt factors for κ and Maxwellian distributions of electrons (Corrigendum)

Temperature-averaged and total free-free Gaunt factors for <i>κ</i> and Maxwellian distributions of electrons <i>(Corrigendum)</i>

Time-dependent Ionization in a Steady Flow in an MHD Model of the Solar Corona and Wind

Abstract Time-dependent ionization is important for diagnostics of coronal streamers and pseudostreamers. We describe time-dependent ionization calculations for a three-dimensional magnetohydrodynamic (MHD) model of the solar corona and inner heliosphere. We analyze how non-equilibrium ionization (NEI) influences emission from a pseudostreamer during the Whole Sun Month interval (Carrington rotation CR1913, 1996 August 22 to September 18). We use a time-dependent code to calculate NEI states, based on the plasma temperature, density, velocity, and magnetic field in the MHD model, to obtain the synthetic emissivities and predict the intensities of the Lyα, O vi, Mg x, and Si xii emission lines observed by the SOHO/Ultraviolet Coronagraph Spectrometer (UVCS). At low coronal heights, the predicted intensity profiles of both Lyα and O vi lines match UVCS observations well, but the Mg x and Si xii emission are predicted to be too bright. At larger heights, the O vi and Mg x lines are predicted to be brighter for NEI than equilibrium ionization around this pseudostreamer, and Si xii is predicted to be fainter for NEI cases. The differences of predicted UVCS intensities between NEI and equilibrium ionization are around a factor of 2, but neither matches the observed intensity distributions along the full length of the UVCS slit. Variations in elemental abundances in closed field regions due to the gravitational settling and the FIP effect may significantly contribute to the predicted uncertainty. The assumption of Maxwellian electron distributions and errors in the magnetic field on the solar surface may also have notable effects on the mismatch between observations and model predictions.

Electrostatic Potential Radiated by a Pulsating Charge in a Two‐Electron Temperature Plasma

AbstractMutual impedance experiments have been developed to constrain the plasma bulk properties, such as density and temperature, of ionospheric and later space plasmas, through the electric coupling between an emitter and a receiver electric antennas. So far, the analytical modeling of such instruments has enabled to treat ionospheric plasmas, where charged particles are usually well characterized by Maxwellian electron distributions. With the growth of planetary exploration, mutual impedance experiments are or will be used to constrain space plasma bulk properties. Space plasmas are usually out of local thermodynamic equilibrium; therefore, new methods to calibrate and analyze mutual impedance experiments are now required in such non‐Maxwellian plasmas. To this purpose, this work aims at modeling the electric potential generated in a two‐electron temperature plasma by a pulsating point charge. A numerical method is developed for the computation of the electrostatic potential in a sum of Maxwellian plasmas. After validating the method, the results are used to build synthetic mutual impedance spectra and quantify the effect of a warm electron population on mutual impedance experiments, in order to illustrate how the method could be applied for recent and future planetary space missions, such as Rosetta, BepiColombo, and JUICE. In particular, we show how it enables to separate the densities and temperatures of two different electron populations using in situ measurements from the RPC‐MIP mutual impedance experiment on board Rosetta.

The generation of Ganymede's diffuse aurora through pitch angle scattering

Abstract. Diffuse auroral intensities of neutral atomic oxygen OI λ1356 Å emission on Ganymede due to whistler mode waves are estimated. Pitch angle diffusion of magnetospheric electrons into the loss cone due to resonant wave–particle interaction of whistler mode waves is considered, and the resulting electron precipitation flux is calculated. The analytical yield spectrum approach is used for determining the energy deposition of electrons precipitating into the atmosphere of Ganymede. It is found that the intensities (4–30 R) calculated from the precipitation of magnetospheric electrons observed near Ganymede are inadequate to account for the observational intensities (≤ 100 R). This is in agreement with the conclusions reached in previous works. Some acceleration mechanism is required to energize the magnetospheric electrons. In the present work we consider the heating and acceleration of magnetospheric electrons by electrostatic waves. Two particle distribution functions (Maxwellian and kappa distribution) are used to simulate heating and acceleration of electrons. Precipitation of a Maxwellian distribution of electrons can produce about 70 R intensities of OI λ1356 Å emission for electron temperature of 150 eV. A kappa distribution can also yield a diffuse auroral intensity of similar magnitude for a characteristic energy of about 100 eV. The maximum contribution to the estimated intensity results from the dissociative excitation of O2. Contributions from the direct excitation of atomic oxygen and cascading in atomic oxygen are estimated to be only about 1 and 2 % of the total calculated intensity, respectively. The findings of this work are relevant for the present JUNO and future JUICE missions to Jupiter. These missions will provide new data on electron densities, electron temperature and whistler mode wave amplitudes in the magnetosphere of Jupiter near Ganymede.

Fast, hot electron production and ion acceleration in a helicon inductive plasma

A large, time-averaged, double layer-like plasma potential drop of 80 V over several hundred Debye lengths has been observed in the magnetic expansion region on the Madison Helicon eXperiment. It is operated in an inductive mode at 900 W and low argon operating pressures (0.12–0.20 mTorr) in the collisionless regime. The plasma space potential drop is due to the formation of a double layer-like structure in the magnetic expansion region and is much higher than the potential drop caused by a Boltzmann expansion. With the plasma potential drop, a locally negative potential ion hole region at lower pressures with a higher electron density than ion density has been observed just the downstream of the potential drop region. Two-temperature Maxwellian electron distributions with a warm (Te≈15 eV) and bulk (Te≈5 eV) components are observed just upstream of the double layer validated through a RF compensated Langmuir probe and an optical emission spectroscopy (OES) diagnostics. In the expansion chamber downstream of the double layer-like potential drop, a single warm (Te≈15 eV) Maxwellian electron distribution is observed via both the Langmuir probe and OES diagnostics. Ion beam energies of 65 eV are also observed downstream of the potential drop.

DEPARTURE OF HIGH-TEMPERATURE IRON LINES FROM THE EQUILIBRIUM STATE IN FLARING SOLAR PLASMAS

ABSTRACT The aim of this study is to clarify if the assumption of ionization equilibrium and a Maxwellian electron energy distribution is valid in flaring solar plasmas. We analyze the 2014 December 20 X1.8 flare, in which the Fe xxi 187 Å, Fe xxii 253 Å, Fe xxiii 263 Å, and Fe xxiv 255 Å emission lines were simultaneously observed by the EUV Imaging Spectrometer on board the Hinode satellite. Intensity ratios among these high-temperature Fe lines are compared and departures from isothermal conditions and ionization equilibrium examined. Temperatures derived from intensity ratios involving these four lines show significant discrepancies at the flare footpoints in the impulsive phase, and at the looptop in the gradual phase. Among these, the temperature derived from the Fe xxii/Fe xxiv intensity ratio is the lowest, which cannot be explained if we assume a Maxwellian electron distribution and ionization equilibrium, even in the case of a multithermal structure. This result suggests that the assumption of ionization equilibrium and/or a Maxwellian electron energy distribution can be violated in evaporating solar plasma around 10 MK.