Initial Electron Density Research Articles

Ion upflow dependence on ionospheric density and solar photoionization

AbstractMotivated by rocket observations showing a variety of different ionospheric responses to precipitation, this paper explores the influence of the background ionospheric density on upflow resulting from auroral precipitation. Simulations of upflow driven by auroral precipitation were conducted using a version of the Varney et al. (2014) model driven by precipitation characterized by observations made during the 2012 Magnetosphere‐Ionosphere Coupling in the Alfvén resonator rocket mission and using a variety of different initial electron density profiles. The simulation results show that increased initial density before the onset of precipitation leads to smaller electron temperature increases, longer ionospheric heating timescales, weaker ambipolar electric fields, lower upflow speeds, and longer upflow timescales but larger upflow fluxes. The upflow flux can increase even when the ambipolar electric field strength decreases due to the larger number of ions that are accelerated. Long‐term observations from the European Incoherent Scatter (EISCAT) Svalbard radar taken during the International Polar Year support the effects seen in the simulations. This correlation between ionospheric density and ion upflows emphasizes the important role of photoionization from solar ultraviolet radiation, which the EISCAT observations show can increase ionospheric density by as much as an order of magnitude during the summer months.

The initiation of lightning in thunderclouds: The possible influence of metastable nitrogen and oxygen molecules in initiating lightning streamers

AbstractAn examination is made of the possible role of metastable molecules of oxygen and nitrogen to contribute to the difference between the observed breakdown field in air of ~25 kV/cm and the electric fields observed in thunderclouds which generally have a maximum of ~3 kV/cm, which suggest much lower breakdown fields. Analysis of steady state equations for electron, ion, and metastable densities indicates that reductions on the order of a factor of 5 are possible in the electric field necessary to sustain a steady state discharge because of (1) electron detachment from negative ions by the oxygen metastable molecules and (2) increased ionization through deexcitation collisions of electrons with metastable nitrogen vibrational states, assuming high vibrational temperatures of the order of the electron temperature. However, solution of the time‐dependent equations for the particle densities indicates that for a uniform discharge, times on the order of 0.1 s are required for the production of sufficient densities of the metastable particles from initial electron densities of ~1011 cm−3. In such times, electron diffusion would have dispersed electrons from any initial narrow electron pulse postulated to be produced by cosmic rays. It is concluded that the role of the metastable molecules, particularly the nitrogen vibrational states, is unlikely to be significant in the initiation of lightning streamers.

Extended Hückel and Slater’s rule initial guess for real space grid-based density functional theory

The Extended Hückel method is commonly used as an initial guess for Gaussian basis set quantum chemistry calculations. In this work, we combine the Extended Hückel method and Slater’s rules to form an initial guess of the electron density and molecular orbitals for all-electron real-space finite element Kohn–Sham density functional theory (FE-KSDFT). With this approach, the first SCF iteration is close in energy to the converged solution, and a factor of two speedup is obtained vs. a naïve initial electron density composed of a superposition of atomic densities. In addition, because the guessed molecular orbitals are already partially self-consistent with the guess density, we can now employ less robust, but fast, block eigensolvers such as Chebyshev-filtered subspace iteration even in the first SCF iteration to achieve 2–10× further gains vs. a conventional Jacobi–Davidson eigensolver.

Weakly relativistic and space charge effects in interaction of high-power microwave with plasma

In the present work, numerical studies on the effects of weakly relativistic ponderomotive force and space charge in the nonlinear interaction of a high-power microwave beam with a plasma are carried out. It is shown that, the profiles of the electron density and dielectric permittivity contain high peaks, and modulation of wavelength occurs in electron density distribution by increasing the microwave energy flux. In addition, it is indicated that the profiles of the electric and magnetic fields in relativistic regime are lengthened more than non-relativistic regime by increasing the initial electron density and the relativistic effects cause the increase in oscillation wavelength of electron density, dielectric permittivity and space charge field, in comparison with the non-relativistic regime. Finally, the results of the research show that the steepening in electron density distributions and their oscillation wavelength are enhanced, when the relativistic effects appear.

Ohmic heating and space charge effects in microwave-plasma interaction

AbstractThe nonlinear propagation of high power microwave beam in unmagnetized collisional plasma is studied taking into account the ponderomotive force, space charge and Ohmic heating effects. It is shown that the amplitude of electron temperature distribution is enhanced by increasing the microwave energy flux, and decreases when the microwave frequency increases. It is also demonstrated that the steepening of the electron density distribution increases when the amplitude of electron temperature profiles reduces and vice versa. Furthermore, by increasing the initial electron density, the amplitude and number of peaks are decreased, but the electron density distribution, the space charge field and the dielectric permittivity profiles are increased.

Theoretical study of the microwave air breakdown at dielectric surface

Microwave air breakdown at dielectric surface is investigated by numerically solving the fluid-based plasma equations coupled with the Maxwell equations. The plasma formation and microwave scattering and absorption by plasma are investigated by one-dimensional (1D) and two-dimensional (2D) models. In the 1D model, it is found that at the initial stage of microwave breakdown, the plasma develops in the whole plasma region. As time increases, the plasma in the upstream grows much faster than in the downstream. Although the electron density distributions for ne = 0 and j = 0 are different, the microwave reflection, absorption and transmission are almost the same. It is found that the electron number density in the upstream region for 20 mm is larger than for 5 mm. In the 2D model, it is found for TE10 mode that the plasmoid first grows in the middle of waveguide until its density becomes large enough to diffract the incident field, then the plasma region moves along the surface to both sides. The plasma region cannot reach the wall of waveguide, where the electric field is smaller than the breakdown threshold. After comparison between the computational and experimental results, it is found that the simulated absorbed power is larger than the measured one, and the transmitted power is smaller than than measured one. The reason is that the initial electron densities in 1D and 2D simulation are both assumed to cover the whole dielectric surface, but the plasma in experiment develops in a very small region.

Interferometry in Femtosecond Laser Plasma Diagnostics

Electron density dynamics in air, nitrogen and argon femtosecond filament plasma channel is studied under different pressure up to hundreds of picoseconds after ionization by transverse interferometry method. It is revealed that initial electron density in nitrogen plasma rises significantly under the pressure over 4 atmospheres. Optical refractive index anisotropy, which precedes and accompanies ionization, related with intense pulse propagation is revealed.

Effect of plasma density profile inhomogeneity on the self-generated magnetic field

The photon orbital and/or spin angular-momentum absorption in absorbing plasma medium causes the circular motion of electrons and generate the azimuthal component of current density. The axial magnetic field generation due to the azimuthal component of generated current density is known as inverse Faraday effect. In this paper, the axial magnetic field generation in an absorbing plasma with ramped density by the propagation of linear polarized Laguerre-Gaussian beam is taken into consideration. It is shown that the positive slope of the initial electron density causes a considerable enhancement of self-generated axial magnetic field. The magnetic field enhancement depends on the twisted light vortex charge number. The rotation angle of the linearly polarized light which can be employed for measurement of the generated magnetic field, is calculated.

Dynamic Anti-plane Crack Analysis in Functional Graded Piezoelectric Semiconductor Crystals

This paper presents a dynamic analysis of an anti-plane crack in func- tionally graded piezoelectric semiconductors. General boundary conditions and sample geometry are allowed in the proposed formulation. The coupled governing partial differential equations (PDEs) for shear stresses, electric displacement field and current are satisfied in a local weak-form on small fictitious subdomains. The derived local integral equations involve one order lower derivatives than the origi- nal PDEs. All field quantities are approximated by the moving least-squares (MLS) scheme. After performing spatial integrations, we obtain a system of ordinary d- ifferential equations for the involved nodal unknowns. It is noted that the stresses and electric displacement field in functionally graded piezoelectric semiconductors exhibit the same singularities at crack tips as in a homogeneous piezoelectric sol- id. The influence of the initial electron density on the intensity factors and energy release rate is also investigated.

Simulation and design of pre-ionization systems for Alborz tokamak

Alborz tokamak is an educational system for studying plasma phenomena in many physics and engineering experiments. A hot filament and a reverse discharge loop are used in the tokamak as the pre-ionization system. The hot cathode prepares a local initial electron density, then the reverse discharge loop trigger the ionization avalanche by means of inducing a toroidal electric field. The parameters of the hot filament are determined in order to produce the desired electron source. Filament temperature is simulated by using three-dimensional finite element method. The average values of filament temperature and electron density at the plasma core (at the end of pre-ionization process) were calculated and are about 2750 K and 1019m−3, respectively. The resultant electron density and equivalent plasma resistivity due to reverse discharge loop are also calculated. In this paper, the simulation results, optimum structural style, the obtained parameters, the temperature of different parts of the filament and produced electron density are presented and discussed.

Microwave propagation with the gas breakdown

By combining the microwave propagation theory and the gas breakdown theory, the microwave propagation with the gas breakdown is analyzed theoretically. Particle-in-cell/Monte Carlo collision (PIC/MCC) simulations are carried out to verify the theoretical results. Based on this theoretical method, the breakdown phenomenon of the pulse microwave is analyzed. The results show that the product values of the initial electron density and the propagation length are the criterion to distinguish the pulse peak decline breakdown and the pulse width reduction breakdown. Furthermore, the energy transmission is also studied, which shows that the total output energy is approximately independent of the input electric field if the electric field is not extremely large.

Automated nucleic acid chain tracing in real time.

The crystallographic structure solution of nucleotides and nucleotide complexes is now commonplace. The resulting electron-density maps are often poorer than for proteins, and as a result interpretation in terms of an atomic model can require significant effort, particularly in the case of large structures. While model building can be performed automatically, as with proteins, the process is time-consuming, taking minutes to days depending on the software and the size of the structure. A method is presented for the automatic building of nucleotide chains into electron density which is fast enough to be used in interactive model-building software, with extended chain fragments built around the current view position in a fraction of a second. The speed of the method arises from the determination of the 'fingerprint' of the sugar and phosphate groups in terms of conserved high-density and low-density features, coupled with a highly efficient scoring algorithm. Use cases include the rapid evaluation of an initial electron-density map, addition of nucleotide fragments to prebuilt protein structures, and in favourable cases the completion of the structure while automated model-building software is still running. The method has been incorporated into the Coot software package.

Ionic Distribution in Plasma for the Process of Electron-Beam Physical Vapor Deposition

This paper investigates ionic distribution generated by electron beam (EB) during Physical Vapor Deposition (PVD). EB-PVD has a wide range of applications in thermal barrier coatings (TBCs) due to favorable characteristics compared with other coating processes. EB-PVD is an important material coating method that utilizes electron beams as heat sources to evaporate materials, which are then deposited on a substrate. Therefore EB-induced ionic distribution dominates the quality and thickness of the final coating on the substrate. Assuming the EB-generated plasma to be only a function of radial direction, the steady-state equations of continuity and motion combined with Posson’s equation were utilized to analyze the plasma distributions along the radial direction. The available experimental data are also used to validate the model. The results show that the coating efficiency can be improved by decreasing the ratio of the electron thermal energy to the initial ion energy and increasing the ratio of the initial ion density to the initial electron density. The uniformity of coating can be achieved by reducing the initial ion density.

Doping effects on ablation enhancement in femtosecond laser irradiation of silicon.

We have conducted an experimental investigation on highly efficient femtosecond laser micromachining of silicon through N-type doping. We found that the material removal amount has a close relationship with the doping concentration rather than with the doping types. The amount of material removal was enhanced gradually as doping densities increased. When the doping density reached higher than 10(18) cm(-3), the ablation threshold was considerably reduced, up to 15%-20%. The results of the experiment indicate that the high density of initial free electrons by doping is the fundamental reason for efficiency improvement, and bandgap shrinkage also plays an important role. The electrons are excited more easily from the valance band to the conduction band and acquire higher initial kinetic energy, which then promotes the material ablation process.

Fracture analysis in piezoelectric semiconductors under a thermal load

In this paper, we solve the in-plane crack problem in piezoelectric semiconductors under a transient thermal load. General boundary conditions and sample geometry are allowed in the proposed formulation. The coupled governing partial differential equations (PDE) for stresses, electric displacement field and current are satisfied in a local weak-form on small fictitious subdomains. All field quantities are approximated by the moving least-squares (MLS) scheme. After performing the spatial integrations, we obtain a system of ordinary differential equations for the nodal unknowns. The influence of initial electron density on the intensity factors and energy release rate is investigated.

Two-dimensional numerical study of two counter-propagating helium plasma jets in air at atmospheric pressure

In this paper, a computational study of two counter-propagating helium plasma jets in ambient air is presented. A two-dimensional fluid model is applied to investigate the physical processes of the two plasma jets interaction (PJI) driven by equal and unequal voltages, respectively. In all studied cases, the PJI results in a decrease of both plasma bullets propagation velocity. When the two plasma jets are driven by equal voltages, they never merge but rather approach each other around the middle of the gas gap at a minimum approach distance, and the minimal distance decreases with the increase of both the applied voltages and initial electron density, but increases with the increase of the relative permittivity. When the two plasma jets are driven by unequal voltages, we observe the two plasma jets will merge at the position away from the middle of the gas gap. The effect of applied voltage difference on the PJI is also studied.

Study of threshold intensity dependence on the physical processes in the breakdown of N2 by CO2 laser radiation

A modified numerical electron cascade model of Gamal and Abdel Moneim (1987 J. Phys. D: Appl. Phys. 20 757), previously developed by Evans and Gamal (1980 J. Phys. D: Appl. Phys. 3 1447), is used to calculate the breakdown threshold irradiance as a function of gas pressure when irradiating molecular nitrogen with a CO2 laser source under two different experimental conditions, namely, the presence and the absence of initial electron density in the focal volume before laser ignition (Camacho J J et al 2007 J. Phys. B: At. Mol. Opt. Phys. 40 4573). The model solves numerically the time-dependent Boltzmann equation for the electron energy distribution function (EEDF) and a set of rate equations that describe the rate of change of the population of excited states. The model is incorporated into a computer program to obtain the threshold irradiance, which is then compared with the measured ones given by Camacho et al. The comparison showed a reasonable agreement for the two experimental conditions. In addition, the study of the EEDF and its parameters revealed different influences on the physical processes in determining the threshold irradiance for nitrogen breakdown in the two selected gas pressure regimes.

Electron density determination in the divertor volume of ASDEX Upgrade via Stark broadening of the Balmer lines

In this article we present the development of a new diagnostic capable of determining the electron density in the divertor volume of ASDEX Upgrade (AUG). It is based on the spectroscopic measurement of the Stark broadening of the Balmer lines. In this work two approaches of calculating the Stark broadening, i.e. the unified theory and the model microfield method, are compared. It will be shown that both approaches yield similar results in the case of Balmer lines with high upper principal quantum numbers n. In addition, for typical AUG parameters the influence of the Zeeman splitting on the high n Balmer lines is found to be negligible. Moreover, an assumption for the Doppler broadening of Tn = 5 eV, which is the maximum Frank–Condon dissociation energy of recycled neutrals, is sufficient. The initial electron density measurements performed using this method are found to be consistent with both Langmuir probe and pressure gauge data.

Plasma ignition and steady state simulations of the Linac4 H− ion source

The RF heating of the plasma in the Linac4 H(-) ion source has been simulated using a particle-in-cell Monte Carlo collision method. This model is applied to investigate the plasma formation starting from an initial low electron density of 10(12) m(-3) and its stabilization at 10(18) m(-3). The plasma discharge at low electron density is driven by the capacitive coupling with the electric field generated by the antenna, and as the electron density increases the capacitive electric field is shielded by the plasma and induction drives the plasma heating process. Plasma properties such as e(-)/ion densities and energies, sheath formation, and shielding effect are presented and provide insight to the plasma properties of the hydrogen plasma.

Nonlinear electron oscillations in a warm plasma

A class of nonstationary solutions for the nonlinear electron oscillations of a warm plasma are presented using a Lagrangian fluid description. The solution illustrates the nonlinear steepening of an initial Gaussian electron density disturbance and also shows collapse behavior in time. The obtained solution may indicate a class of nonlinear transient structures in an unmagnetized warm plasma.