Electron Impact Ionization Rate Research Articles

Impact of the transport of magnetospheric electrons on the composition of the Triton atmosphere

Due to its inclined orbit and the complex geometry of the magnetic field of Neptune, Triton experiences a highly variable magnetic environment. As precipitation of magnetospheric electrons is thought to have a large impact on the Triton atmosphere, a better understanding of the interaction between its atmosphere and the magnetosphere of Neptune is important. We aim to couple a model of the Triton atmosphere with an electron transport model to compute the impact of a varying electron precipitation on the atmospheric composition. We coupled a recent photochemical model of the Triton atmosphere with the electron transport model TRANSPlanets. The inputs of this code were determined from Voyager 2 observations and previous studies. The main inputs were the electron precipitation flux, the orbital scaling factor, and the magnetic field strength. The electron-impact ionization and electron-impact dissociation rates computed by TRANSPlanets were then used in the photochemical model. We also analyzed the model uncertainties. The coupling of the two models enabled us to find an electron density profile, as well as N$_2$ and N number densities, that are consistent with the Voyager 2 observations. We found that photoionization and electron-impact ionization are of the same order, in contrast to the results of previous photochemical models. However, we emphasize that this result depends on the hypotheses we used to determine the input variables of TRANSPlanets. Our model would greatly benefit from new measurements of the magnetic environment of Triton, as well as of the electron fluxes in the Neptune magnetosphere.

Nitrogen admixture-driven electron cooling and plasma bullet dynamics in atmospheric-pressure dc nanosecond-pulsed argon jet plasmas

We present experimental measurements of the electron temperature and density profiles and analyze the dynamics of a plasma bullet at volumetric concentrations of nitrogen admixture, 0%–3%, in an atmospheric-pressure nanosecond-pulsed argon jet plasma. Time-resolved Thomson scattering measurements taken 2.5 mm from the exit plane reveal that the temporal maximum of electron temperature and density reduced by as much as 55% and 29%, respectively, when mixing only 3% nitrogen to pure argon. These trends were consistent across axial locations from 2.5 to 14 mm from the exit plane for both electron temperature and density at nitrogen admixture plasmas. Moreover, the propagation velocity and length of the plasma bullet decreased by 13% while the radius by 23% at 3%-nitrogen admixture when compared to the pure argon jet case. The analysis suggests that the nitrogen admixture causes electron cooling due to inelastic energy losses, which results in a reduced electron density and propagation velocity due to a decrease in the electron-impact ionization rate. It is therefore inferred that the electron cooling mechanism and reduced density at nitrogen admixture will significantly impact the electron-impact excitation rate coefficient of nitrogen as well as the concentration of the precursor species such as N2(A3Σu+).

Effect of CF4 concentration on the discharge dynamics and reactive species distribution of atmospheric nanosecond pulsed He plasma jet

Based on a two dimensional self-consistent model, the effect of different CF4 concentrations on the discharge dynamics characteristics, F-containing species distribution and the distribution of the axial and radial ionization rates of He atmospheric pressure plasma jet (APPJ) has been investigated. In addition, the accuracy of the simulation is verified by comparing the experimental results. The ionization rate of He APPJ enhances with the increase of CF4 from 0% to 2% because of the Penning ionization of CF4 with metastable He while decreases in the case of 3% CF4 owing to the electronegativity and the electron excitation energy loss of CF4. The increase of CF4 concentration and the decrease of the electron density leads to the decrease of CF4 + density at 3% CF4. CF3 + density is determined by the Penning ionization rate of CF4 and the reaction rate of He+ + CF4 → CF3 + + F + He. As CF4 concentration increases from 2% to 3%, the decrease of metastable He and He+ concentration results in the significant decrease of CF3 + density. The densities of CF3 − and F− keep increasing from 1% to 3% CF4 due to the effect of the electron attachment reaction of CF4. With the addition of CF4, the ionization reaction of CF4 is gradually playing the major role in the streamer head, the Penning ionization of CF4 is the main source of electron ionization inside the jet and dominates in the formation of ionization wave tail. As the concentration of CF4 increases from 0% to 2%, the electron impact ionization rates of He and CF4 near the axis increase, resulting in the more uniform radial ionization rate distribution of He + 2% CF4 APPJ.

Maxwellian and non-Maxwellian rate coefficients of the tungsten ions: W46+–W55+

This study focuses on the significance of suprathermal (‘hot’) electrons in the tokamak device. Hot electrons, which follow a non-Maxwellian energy distribution, are high-energy electrons that exert a substantial influence on various processes taking place within the plasma. Our aim was to investigate the influence of non-Maxwellian distribution on the rate coefficients of highly charged tungsten ions. This paper presents Maxwellian and non-Maxwellian electron impact ionization rate coefficients for W46+ to W55+ ions. The cross sections were calculated using the fully relativistic flexible atomic code with level-to-level distorted-wave method. We found that even for a small fraction of hot electrons, the contribution of hot electrons to the rate coefficients is still dominant at low bulk temperature.

Numerical study of discharge characteristics of an atmospheric pressure plasma jet with a coaxial dual-channel inlet

A two-dimensional axisymmetric fluid model was applied to investigate the influence of N2 flow velocity on the discharge characteristics of a He plasma jet with a coaxial dual-channel inlet. Helium working gas flowed in the annular space of a coaxial tube and N2 flowed in a central stainless steel tube powered by a DC voltage. When N2 flow velocity increases from 0 m/s, the jet appears to be stratified, forming the outer side and inner side of the jet, and the electron density on the outside of the jet is much higher than that on the inside. For different N2 flow velocities, the peak densities of He+ and N2(c3π) appear in the jet head, while the peak densities of He* and N2+ both appear at the dielectric nozzle and the jet head. When N2 flow velocity is low, the Penning ionization rate is lower than the electron impact ionization rate, but when N2 flow velocity is high, it is just the opposite, which can increase the concentration of reactive species and contribute to the practical application of the jet. N2 flow velocity not only changes the length and structure of the jet but also controls the uniformity of the distribution of reactive species in the jet, which indicates that there is an optimal N2 flow velocity to make the jet longer and more uniform in space, which will greatly promote the practicality and flexibility of the plasma jet and also provide meaningful insights for optimizing and controlling the characteristics of the plasma jet.

Influence of helium metastable states on the E-H mode transition in an inductively coupled plasma

A helium (He) inductively coupled plasma (ICP) source combined with two diode laser systems with the wavelengths of 396.5 nm (21S–41P) and 1082.9 nm (23S–23P) was configured to investigate the influence of He metastable states on the E-H mode transition. The population densities of 21S and 23S states were measured by laser absorption spectroscopy as the functions of RF power, He pressure, and the distance from RF antenna. Correlations between the E-H mode transition and the population density ratio of the 23S metastable state to the 21S metastable state as well as the line intensity ratio (LIR) of the 33D-23S transition to the 31P–21S transition were investigated. The E-H mode transition of He ICP was analyzed by the electron impact ionization rate and it was confirmed that the 23S metastable state plays a key role in the E-H mode transition.

Estimation of Electron Impact Ionization Rates of Li Using a Non-Maxwellian Distribution Function

We report an estimate of the cross-section and rate of electron-impact ionization of Li. The FAC code (Flexible Atomic Code) is used in order to determine the cross-section and to calculate the level of energy. We evaluate the effect of electron energy distribution functions on the measurement of the ionization rate for a non-Maxwellian energy distribution, if the fraction of hot electrons is small. In several types of plasma, it has been observed that certain (hot) electrons are governed by a non-Maxwellian energy distribution. These electrons affect the line spectra and other characteristics of plasma. By using a non-Maxwellian distribution of energies, we revealed the sensitivity of the electron-impact ionization rate of Li to types of the electron energy distribution and to the fraction of hot electrons.

Solar cycle and seasonal variability of the nightside ionosphere of Mars: Insights from five years of MAVEN observations

Using Mars Atmosphere and Volatile EvolutioN (MAVEN) observations from 2014 to 2019, we characterize how the structure and composition of the nightside ionosphere varies on solar cycle and seasonal timescales. At fixed altitudes between 150 and 200 km, plasma densities vary substantially on these timescales in response to variations in fixed thermospheric pressure levels, which cause the ionosphere to rise and fall in altitude. Additionally, solar cycle and seasonal trends in the electron impact ionization (EII) rate affects nightside densities; higher densities are observed near solar maximum and Mars perihelion (when EII rates are largest), and lower densities are observed near solar minimum and Mars aphelion (when EII rates are smallest). Lastly, densities in the high-altitude (>200 km) nightside ionosphere vary significantly over the solar cycle: topside O+ densities vary by a factor of ∼50 and topside O2+ densities vary by a factor of ∼40. Topside ion densities were relatively stable throughout the solar minimum of 2018–2019.

Velocity-Space Hybridization of Direct Simulation Monte Carlo and a Quasi-Particle Boltzmann Solver

This paper presents a new method for modeling rarefied gas flows based on hybridization of direct simulation Monte Carlo (DSMC) and discrete velocity method (DVM)-based quasi-particle representations of the velocity distribution function. It is aimed at improving the resolution of the tails of the distribution function (compared with DSMC) and computational efficiency (compared with DVM). Details of the method, such as the collision algorithm and the particle merging scheme, are discussed. The hybrid approach is applied to the study of noise in a Maxwellian distribution, computation of electron-impact ionization rate coefficient, as well as numerical simulation of a supersonic Couette flow. The hybrid-based solver is compared with pure DSMC and DVM approaches in terms of accuracy, computational speed, and memory use. It is shown that such a hybrid approach can provide a lower computational cost than a pure DVM approach, while being able to retain accuracy in modeling high-velocity tails of the distribution function. For problems where trace species have a significant impact on the flow physics, the proposed method is shown to be capable of providing better computational efficiency and accuracy compared with standard fixed-weight DSMC.

Magnetron sputtering: determining scaling relations towards real power discharges using 3D particle-in-cell Monte Carlo models

Plasma simulation of glow-magnetized discharges with the particle-in-cell Monte Carlo (PICMC) method is constraint to low current densities because of otherwise huge computational requirements. The present work aims to show, how it is nevertheless possible to extrapolate information for higher current densities similar to realistic lab or industrial conditions by applying a scaling strategy on the simulation. This is demonstrated for a DC magnetron sputtering (DCMS) case study involving the following species: Ar, Ar+, Ti, Ti+ and electrons. The evolution of the electron density is extracted from the simulation and compared with experimental values obtained with a Langmuir probe. A linear relationship between the electron density and the discharge current is highlighted and explained by studying the reaction rates of both ionization and excitation collisions. This allows to scale the reaction rates with the discharge parameters: the Ar-electron impact ionization and excitation rates scale linearly with the discharge current, while the electron impact ionization rate of sputtered species scales quadratically with the discharge current. The simulations also feature propagating plasma instabilities, so-called spokes, but in average, the above-mentioned scaling laws hold. Consequently, the flux of particles at the substrate during a plasma deposition process at realistic power density can be extrapolated from a 3D PICMC simulation at lower power density. Finally, the validity domain of the scaling strategy is discussed in the light of the model constraints.

Model calculation of ionization efficiency in the Martian dayside ionosphere using MAVEN observations

ABSTRACT The ionization efficiency (η), which is defined as the ratio of the electron impact to the photon impact ionization rates, is calculated for the dayside Martian ionosphere using Mars Atmosphere Volatile EvolutioN (MAVEN) observations of neutral density and solar flux as input to the model. To calculate the secondary (electron impact) ionization rates, the photoelectron flux is computed using the Analytical Yield Spectrum (AYS) approach. Model calculations suggest that η should increase consistently as altitude decreases in the lower ionosphere. However, when the secondary ionization rates are calculated using the SWEA/MAVEN measured photoelectron flux, the ionization efficiency remains constant in the region 160–250 km. This behaviour is not in agreement with the theory of the altitude variation of ionization efficiency. The reason for this discrepancy is examined in detail. It is suggested that this disagreement is caused by the altitude-independent nature of MAVEN measured photoelectron flux. In two MAVEN deep dip orbits SWEA measured flux showed an altitudinal variation and the η curve estimated using these observations showed the variation in agreement with the theory. This suggests that the ionization efficiency calculated using SWEA/MAVEN measured photoelectron flux will show an altitude-dependent behaviour only when the instrument can resolve the altitudinal variation in the electron flux. We also calculated the ionization efficiency for two MAVEN dayside deep dip campaigns. The efficiency of CO2 and O showed an increase of 30 per cent and 60 per cent, respectively, at 130 km as compared to its value at 200 km.

Two-temperature Collisional-radiative Modeling of Partially Ionized O2-Ar Mixtures over 8000-10,000 K Behind Reflected Shock Waves.

The collisional excitation kinetics of atomic oxygen was studied behind reflected shock waves using tunable diode laser absorption spectroscopy. A test gas mixture of 1% O2/Ar was shock-heated to temperatures between 8000 and 10,000 K and pressures between 0.15 and 1 atm. The time evolution of the atomic oxygen population in the 3 s 5S0 state was monitored by laser absorption at 777.2 nm. The measured O(3 s 5S0) population revealed multistage behavior that was not observed in previous measurements over a temperature range of 5300-7200 K. To interpret the multistage behavior, a three-level collisional-radiative model for atomic oxygen excitation kinetics was developed. The model utilized two independent temperatures, that is, heavy particle translational temperature Ttr and electron translational temperature Te, to describe the fundamental rate constants of atomic oxygen excitation because of collisions with heavy particles and electrons, respectively. The heavy particle excitation rate was inferred from the early stage of the measurement to be k(3P →5S0) = 3.4 × 10-27 (T/K)0.5(1.061 × 105 + 2 (T/K)) exp(-1.061 × 105 K/T) ± 50% m3 s-1. The electron impact excitation rate constant of oxygen, electron impact, and heavy particle impact ionization rate constants of Argon were modified in the model to match the experimental population time histories. The modified rate parameters are also reported for the temperature range explored in the current study.

Measurement of ionization rate of charge carriers and breakdown characteristics of CVD-grown 4H-SiC diodes under steady magnetic field

The impact ionization rate of electrons and holes in (0001)-oriented 4H-SiC has been measured under steady oblique magnetic field and hence, the influence of magnetic field on the breakdown characteristics of 4H-SiC diodes has been investigated. A set of three diodes having p+–n–n+ structure and another set of three diodes having n+–p–p+ structure have been grown on (0001)-oriented n+ and p+ 4H-SiC substrates using chemical vapor deposition (CVD) technique with different epitaxial layer thicknesses for carrying out the measurements within the electric field range of 2.5 × 108–4.0 × 108 V m−1. The ionization rate data are extracted from the photomultiplication measurements using ultra-violet light source of 260-nm wavelength. The ionization rates are found to be decreased with the increase of the magnetic field strength, especially at low electric fields; this decrement is found to be more pronounced in the ionization rate of electrons than the same of holes. A theoretical model developed by the authors is used to calculate the breakdown voltage of the 4H-SiC diodes. The numerical results are compared with the experimentally measured breakdown characteristics and those are found to be in good agreement within the externally applied magnetic field strengths of 0–1500 Gauss. Breakdown voltage of a diode is found to be increased due to the presence of magnetic field; the maximum values of these said increments are found to be around 5% and 2.6% in p+–n–n+ and n+–p–p+ diodes, respectively. Better magnetic field sensitivity is observed in the diodes having broader epitaxial layer widths.

Relativistic electron impact ionization cross sections of carbon ions and application to an optically thin plasma

Context.Ionization through electron impact is a fundamental process associated with the evolution of the ionic structure and emissivity of astrophysical plasmas. Over several decades substantial efforts have been made to measure and calculate the ionization cross sections of ionization through electron impact of different ions shell by shell, in particular, of carbon ions. Spectral emission codes use electron-impact ionization cross sections and/or rates taken from different experimental and theoretical sources. The theoretical cross sections are determined numerically and include a diversity of quantum mechanical methods. The electron-impact ionization database therefore is not uniform in the methods, which makes it hard to determine the reason for the deviations with regard to experimental data. In many cases only total ionization rates for Maxwell–Boltzmann plasmas are available, which makes calculating inner-shell ionization in collisional-radiative models using thermal and nonthermal electron distribution functions difficult. A solution of this problem is the capability of generating the cross sections with an analytical method using the minimum number of atomic parameters. In this way, uniformity in the database is guaranteed, and thus deviations from experiments are easily identified and traced to the root of the method.Aims.The modified relativistic binary encounter Bethe (MRBEB) method is such a simple analytical scheme based on one atomic parameter that allows determining electron-impact ionization cross sections. This work aims the determination of K- and L-shell cross sections of the carbon atom and ions using the MRBEB method and show their quality by: (i) comparing them with those obtained with the general ionization processes in the presence of electrons and radiation (GIPPER) code and the flexible atomic code (FAC), and (ii) determining their effects on the ionic structure and cooling of an optically thin plasma.Methods.The MRBEB method was used to calculate the inner-shells cross sections, while the plasma calculations were carried out with the collisional+photo ionization plasma emission software (CPIPES). The mathematical methods used in this work comprise a modified version of the double-exponential over a semi-finite interval method for numerical integrations, Gauss-elimination method with scaled partial pivoting for the solution of systems of linear equations, and an iterative least-squares method to determine the fits of ionization cross sections.Results.The three sets of cross sections show deviations among each other in different energy regions. The largest deviations occur near and in the peak maximum. Ion fractions and plasma emissivities of an optically thin plasma that evolves under collisional ionization equilibrium, derived using each set of cross sections, show deviations that decrease with increase in temperature and ionization degree. In spite of these differences, the calculations using the three sets of cross sections agree overall.Conclusions.A simple model like the MRBEB is capable of providing cross sections similar to those calculated with more sophisticated quantum mechanical methods in the GIPPER and FAC codes.

Impact Ionization Rate of Electrons in Bilayer Graphene Nanoribbons

An analytical model is developed for calculating the electric field-dependent electron ionization rate in bilayer graphene nanoribbons. All probable arrangements of optical phonon and inter-electron scattering phenomena before the occurrence of an ionizing collision event have been considered in this model, which is viewed as the multistage scattering model of impact ionization. This model considers both lucky ballistic and lucky drift electrons moving under an applied electric field. The numerical results calculated by using this model are compared with the results obtained from an earlier developed analytical model.

Atomic modeling of photoionization fronts in nitrogen gas

Photoionization fronts play a dominant role in many astrophysical environments but remain difficult to achieve in a laboratory experiment. Recent papers have suggested that experiments using a nitrogen medium held at ten atmospheres of pressure which is irradiated by a source with a radiation temperature of TR ∼ 100 eV can produce viable photoionization fronts. We present a suite of one-dimensional numerical simulations using the Helios multimaterial radiation hydrodynamics code that models these conditions and the formation of a photoionization front. We study the effects of varying the atomic kinetics and radiative transfer model on the hydrodynamics and ionization state of the nitrogen gas, finding that more sophisticated physics, in particular, a multi-angle long characteristic radiative transfer model and a collisional-radiative atomics model, dramatically changes the atomic kinetic evolution of the gas. A photoionization front is identified by computing the ratios between the photoionization rate, the electron impact ionization rate, and the total recombination rate. We find that due to the increased electron temperatures found using more advanced physics that photoionization fronts are likely to form in our nominal model. We report the results of several parameter studies. In one of these, the nitrogen pressure is fixed at ten atmospheres and the source radiation temperature is varied, while in another, the temperature is fixed at 100 eV and the nitrogen pressure is varied. Lower nitrogen pressures increase the likelihood of generating a photoionization front while varying the peak source temperature has little effect.

Enhancement of surface discharge in catalyst pores in dielectric barrier discharges

The generation of high-density plasmas on the surface of porous catalysts is very important for plasma catalysis, as it determines the active surface of the catalyst that is available for the reaction. In this work, we investigate the mechanism of surface and volume plasma streamer formation and propagation near micro-sized pores in dielectric barrier discharges operating in air at atmospheric pressure. A two-dimensional particle-in-cell/Monte Carlo collision model is used to model the individual kinetic behavior of plasma species. Our calculations indicate that the surface discharge is enhanced on the surface of the catalyst pores compared with the microdischarge inside the catalyst pores. The reason is that the surface ionization wave induces surface charging along the catalyst pore sidewalls, leading to a strong electric field along the pore sidewalls, which in turn further enhances the surface discharge. Therefore, highly concentrated reactive species occur on the surfaces of the catalyst pores, indicating high-density plasmas on the surface of porous catalysts. Indeed, the maximum electron impact excitation and ionization rates occur on the pore surface, indicating the more pronounced production of excited state and electron-ion pairs on the pore surface than inside the pore, which may profoundly affect the plasma catalytic process.

Effect of acoustic phonon scattering on impact ionization rate of electrons in monolayer graphene nanoribbons

In this paper, we have studied the behavior of impact ionization rate of electrons in monolayer graphene nanoribbons due to various scattering mechanisms. The effects have been analyzed using the comprehensive analytical expressions, which result from all the possible inelastic interactions before the ionizing collision inside the graphene nanoribbons structures. These interactions include scattering of electrons due to acoustic phonon, inter-carrier scattering and optical phonon. Also, the obtained results are compared with previously reported data. The overestimation in earlier data has been analyzed in detail through the developed analytical expression. In addition to this, the percentage change in electron ionization rate has been calculated by considering it with and without acoustic phonon scattering.

Laboratory Photoionization Fronts in Nitrogen Gas: A Numerical Feasibility and Parameter Study

Abstract Photoionization fronts play a dominant role in many astrophysical situations but remain difficult to achieve in a laboratory experiment. We present the results from a computational parameter study evaluating the feasibility of the photoionization experiment presented in the design paper by Drake et al. in which a photoionization front is generated in a nitrogen medium. The nitrogen gas density and the Planckian radiation temperature of the X-ray source define each simulation. Simulations modeled experiments in which the X-ray flux is generated by a laser-heated gold foil, suitable for experiments using many kJ of laser energy, and experiments in which the flux is generated by a “z-pinch” device, which implodes a cylindrical shell of conducting wires. The models are run using crash, our block-adaptive-mesh code for multimaterial radiation hydrodynamics. The radiative transfer model uses multigroup, flux-limited diffusion with 30 radiation groups. In addition, electron heat conduction is modeled using a single-group, flux-limited diffusion. In the theory, a photoionization front can exist only when the ratios of the electron recombination rate to the photoionization rate and the electron-impact ionization rate to the recombination rate lie in certain ranges. These ratios are computed for several ionization states of nitrogen. Photoionization fronts are found to exist for laser-driven models with moderate nitrogen densities (∼1021 cm−3) and radiation temperatures above 90 eV. For “z-pinch”-driven models, lower nitrogen densities are preferred (<1021 cm−3). We conclude that the proposed experiments are likely to generate photoionization fronts.

Density Structures, Dynamics, and Seasonal and Solar Cycle Modulations of Saturn's Inner Plasma Disk

AbstractWe present statistical results from the Cassini Radio and Plasma Wave Science (RPWS) Langmuir probe measurements recorded during the time interval from orbit 3 (1 February 2005) to 237 (29 June 2016). A new and improved data analysis method to obtain ion density from the Cassini LP measurements is used to study the asymmetries and modulations found in the inner plasma disk of Saturn, between 2.5 and 12 Saturn radii (1 RS=60,268 km). The structure of Saturn's plasma disk is mapped, and the plasma density peak, nmax, is shown to be located at ∼4.6 RS and not at the main neutral source region at 3.95 RS. The shift in the location of nmax is due to that the hot electron impact ionization rate peaks at ∼4.6 RS. Cassini RPWS plasma disk measurements show a solar cycle modulation. However, estimates of the change in ion density due to varying EUV flux is not large enough to describe the detected dependency, which implies that an additional mechanism, still unknown, is also affecting the plasma density in the studied region. We also present a dayside/nightside ion density asymmetry, with nightside densities up to a factor of 2 larger than on the dayside. The largest density difference is found in the radial region 4 to 5 RS. The dynamic variation in ion density increases toward Saturn, indicating an internal origin of the large density variability in the plasma disk rather than being caused by an external source origin in the outer magnetosphere.