Ion Density Profiles Research Articles

Local electron and ion density control using passive resonant coils in inductively coupled plasma

Control of local electron and ion density using passive resonant coils is experimentally investigated in an inductive argon discharge. Four passive resonant coils are installed under a powered coil; each coil has a fan shape and good azimuthal symmetry. Electron energy probability functions and two-dimensional ion density profiles were measured under both resonant and non-resonant regimes. At non-resonance, almost all of the current flows through the powered coil located in the center of the reactor, and the profiles of the electron and ion density are convex. However, at resonance, a large current flows through the passive resonant coil, and dramatic changes are observed in the electron and ion density profiles. At resonance, the electron and ion densities near the passive resonant coil are increased by 300% compared to the non-resonant condition, and radial distributions become almost flat. Experimental results show that the electron and ion density profiles can be effectively controlled by a passive resonant coil at both low pressure (5 mTorr) and high pressure (50 mTorr). These changes in electron and ion density profiles can be understood by the changes of the electron heating and ionization regions.

Interaction between Charge-Regulated Metal Nanoparticles in an Electrolyte Solution

We present a theory which allows us to calculate the interaction potential between charge-regulated metal nanoparticles inside an acid-electrolyte solution. The approach is based on the recently introduced model of charge regulation which permits us to explicitly-within a specific microscopic model-relate the bulk association constant of a weak acid to the surface association constant for the same weak acid adsorption sites. When considering metal nanoparticles we explicitly account for the effect of the induced surface charge in the conducting core. To explore the accuracy of the approximations, we compare the ionic density profiles of an isolated charge-regulated metal nanoparticle with explicit Monte Carlo simulations of the same model. Once the accuracy of the theoretical approach is established, we proceed to calculate the interaction force between two charge-regulated metal nanoparticles by numerically solving the Poisson-Boltzmann equation with charge regulation boundary condition. The force is then calculated by integrating the electroosmotic stress tensor. We find that for metal nanoparticles the charge regulation boundary condition can be well approximated by the constant surface charge boundary condition, for which a very accurate Derjaguin-like approximation was recently introduced. On the other hand, a constant surface potential boundary condition often used in colloidal literature, shows a significant deviation from the charge regulation boundary condition for particles with large charge asymmetry.

Ionic structure and decay length in highly concentrated confined electrolytes

We use molecular dynamics simulations of the primitive model of electrolytes to study the ionic structure in aqueous monovalent electrolyte solutions confined by charged planar interfaces over a wide range of electrolyte concentrations, interfacial separations, surface charge densities, and ion sizes. The investigations are inspired by recent experiments that have directly measured the increase in the decay length for highly concentrated electrolytes with an increase in concentration. The behavior of ions in the nanoconfinement created by the interfaces is probed by evaluating the ionic density profiles, net charge densities, integrated charges, and decay lengths associated with the screening of the charged interface. The results show the presence of two distinct regimes of screening behavior as the concentration is changed from 0.1M to 2.5M for a wide range of electrolyte systems generated by tuning the interfacial separation, surface charge density, and ionic size. For low concentrations, the integrated charge exhibits a monotonic decay to 0 with a decay length that decreases sharply with increasing concentration. For high concentrations (≳1M), the integrated charge has a non-monotonic behavior signaling charge inversion and formation of structured layers of ions near the interfaces. The decay length under these conditions rises with increasing concentration. To complement the simulation results, a variational approach is developed that produces charge densities with characteristics consistent with those observed in simulations. The results demonstrate the relation between the rise in the strength of steric correlations and the changes in the screening behavior.

Rate coefficients for the reactions of CO[formula omitted] with O: Lessons from MAVEN at Mars

In the lower ionosphere of Mars, the relative density distributions of the major ions CO2+, O2+, and O+ are largely determined by three reactions, including the two channels of the reaction of CO2+ with O, which yield either O2+ (R1) or O+ (R2), and the reaction of O+ with CO2, which yields solely O2+ (R3). There have been only two measurements of the rate coefficients for reactions (R1) and (R2) in the last 50 years, and they are very different (Fehsenfeld et al., 1970; Tenewitz et al., 2018). Although we have carried out a fairly thorough exploration of parameter space in our quest to fit the density profiles of the major ions and O atoms as measured by instruments on the MAVEN spacecraft, we report here only a small fraction of that exploration. In this investigation, we have used the atmosphere of Mars as a laboratory to distinguish between the two sets of rate coefficients. We find that the best fits to the O2+, CO2+, and O+ densities are for the three models in which the Fehsenfeld et al. rate coefficients are adopted, each of which has its advantages. We conclude that the favored O density profile is 1.5±0.5 times the measured profile, and that the rate coefficients measured 50 years ago are much better than those recently measured at explaining the density profiles of O2+, CO2+, and O+ in the Martian ionosphere as we know it.

Macroscopic surface charges from microscopic simulations.

Attaining accurate average structural properties in a molecular simulation should be considered a prerequisite if one aims to elicit meaningful insights into a system's behavior. For charged surfaces in contact with an electrolyte solution, an obvious example is the density profile of ions along the direction normal to the surface. Here, we demonstrate that, in the slab geometry typically used in simulations, imposing an electric displacement field D determines the integrated surface charge density of adsorbed ions at charged interfaces. This allows us to obtain macroscopic surface charge densities irrespective of the slab thickness used in our simulations. We also show that the commonly used Yeh-Berkowitz method and the "mirrored slab" geometry both impose vanishing integrated surface charge densities. We present results both for relatively simple rocksalt (1 1 1) interfaces and the more complex case of kaolinite's basal faces in contact with an aqueous electrolyte solution.

Quasi-linear heat transport induced by ITG turbulence in the presence of impurities

The impurity effects on turbulent transport induced by ion temperature gradient (ITG) turbulence are numerically studied in tokamak plasmas, using a gyrokinetic quasi-linear model. The characteristics of the instability and heat fluxes in the presence of impurity ions are investigated for a broad parameter regime, including temperature and density gradients of main and impurity ions, concentration, charge and mass numbers of impurity ions, magnetic shear as well as wave vector spectrum. The heat fluxes are demonstrated to depend not only on the saturation amplitude of the instability but also on the phase shift between and . The peaking factor of temperature/density profile, defined as the ratio of major radius to gradient scale length when the total turbulent heat flux equals zero, is fitted with linear/quadratic functions. In addition, the contributions from diagonal and off-diagonal terms to heat fluxes are identified in detail, i.e. the main ion heat diffusion are proved to be dominated by off-diagonal (diagonal) terms for regions of weak (strong) ITG. In general, steep temperature gradients of main ions as well as hollow density profiles of impurity ions significantly enhance instability and heat fluxes. However, it is interesting to find that the effect of impurity ions with positive density gradient may transit from the enhancement to reduction of the quasi-linear heat flux of main ions in regions of steep ITG, corresponding to transport barriers (e.g. pedestal of H-mode and I-mode plasmas). Both strong and weak positive magnetic shear decrease heat transport.

THE ROLE OF SUPERTHERMAL ELECTRONS ON THE ESCAPING IONS FROM THE UPPER ATMOSPHERE OF TITAN AND VENUS

The observed escape of ions from the ionosphere of planets and moons is of interest during the last few years. Different mechanisms that can explain the escape of charged particles were assumed. One of the recent mechanisms to explain the ionospheric loss is the plasma expansion approach. In the present work, we investigate the ionic loss from the moon Titan and planet Venus, where their ionospheres suffer from a continuous loss reaches to several tons of ions per day. For this purpose, we use a suitable set of hydrodynamic fluid equations to describe the plasma system in either Titan or Venus. Interestingly, it is observed that superthermal electrons exist in both cases (i.e. Titan and Venus ionosphere). The self-similar transformation is used to reduce the basic equations to ordinary differential equations which can be solved numerically to obtain the profiles of the plasma ion density, velocity, and potential during the expansion interval. Furthermore, it is of interest to examine the role of the superthermal electrons on the expansion profile properties.

Project AMIGA: The Circumgalactic Medium of Andromeda*

Abstract Project AMIGA (Absorption Maps In the Gas of Andromeda) is a survey of the circumgalactic medium (CGM) of Andromeda (M31, ≃ 300 kpc) along 43 QSO sightlines at impact parameters 25 ≤ R ≤ 569 kpc (25 at R ≲ ). We use ultraviolet absorption measurements of Si ii, Si iii, Si iv, C ii, and C iv from the Hubble Space Telescope/Cosmic Origins Spectrograph and O vi from the Far Ultraviolet Spectroscopic Explorer to provide an unparalleled look at how the physical conditions and metals are distributed in the CGM of M31. We find that Si iii and O vi have a covering factor near unity for R ≲ 1.2 and ≲1.9 , respectively, demonstrating that M31 has a very extended ∼104–105.5 K ionized CGM. The metal and baryon masses of the 104–105.5 K CGM gas within are ≳108 and ≳4 × 1010 (Z/0.3 Z ⊙)−1 M ⊙, respectively. There is not much azimuthal variation in the column densities or kinematics, but there is with R. The CGM gas at R ≲ 0.5 is more dynamic and has more complicated, multiphase structures than at larger radii, perhaps a result of more direct impact of galactic feedback in the inner regions of the CGM. Several absorbers are projected spatially and kinematically close to M31 dwarf satellites, but we show that those are unlikely to give rise to the observed absorption. Cosmological zoom simulations of ∼L* galaxies have O vi extending well beyond as observed for M31 but do not reproduce well the radial column density profiles of the lower ions. However, some similar trends are also observed, such as the lower ions showing a larger dispersion in column density and stronger dependence on R than higher ions. Based on our findings, it is likely that the Milky Way has a ∼104–105.5 K CGM as extended as for M31 and their CGM (especially the warm–hot gas probed by O vi) are overlapping.

Solar and Magnetic Control of Minor Ion Peaks in the Dayside Martian Ionosphere

AbstractThe Neutral Gas and Ion Mass Spectrometer of the Mars Atmosphere and Volatile Evolution provides a large data set to explore the ion composition and structure of the Martian ionosphere. Here, the dayside measurements are used to investigate the minor ion density profiles with distinctive peaks above 150 km, revealing a systematic trend of decreasing peak altitude with increasing ion mass. We specifically focus on a subset of species including O+, N /CO+, C+, N+, He+, and O++, all of which are mainly produced via direct photoionization of parent neutrals. Our analysis reveals weak or no variation with solar zenith angle (SZA) in both peak density and altitude, which is an expected result because these ion peaks are located within the optically thin regions subject to the same level of solar irradiance independent of SZA. In contrast, the solar cycle variations of peak density and altitude increase considerably with increasing solar activity, as a result of enhanced photoionization frequency and atmospheric expansion at high solar activities. He+ serves as an exception in that its peak density increases toward large SZA and meanwhile shows no systematic variation with solar activity. The thermospheric He distribution on Mars should play an important role in determining these observed variations. Finally, the peak altitudes for all species are elevated by at least several km within the weakly magnetized regions, possibly attributable to the suppression of vertical diffusion by preferentially horizontal magnetic fields in these regions.

Simulations of electrolyte between charged metal surfaces.

We present a new method for simulating ungrounded charged metal slabs inside an electrolyte solution. The ions are free to move between the interior and exterior regions of the slab-electrolyte system. This leads to polarization of both sides of each slab, with a distinct surface charge induced on each surface. Our simulation method is based on the exact solution of the Poisson equation using periodic Green functions. To efficiently perform the calculations, we decouple the electrostatic energy due to surface polarization from that of purely Coulomb interaction between the ions. This allows us to combine a fast 3D Ewald summation technique with an equally fast calculation of polarization. As a demonstration of the method, we calculate ionic density profiles inside an electrolyte solution and explore charge neutrality violation in between charged metal slabs.

Measurement of Absolute Carbon Ion Density Profile Based on CXRS Diagnostic on HL-2A Tokamak

Measurement of Absolute Carbon Ion Density Profile Based on CXRS Diagnostic on HL-2A Tokamak

Control of electron and ion density profiles via virtual ground position control in an inductively coupled plasma

The effects of capacitive coupling on electron and ion density profiles are studied in an argon inductively coupled plasma. Electron energy probability functions and two-dimensional ion density profiles were measured by changing the termination capacitance from 200 to 1000 pF. Experimental results show that a termination capacitor creates a virtual ground on a coil, and the virtual ground suppresses the local capacitive coupling. At 2 mTorr (non-local electron kinetics), there is little change in the azimuthal electron density distribution for different termination capacitances. However, at 50 mTorr (local electron kinetics), the virtual ground causes each mode (E-mode and H-mode) to have the maximum and minimum points in the azimuthal electron density distribution. As the termination capacitance increases, the virtual ground moves along the coil and the maximum and minimum points of the electron density also move with the virtual ground. These effects are explained by electron dynamics and the power transfer mechanism in each mode (E-mode and H-mode).

Design of a 10 GHz minimum-B quadrupole permanent magnet electron cyclotron resonance ion source

This paper presents a simulation study of a permanent magnet electron cyclotron resonance ion source (ECRIS) with a minimum-B quadrupole magnetic field topology. The magnetic field is made to conform to conventional ECRIS with Bmin/BECR of 0.67 and a last closed magnetic isosurface of 1.86BECR at 10 GHz. The distribution of magnetic field gradients parallel to the field, affecting the electron heating efficiency, cover a range from 0 to 13 T/m, being similar to conventional ECRIS. Therefore it is expected that the novel ion source produces warm electrons and high charge state ions in significant number. Single electron tracking simulations are used to estimate plasma flux distribution on the plasma chamber walls and to provide an estimate of the ion density profile at the extraction slit then used in ion optical simulations demonstrating high transmission through the low energy beam transport. The designed ion source is intended to study if the quadrupole field topology could produce high charge state beams in comparable intensities to conventional ECRIS and efficiently transport them through a low energy beamline, thus paving the way for a superconducting ARC-ECRIS using the same field topology. Furthermore, the prospects of the presented ion source design as an injector of a single-ended accelerator for ion beam analysis are discussed.

Measurement of molybdenum ion density for L-mode and H-mode plasma discharges in the EAST tokamak**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFE031300), the Key Program of Research and Development of Hefei Science Center of China (Grant No. 2017HSC-KPRD002), the National Natural Science Foundation of China (Grant No. 11805231), the Natural Science Foundation of

We report the measurement of total molybdenum ion density for L-mode and H-mode plasmas on EAST using spectral lines observation and calculation based on an impurity transport code. A flat-filed extreme ultraviolet spectrometer with some spatial resolution is used to obtain the radial profiles of molybdenum spectral line emissions. The absolute calibration for the extreme ultraviolet spectrometer is finished by comparing the calculated bremsstrahlung intensity with the readings of CCD detector. Molybdenum ion transport study is performed using the radial ion density profiles and one-dimensional impurity transport code STRAHL. The total molybdenum density profiles are determined from the transport analysis. The molybdenum density during L-mode and H-mode phases are obtained, which are about 3 and 4 orders of magnitude smaller than the electron density, respectively. An inward pinch is found during the H-mode phase that leads to the peaked profile of molybdenum density.

Properties of bulk liquid Pd and Pt and their free liquid surface studied with first principles techniques

We have performed first principles computer simulations in order to study the structural and dynamic properties of bulk liquid Pd and Pt near their melting points. We find good agreement with the available experimental static structure and transport properties, and furthermore we provide more detailed information that is not available from experiments. Additional simulations have also been undertaken so as to study the free liquid surface of both liquid metals. The calculated longitudinal ionic density profile exhibits an oscillatory behavior whose properties have been analyzed. For both metals, the associated intrinsic surface structure factor presents a marked maximum related to surface layering.

Simulation of helium supersonic molecular beam injection in tokamak plasma**Project supported by the Chunhui Program of the Ministry of Education of China (Grant No. Z2017091), the Sichuan Provincial Science Foundation for Distinguished Young Leaders of Disciplines in Science and Technology, China (Grant Nos. 2019JDJQ0051 and 2019JDJQ0050), the National Natural Science Foundation of China (Grant Nos. 11575055 and 11605143), the Fund

To study helium (He) supersonic molecular beam injection (SMBI) into H-mode tokamak plasma, a simplified multicomponent-plasma model under the assumption of quasi-neutral condition is developed and implemented in the frame of BOUT ++. The simulation results show that He species propagate inwards after He SMBI, and are deposited at the bottom of the pedestal due to intensive ionization and weak spreading speed. It is found that almost all injected helium particles strip off all the bounded electrons. He species interact intensively with background plasma along the injection path during He SMBI, making deuterium ion density profile drop at the He-deposited location and resulting in a large electron temperature decreasing, but deuterium ion temperature decreasing a little at the top of the pedestal.

Machine learning surrogates for molecular dynamics simulations of soft materials

Molecular dynamics (MD) simulations accelerated by high-performance computing (HPC) methods are powerful tools to investigate and extract the microscopic mechanisms characterizing the properties of soft materials such as self-assembled nanoparticles, virus capsids, confined electrolytes, and polymeric fluids. In this paper, we extend the idea developed in our earlier work of integrating machine learning (ML) methods with HPC-accelerated MD simulations of soft materials in order to enhance their predictive power and advance their applications for research and educational activities. Parallelized MD simulations of self-assembling ions in nanoconfinement are employed to demonstrate our approach. We find that an artificial neural network-based regression model successfully learns nearly all the interesting features associated with the output ionic density profiles over a broad range of ionic system parameters. The ML model generates predictions that are in excellent agreement with the results from MD simulations. The inference time associated with the ML model is over a factor of 10,000 smaller than the corresponding parallel MD simulation time. Through this demonstration, we introduce a “machine learning surrogate” for MD simulations of soft-matter systems. We develop and deploy a web application on nanoHUB to realize the advantages associated with the ML surrogate. The results demonstrate that the performance of MD simulations can be further enhanced by using ML, enabling rapid and accurate simulation-driven exploration of the soft material design space.

Influence of Polarizability on the Prediction of the Electrical Double Layer Structure in a Clay Mesopore: A Molecular Dynamics Study

The prediction of the water and ion density profiles in the electrical double layer (EDL) is a key insight for macroscopic models that intend to model transport and swelling properties in clay materials. Unfortunately, the structure of the EDL, and especially the ion distribution, cannot be probed directly by measurements, because EDL features are inherently disturbed by direct measurement techniques. In recent years, molecular dynamics has provided a growing set of information on the properties of the EDL, including the properties of the diffuse layer, located beyond the Stern or compact layer, and in which a diffuse cloud of ions screens the remaining uncompensated surface charge. Molecular dynamics results are, however, dependent on the force field used to run the simulations. In this study, we investigated the influence of the choice of a force field on the structural and dynamic properties of the EDL present at montmorillonite surface/water interfaces in a 50 Å wide slit-shaped mesopore. Simulations were run in the presence and in the absence of added NaCl, and the results were compared to ion distributions in the diffuse layer predicted with a Poisson–Boltzmann model. The simulations evidenced the strong influence of the consideration of polarizability of ions and water molecules on the predicted structural and dynamic properties of the EDL. While nonpolarizable force fields gave results in good agreement with the prediction of the Poisson–Boltzmann theory, our tested polarizable force field, PIM, showed very significant deviations, which could be mainly attributed to the enhanced formation of ion pairs.

Theoretical study of pre-ionization by inductive field in tokamaks

A one-dimensional fluid model is developed for a numerical study of pre-ionization in a tokamak device under a given external inductive field. The calculation space is a magnetic field line connecting the bottom and the top limiters, and the spatial profiles of ion and electron densities and their time evolutions are calculated, assuming density growth by the Townsend avalanche. It was found that at a low initial density, electrons are evacuated and exponential growth can be stopped (i.e. stagnation phase), and at a high density, formation of an ambipolar diffusion state may suppress further avalanche by compensating the external inductive field. The numerical study revealed that these two obstacles for pre-ionization can be avoided or mitigated under a certain condition. A high ion impact secondary electron emission efficiency at the limiter, a long field line length and a large Townsend’s coefficient are preferable to avoid the stagnation phase or to resume growth from the stagnation phase. The ambipolar diffusion state can be mitigated or avoided using the following four mechanisms: drift induced diffusion, and curvature drifts, a high ion impact secondary electron emission efficiency and AC inductive field. Among them, only the first is practically useful in weak inductive field large devices. Various analytical expressions describing these behaviors and the conditions for a successful pre-ionization are obtained.

Borophene vs. graphene interfaces: Tuning the electric double layer in ionic liquids

In this work we perform molecular dynamics simulations of mixtures of a prototypical protic ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), with lithium tetrafluoroborate (LiBF4), confined between two borophene walls of three different surface charges, −1, 0 and +1 e/nm2, where e is the elementary charge. The properties of the system are analyzed by means of ionic density profiles, angular orientations of [BMIM]+ cations close to the wall and vibrational densities of states for the salt cations close to the walls. The lateral structure of the first layer close to the surface is also studied on one hand, calculating Minkowski parameters and the Shannon entropy of the patterns of the 2D density maps of the anions placed there and, on the other hand, computing the 2D-Fourier transform of the positions of these anions. Our results are compared with those obtained previously for the same mixtures confined between two graphene walls. Although similarities exist between both cases, interesting differences are observed in the lateral structure that the ionic liquid adopts near borophene interfaces due to their strong anisotropy. In particular, we have observed that borophene induces more markedly ordered 2D patterns in the innermost layer of the ionic liquid electric double layer, specially when they are charged. It is this feature that makes borophene a potential candidate for battery electrode applications with possibilities beyond those of graphene.