Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “IONIZATION”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,135 records · Page 63
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Effect of the radio frequency discharge on the dust charging process in a weakly collisional and fully ionized plasma

A close analysis of dust charging process in the presence of radio frequency (RF) discharge on low pressure and fully ionized plasma for both weak and strong discharge's electric field is considered. When the electromagnetic waves pass throughout fully ionized plasma, the collision frequency of the plasma is derived. Moreover, the disturbed distribution function of plasma particles in the presence of the RF discharge is obtained. In this article, by using the Krook model, we separate the distribution function in two parts, the Maxwellian part and the perturbed part. The perturbed part of distribution can make an extra current, so-called the accretion rate of electron (or ion) current, towards a dust particle as a function of the average electron-ion collision frequency. It is proven that when the potential of dust grains increases, the accretion rate of electron current experiences an exponential reduction. Furthermore, the accretion rate of electron current for a strong electric field is relatively smaller than that for a weak electric field. The reasons are elaborated.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Hard disk drive based microsecond x-ray chopper for characterization of ionization chambers and photodiodes

A fast X-ray chopper capable of producing ms long X-ray pulses with a typical rise time of few μs was realized. It is ideally suited to investigate the temporal response of X-ray detectors with response times of the order of μs to ms, in particular, any kind of ionization chambers and large area photo diodes. The drive mechanism consists of a brushless DC motor and driver electronics from a common hard disk drive, keeping the cost at an absolute minimum. Due to its simple construction and small dimensions, this chopper operates at home lab based X-ray tubes and synchrotron radiation sources as well. The dynamics of the most important detectors used in time resolved X-ray absorption spectroscopy, namely, ionization chambers and Passivated Implanted Planar Silicon photodiodes, were investigated in detail. The results emphasize the applicability of this X-ray chopper.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Photoelectron circular dichroism in the multiphoton ionization by short laser pulses. I. Propagation of single-active-electron wave packets in chiral pseudo-potentials

A theoretical method to study the angle-resolved multiphoton ionization of polyatomic molecules is developed. It is based on the time-dependent formulation of the Single Center (TDSC) method and consists in the propagation of single-active-electron wave packets in the effective molecular potentials in the presence of intense laser pulses. For this purpose, the time-dependent Schrödinger equation for one electron, moving in a molecular field and interacting with an arbitrary laser pulse, is solved in spherical coordinates by an efficient numerical approach. As a test, the method is applied to the one- and two-photon ionizations of a model methane-like chiral system by circularly polarized short intense high-frequency laser pulses. Thereby, we analyze the photoelectron circular dichroism (PECD) in the momentum distribution. The considered model application illustrates the capability of the TDSC method to study multiphoton PECD in fixed-in-space and randomly oriented chiral molecules.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Properties of a weakly ionized NO gas sensor based on multi-walled carbon nanotubes

Nitric oxide NO is one of the major targets for environmental monitoring, but the existing NO sensors are limited by their low sensitivity and narrow test range. Here, a NO gas sensor employing multiwalled carbon nanotubes (MWCNTs) was fabricated, and its properties in NO–N{sub 2} mixture were investigated from both emission and ionization. The current I{sub e} passing through the nanotubes cathode was found to decrease with increasing NO concentration and increase linearly in different slopes with the extracting voltage U{sub e}. It is shown that the Schottky barrier of the MWCNTs calculated by I{sub e} increased with NO concentration due to the adsorption of NO gas, which restrained the electron emission and consequently weakened the ionization. The positive ion currents I{sub c} passing through the collecting electrode at different voltages of U{sub e} were found to monotonically decrease with increasing NO concentration, which was induced by both of the reduced electron emission and the consumption of the two excited metastable states N{sub 2}(A{sup 3}∑{sub u}{sup +}) and N{sub 2}(a′{sup 1}∑{sub u}{sup −}) by NO. The sensor exhibited high sensitivity at the low temperature of 30 °C. The calculated conductivity was found to be able to take place of I{sub c} for NO detection in a wide voltage range of 80–150 V U{sub e}.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Time-resolved laser-induced fluorescence measurement of ion and neutral dynamics in a Hall thruster during ionization oscillations

The paper presents spatially and temporally resolved laser-induced fluorescence (LIF) measurements of the xenon ion and neutral velocity distribution functions in a 400 W Hall thruster during natural ionization oscillations at 23 kHz, the so-called “breathing mode.” Strong fluctuations in measured axial ion velocity throughout the discharge current cycle are observed at five spatial locations and the velocity maxima appear in the low current interval. The spatio-temporal evolution of the ion velocity distribution function suggests a propagating acceleration front undergoing periodic motion between the thruster exit plane and ~1 cm downstream into the plume. In conclusion, the ion LIF signal intensity oscillates almost in phase with the discharge current, while the neutral fluorescence signal appears out of phase, indicating alternating intervals of strong and weak ionization.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Equation of state of dense neon and krypton plasmas in the partial ionization regime

The compression behaviors of dense neon and krypton plasmas over a wide pressure-temperature range are investigated by self-consistent fluid variational theory. The ionization degree and equation of state of dense neon and krypton are calculated in the density-temperature range of 0.01–10 g/cm{sup 3} and 4–50 kK. A region of thermodynamic instability is found which is related to the plasma phase transition. The calculated shock adiabat and principal Hugoniot of liquid krypton are in good agreement with available experimental data. The predicted results of shock-compressed liquid neon are presented, which provide a guide for dynamical experiments or numerical first-principle calculations aimed at studying the compression properties of liquid neon in the partial ionization regime.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Recommended direct simulation Monte Carlo collision model parameters for modeling ionized air transport processes

A systematic approach for calibrating the direct simulation Monte Carlo (DSMC) collision model parameters to achieve consistency in the transport processes is presented. The DSMC collision cross section model parameters are calibrated for high temperature atmospheric conditions by matching the collision integrals from DSMC against ab initio based collision integrals that are currently employed in the Langley Aerothermodynamic Upwind Relaxation Algorithm (LAURA) and Data Parallel Line Relaxation (DPLR) high temperature computational fluid dynamics solvers. The DSMC parameter values are computed for the widely used Variable Hard Sphere (VHS) and the Variable Soft Sphere (VSS) models using the collision-specific pairing approach. The recommended best-fit VHS/VSS parameter values are provided over a temperature range of 1000-20 000 K for a thirteen-species ionized air mixture. Use of the VSS model is necessary to achieve consistency in transport processes of ionized gases. The agreement of the VSS model transport properties with the transport properties as determined by the ab initio collision integral fits was found to be within 6% in the entire temperature range, regardless of the composition of the mixture. The recommended model parameter values can be readily applied to any gas mixture involving binary collisional interactions between the chemical species presented for the specified temperature range.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Three species one-dimensional kinetic model for weakly ionized plasmas

A three species one-dimensional kinetic model is presented for a spatially homogeneous weakly ionized plasma subjected to the action of a time varying electric field. Planar geometry is assumed, which means that the plasma evolves in the privileged direction of the field. The energy transmitted to the electric charges is channelized to the neutrals thanks to collisions, a mechanism that influences the plasma dynamics. Charge-charge interactions have been designed as a one-dimensional collision term equivalent to the Landau operator used for fully ionized plasmas. Charge-neutral collisions are modelled by a conservative drift-diffusion operator in the Dougherty's form. The resulting set of coupled integro-differential equations is solved with the stable and robust propagator integral method. This semi–analytical method feasibility accounts for non–linear effects without appealing to linearisation or simplifications, providing conservative physically meaningful solutions even for initial or emerging sharp velocity distribution function profiles. It is found that charge-neutral collisions exert a significant effect since a quite different plasma evolution arises if compared to the collisionless limit. In addition, substantial differences in the system motion are found for constant and temperature dependent collision frequencies cases.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Microhydration of LiOH: Insight from electronic decays of core-ionized states

We compute and compare the autoionization spectra of a core-ionized LiOH molecule both in its isolated and microhydrated states. Stepwise microhydration of LiOH leads to gradual elongation of the Li–OH bond length and finally to molecular dissociation. The accompanying changes in the local environment of the OH{sup −} and Li{sup +} counterions are reflected in the computed O 1s and Li 1s spectra. The role of solvent water molecules and the counterion in the spectral shape formation is assessed. Electronic decays of the microhydrated LiOH are found to be mostly intermolecular since the majority of the populated final states have at least one outer-valence vacancy outside the initially core-ionized ion, mainly on a neighboring water molecule. The charge delocalization occurs through the intermolecular Coulombic and electron transfer mediated decays. Both mechanisms are highly efficient that is partly attributed to hybridization of molecular orbitals. The computed spectral shapes are sensitive to the counterion separation as well as to the number and arrangement of solvent molecules. These sensitivities can be used for studying the local hydration structure of solvated ions in aqueous solutions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Photodissociation of aligned CH3I and C6H3F2I molecules probed with time-resolved Coulomb explosion imaging by site-selective extreme ultraviolet ionization

We explore time-resolved Coulomb explosion induced by intense, extreme ultraviolet (XUV) femtosecond pulses from a free-electron laser as a method to image photo-induced molecular dynamics in two molecules, iodomethane and 2,6-difluoroiodobenzene. At an excitation wavelength of 267 nm, the dominant reaction pathway in both molecules is neutral dissociation via cleavage of the carbon–iodine bond. This allows investigating the influence of the molecular environment on the absorption of an intense, femtosecond XUV pulse and the subsequent Coulomb explosion process. We find that the XUV probe pulse induces local inner-shell ionization of atomic iodine in dissociating iodomethane, in contrast to non-selective ionization of all photofragments in difluoroiodobenzene. The results reveal evidence of electron transfer from methyl and phenyl moieties to a multiply charged iodine ion. In addition, indications for ultrafast charge rearrangement on the phenyl radical are found, suggesting that time-resolved Coulomb explosion imaging is sensitive to the localization of charge in extended molecules.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2018

Ionization instability induced striations in low frequency and pulsed He/H 2 O atmospheric pressure plasmas

In previous work [Kawamura et al., Plasma Sources Sci. Technol. 25, 054009 (2016)] and [Kawamura et al., J. Phys. D: Appl. Phys. 50, 145204 (2017)], 1D kinetic particle-in-cell (PIC) simulations of narrow gap (1 to 4 mm), high frequency (27 MHz) or dc-driven, He/2%H 2 O atmospheric pressure plasmas (APPs) showed an ionization instability resulting in standing striations (spatial oscillations) in the bulk plasma. We developed a steady-state striation theory which showed that the striations are due to non-local electron kinetics. In both the high frequency and dc-driven cases, the equilibrium electron density n 0 in the plasma bulk was stationary. In this work, we first conduct 1D PIC simulations of a 1 mm gap He/2%H 2 O APP, driven by a sinusoidal current at a low frequency of f = 50 kHz such that ω = 2 πf is well below the ionization frequency ν iz . In this case, n 0 varies with time, and we observe a time-varying instability which quasistatically depends on n 0 ( t ). At each phase of the rf cycle, the discharge resembles a dc discharge at the same n 0 . At higher frequencies (200 kHz–1 MHz), ω approaches ν iz , and quasistatic equilibrium at each phase breaks down. The discharge is also driven with a 200 kHz, 50% duty cycle square wave pulse with a short rise and fall time of 0.1 μs in an attempt to directly measure the striation growth rate s during the on-cycle before it saturated. However, the spike in current during the rise time leads to a spike in electron temperature T e and hence ν iz and s at the beginning of the rise which saturated during the beginning of the on-cycle. To predict the instability growth rate and saturation during and after the current spike, we extend our striation theory to include time-varying n 0 , T e , ν iz , as well as terms for the nonlinear saturation and noise floor of the striation amplitude. Furthermore, the time-varying global model predictions are compared to the PIC simulations, showing reasonable agreement.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

An experimental and theoretical study of the C 1s ionization satellites in CH 3 I

We report the C 1s ionization spectrum of CH 3 I has been studied both experimentally and theoretically. Synchrotron radiation has been employed to record polarization dependent photoelectron spectra at a photon energy of 614 eV. These spectra encompass the main-line due to the C 1s single-hole state and the peaks associated with the shake-up satellites. Vertical ionization energies and relative photoelectron intensities have been computed using the fourth-order algebraic-diagrammatic construction approximation scheme for the one-particle Green’s function and the 6-311++G** basis set. The theoretical spectrum derived from these calculations agrees qualitatively with the experimental results, thereby allowing the principal spectral features to be assigned. According to our calculations, two 2 A 1 shake-up states of the C 1s -1 σ CI → σ$*\atop{CI}$ type with singlet and triplet intermediate coupling of the electron spins (S' = 0, 1) play an important role in the spectrum and contribute significantly to the overall intensity. Both of these states are expected to have dissociative diabatic potential energy surfaces with respect to the C–I separation. Whereas the upper of these states perturbs the manifold of Rydberg states, the lower state forms a band which is characterized by a strongly increased width. Our results indicate that the lowest shake-up peak with significant spectral intensity is due to the pair (S' = 0, 1) of 2 E (C 1s -1 I 5p → σ$*\atop{CI}$) states. We predict that these 2 E states acquire photoelectron intensity due to spin-orbit interaction. Lastly, such interactions play an important role here due to the involvement of the I 5p orbitals.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Multi-mode ionization instability induced striations in RF driven He/H 2 O atmospheric pressure plasma (APP) discharges

Previously [E. Kawamura et al., Plasma Sources Sci. Technol. 25, 054009 (2016) and E. Kawamura et al., J. Phys. D: Appl. Phys. 50, 145204 (2017)], one dimensional (1D) particle-in-cell (PIC) simulations of 1 to 4mm gap, He/2%H 2 O atmospheric pressure plasma discharges showed an ionization instability resulting in bulk striations. Assuming that the ionization rate coefficient K iz is related to the root mean square electric field E by K iz &prop; E q , a striation theory showed that q<0 is a necessary condition for the instability. A local calculation yielded q>0, implying that nonlocal electron kinetics are required for the instability. Wider gaps can fit in a wider range of wavelengths, resulting in multi-mode striations. Previously, we assumed a single mean q value for each discharge, and did not calculate q for each mode separately. Here, we develop a wavelength resolved global striation model and apply it to 1D PIC simulations of 4mm gap discharges with input currents J rf = 0.04 to 0.30A/cm 2 . We first examine a base case at 0.23A/cm 2 and observe a mixture of unstable modes within a window of wavelengths λ. At shorter λ, the modes are suppressed by diffusion. At longer λ, we observe a transition to locality in which q becomes less negative with increasing λ, approaching its local positive value and stabilizing the modes. The window of unstable modes shifts to shorter λ with increasing J rf , causing the modes to be suppressed by diffusion at higher J rf . At lower J rf , the decrease in bulk plasma density with decreasing J rf suppresses the striations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Diffusion regime of electron–electron collisions in weakly ionized plasmas

We consider weakly ionized plasma where an elastic scattering of electrons on neutrals changes the individual acts and the rate of electron-electron collisions significantly. In this case, the kinetics of electron thermalization is very different from that in fully ionized plasma. Here, the colliding electrons do not move freely. They, instead, diffuse because of fast scattering on neutrals. We demonstrate how a proper account of this diffusion enables one to estimate the characteristic time of electron thermalization. We also present a rigorous derivation of the kinetic equation for electrons by using Bogolyubov’s method based on Liouville equations for multi-particle distribution functions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Fluid and hybrid simulations of the ionization instabilities in Hall thruster

Low-frequency axial oscillations in the range of 5–50 kHz stand out as a pervasive feature observed in many types of Hall thrusters. While it is widely recognized that the ionization effects play the central role in this mode, as manifested via the large-scale oscillations of neutral and plasma density, the exact mechanism(s) of the instabilities remain unclear. To gain further insight into the physics of the breathing mode and evaluate the role of kinetic effects, a one-dimensional time-dependent full nonlinear low-frequency model describing neutral atoms, ions, and electrons is developed in full fluid formulation and compared to the hybrid model in which the ions and neutrals are kinetic. Both models are quasi-neutral and share the same electron fluid equations that include the electron diffusion, mobility across the magnetic field, and the electron energy evolution. The ionization models are also similar in both approaches. Further, the predictions of fluid and hybrid simulations are compared for different test cases. Two main regimes are identified in both models: one with pure low-frequency behavior and the other one, where the low-frequency oscillations coexist with high-frequency oscillations in the range of 100–200 kHz, with the characteristic time scale of the ion channel fly-by time, 100–200 kHz. The other test case demonstrates the effect of a finite temperature of injected neutral atoms, which has a substantial suppression effect on the oscillation amplitude.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Details of the neutral energy distribution and ionization source using spectrally resolved Balmer-alpha measurements on DIII-D

Spectrally resolved passive Balmer- α (D- α, H- α) measurements from the DIII-D 16 channel edge main-ion charge exchange recombination system confirm the presence of higher energy neutrals (“thermal” neutrals) in addition to the cold neutrals that recycle off the walls in the edge region of DIII-D plasmas. Charge exchange between thermal ions and edge neutrals transfers energy and momentum between the populations giving rise to thermal neutrals with energies approximating the ions in the pedestal region. Multiple charge exchange events in succession allow an electron to effectively take a random walk, transferring from ion to ion, providing a pathway of increasing energy and velocity, permitting a neutral to get deeper into the plasma before a final ionization event that contributes to the ion and electron particle fueling. Spectrally resolved measurements provide information about the density and velocity distribution of these neutrals, which has been historically valuable for validating Monte Carlo neutral models, which include the multi stage charge exchange dynamics. Here, in this study, a multi-channel set of such measurements is used to specifically isolate the details of the thermal neutrals that are responsible for fueling inside the pedestal top. Being able to separate the thermal from the cold emission overcomes several challenges associated with optical filter-based neutral density measurements. The neutral dynamics, deeper fueling by the thermal neutrals, and spectral measurement are modeled with the FIDASIM Monte Carlo collisional radiative code, which also produces synthetic spectra with a shape that is in close agreement with the measurements. By scaling the number of neutrals in the simulation to match the intensity of the thermal emission, we show it is possible to obtain local neutral densities and ionization source rates.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Thermal Weibel instability induced magnetic fields co-exist with linear wakes in laser-ionized plasmas

When a moderately intense, few-picosecond-long laser pulse ionizes gas to produce an underdense plasma column, a linear relativistic plasma wave or wake can be excited by the self-modulation instability that may prove useful for multi-bunch acceleration of externally injected electrons or positrons to high energies in a short distance. At the same time, due to the anisotropic temperature distributions of the ionized plasma electrons, the Weibel instability can self-generate magnetic fields throughout such a plasma on a few picoseconds timescale that can persist even longer than the lifetime of the wake. In the present paper, we first show using simulations that both these effects do indeed co-exist in space and time in the plasma. Using our simulations, we make preliminary estimates of the contribution to the transverse emittance growth of an externally injected beam due to the Weibel magnetic fields in a few-millimeter-long plasma. We then present the results of an experiment that has allowed us to measure the spatiotemporal evolution of the magnetic fields using an ultrashort relativistic electron probe beam. Both the topology and the lifetime of the Weibel instability induced magnetic fields in the experiment are in reasonable agreement with the fields induced by the Weibel instability in the simulations.

43 PARTICLE ACCELERATORS↗
DOE OSTI2024

The ICRP, MELODI, and ALLIANCE workshop on effects of ionizing radiation exposure in offspring and next generations: a summary of discussions

Task Group 121 – Effects of ionizing radiation exposure in offspring and next generations – is a task group under the Committee 1 of the International Commission on Radiological Protection (ICRP), approved by the Main Commission on 18th November 2021. The main goals of Task Group 121 are to (1) review and update the scientific literature of relevance to radiation-related effects in the offspring of parent(s) exposed to ionizing radiation in both human and non-human biota; (2) to assess preconceptional and intrauterine effects of radiation exposure and related morbidity and mortality; and, (3) to provide advice about the level of evidence and how to consider these preconceptional and postconceptional effects in the system of radiological protection for humans and non-human biota. The Task Group is reviewing relevant literature since Publication 90 ‘Biological effects after prenatal irradiation (embryo and fetus)’ (2003) and will include radiation-related effects on future generations in humans, animals, and plants. This review will be conducted to account for the health effects on offspring and subsequent generations in the current system of radiological protection. Radiation detriment calculation will also be reviewed. Finally, preliminary recommendations will be made to update the integration of health effects in offspring and next generations in the system of radiological protection. A Workshop, jointly organized by ICRP Task Group 121 and European Radiation Protection Research Platforms MELODI and ALLIANCE was held in Budapest, Hungary, from 31st May to 2nd June 2022. Participants discussed four important topics: (1) hereditary and epigenetic effects due to exposure of the germ cell line (preconceptional exposure), (2) effects arising from exposure of the embryo and fetus (intrauterine exposure), (3) transgenerational effects on biota, and (4) its potential impact on the system of radiological protection. Based on the discussions and presentations during the breakout sessions, newer publications, and gaps on the current scientific literature were identified. For instance, there are some ongoing systematic reviews and radiation epidemiology reviews of intrauterine effects. There are newer methods of Monte Carlo simulation for fetal dosimetry, and advances in radiation genetics, epigenetics, and radiobiology studies. While the current impact of hereditary effects on the global detriment was reported as small, the questions surrounding the effects of radiation exposure on offspring and the next generation are crucial, recurring, and with a major focus on exposed populations. Here, this article summarizes the workshop discussions, presentations, and conclusions of each topic and introduces the special issue of the International Journal of Radiation Biology resulting from the discussions of the meeting.

63 RADIATION, THERMAL, AND OTHER ENVIRON. POLLUTAN↗