Distribution Of Positive Ions Research Articles

Influence of positive ions on the beamlet optics for negative-ion neutral beam injectors

Neutral beam injectors are based on the neutralization of ion beams accelerated at the desired energy. In the case of the ITER heating and diagnostic neutral beams, the target heating power translates into stringent requirements on the acceptable beamlet divergence and aiming to allow the beam to reach the fusion plasma. The beamlets composing the accelerated beam are experimentally found to feature a transverse velocity distribution exhibiting two Gaussian components: the well-focused one is referred to as the core component while the rest of the beam, the halo, describes beam particles with much worse optics. The codes that simulate beam extraction and acceleration usually assume that the negative ions move towards the plasma meniscus with a laminar flow (no transverse velocity) or that the transverse velocity distribution can be modelled as a Maxwellian and that the current density is uniformly illuminating the meniscus; under such approximations, the presence of highly divergent components cannot be explained. In this work, we develop a simple test-particle tracing code with Monte Carlo collisions, named ICARO (for Ions Coming Around), to study the transport of negative ions in the extraction region and derive the spatial and velocity distribution of the negative ions at the meniscus (i.e. the plasma boundary where a beamlet is extracted). In particular, the origin of the beamlet halo and its dependence on the source parameters are discussed, highlighting as a key parameter the energy distribution of positive ions in the source plasma.

Experimental and kinetic modeling study of the positive ions in premixed ethylene flames over a range of equivalence ratios

Understanding the ion chemistry in flames is crucial for developing ion sensitive technologies for controlling combustion processes. In this work, we measured the spatial distributions of positive ions in atmospheric-pressure burner-stabilized premixed flames of ethylene/oxygen/argon mixtures in a wide range of equivalence ratios ϕ = 0.4÷1.5. A flame sampling molecular beam system coupled with a quadrupole mass spectrometer was used to obtain the spatial distributions of cations in the flames, and a high mass resolution time-of-flight mass spectrometer was utilized for the identification of the cations having similar m/z ratios. The measured profiles of the flame ions were corrected for the contribution of hydrates formed during sampling in the flames slightly upstream the flame reaction zone. We also proposed an updated ion chemistry model and verified it against the experimental profiles of the most abundant cations in the flames. Our model is based on the kinetic mechanism available in the literature extended with the reactions for C3H5+ cation. Highly accurate W2-F12 quantum chemical calculations were used to obtain a reliable formation enthalpy of C3H5+. The model was found to reproduce properly the measured relative abundance of the key oxygenated cations (viz., CH5O+, C2H3O+) in the whole range of equivalence ratios employed, and the C3H5+ cation abundance in the richest flame with ϕ=1.5, but significantly underpredicts the relative mole fraction of C3H3+, which becomes a key species under fuel-rich conditions. Apart from this, several aromatic and cyclic CxHy cations dominating under fuel-rich conditions were identified. We also considered the most important directions for the further refinement of the mechanism.

Alpha particle thermodynamics in the inner heliosphere fast solar wind

Context.Plasma processes occurring in the corona and solar wind can be probed by studying the thermodynamic properties of different ion species. However, most in situ observations of positive ions in the solar wind are taken at 1 AU, where information on their solar source properties may have been irreversibly erased.Aims.In this study we aim to use the properties of alpha particles at heliocentric distances between 0.3 AU and 1 AU to study plasma processes occurring at the points of observation, and to infer processes occurring inside 0.3 AU by comparing our results to previous remote sensing observations of the plasma closer to the Sun.Methods.We reprocessed the original Helios positive ion distribution functions, isolated the alpha particle population, and computed the alpha particle number density, velocity, and magnetic field perpendicular and parallel temperatures. We then investigated the radial variation of alpha particle temperatures in fast solar wind observed between 0.3 AU and 1 AU.Results.Between 0.3 AU and 1 AU alpha particles are heated in the magnetic field perpendicular direction and cooled in the magnetic field parallel direction. Alpha particle evolution is bounded by the alpha firehose instability threshold, which provides one possible mechanism to explain the observed parallel cooling and perpendicular heating. Closer to the Sun our observations suggest that the alpha particles undergo heating in the perpendicular direction, whilst the large magnetic field parallel temperatures observed at 0.3 AU may be due to the combined effect of double adiabatic expansion and alpha particle deceleration inside 0.3 AU.

A Molecular Dynamics Study to Assess the Positive Ion Distribution and the Effects of Protonation on the N-terminus Region of a Xylanase

Xylanase enzyme families play an important role in many industries due to its ability to digest the hemicellulose within plant cell walls. Developing the enzyme that can withstand a highly acidic environment in a stomach requires an extended information in molecular scale for further protein engineering. In this study, an atomistic molecular dynamics simulation was performed to assess the positive ion distribution pattern on the N-terminus region of a Xylanase enzyme molecule. Then, another series of simulations were performed for the enzyme protonated at the aspartate residues 11 and/or 32 identified by the previous calculation. Conformational analysis showed that ASP32 protonation caused a significant disruption at the N-terminus region, as the nearby arginine residue 31 (ARG31) became free from the Coulomb interaction with the negatively charged ASP32 sidechain and cause the N-terminus region to misfold.

Sheath criterion in constant mean free path collisional plasma with two distinct temperature q-nonextensive electrons

A constant mean free path collisional plasma sheath model is investigated in the presence of two-temperature q-nonextensive electrons and fluid ions. By using Sagdeev potential technique, a modified Bohm sheath criterion is derived and also verified. It is shown that the density distribution of positive ions reduces monotonically when the Bohm velocity lies between the derived limits. The effect of collision on the plasma sheath profiles viz. density, potential, net space charge density and positive ion velocity in the sheath in presence of the proposed plasma configuration is investigated. It is also shown that the increasing values of ion-neutral collisionality leads to a decrease of sheath thickness, increase of sheath potential and net space charge density in the sheath region.

Energy–angle dispersion of accelerated heavy ions at 67P/Churyumov–Gerasimenko: implication in the mass-loading mechanism

The Rosetta spacecraft studied the comet 67P/Churyumov–Gerasimenko for nearly two years. The Ion Composition Analyzer instrument on board Rosetta observed the positive ion distributions in the environment of the comet during the mission. A portion of the comet's neutral coma is expected to get ionized, depending on the comet's activity and position relative to the Sun, and the newly created ions are picked up and accelerated by the solar wind electric field, while the solar wind flow is deflected in the opposite direction. This interaction, known as the mass-loading mechanism, was previously studied by comparing the bulk flow direction of both the solar wind protons and the accelerated cometary ions with respect to the direction of the magnetic and the convective solar wind electric field. In this study, we show that energy–angle dispersion is occasionally observed. We report two types of dispersion: one where the observed motion is consistent with ions gyrating in the local magnetic field and another where the energy–angle dispersion is opposite to that expected from gyration in the local magnetic field. Given that the cometary ion gyro-radius in the undisturbed solar wind magnetic and electric field is expected to be too large to be detected in this way, our observations indicate that the local electric field might be significantly smaller than that of the undisturbed solar wind. We also discuss how the energy–angle dispersion, which is not consistent with gyration, may occur due to spatially inhomogeneous densities and electric fields.

Chemistry in glow discharges of H2/O2 mixtures: diagnostics and modelling

The chemistry of low pressure H2+O2 discharges with different mixture ratios has been studied in a hollow cathode dc reactor. Neutral and positive ion distributions have been measured by mass spectrometry, and Langmuir probes have been used to provide charge densities and electron temperatures. A simple zero order kinetic model including neutral species and positive and negative ions, which takes into account gas-phase and heterogeneous chemistry, has been used to reproduce the global composition of the plasmas over the whole range of mixtures experimentally studied, and allows for the identification of the main physico-chemical mechanisms that may explain the experimental results. To our knowledge, no combined experimental and modelling studies of the heavy species kinetics of low pressure H2 + O2 plasmas including ions has been reported before.As expected, apart from the precursors, H2O is detected in considerable amounts. The model also predicts appreciable concentrations of H and O atoms and the OH radical. The relevance of the metastable species O(1D) and O2(a 1Δg) is analysed. Concerning the charged species, positive ion distributions are dominated by H3O+ for a wide range of intermediate mixtures, while and are the major ions for the higher and lower H2/O2 ratios, respectively. The mixed ions OH+, H2O+ and are also observed in small amounts. Negative ions are shown to have a limited relevance in the global chemistry; their main contribution is the reduction of the electron density available for electron impact processes.

Influence of water vapour on the height distribution of positive ions, effective recombination coefficient and ionisation balance in the quiet lower ionosphere

Abstract. Mesospheric water vapour concentration effects on the ion composition and electron density in the lower ionosphere under quiet geophysical conditions were examined. Water vapour is an important compound in the mesosphere and the lower thermosphere that affects ion composition due to hydrogen radical production and consequently modifies the electron number density. Recent lower-ionosphere investigations have primarily concentrated on the geomagnetic disturbance periods. Meanwhile, studies on the electron density under quiet conditions are quite rare. The goal of this study is to contribute to a better understanding of the ionospheric parameter responses to water vapour variability in the quiet lower ionosphere. By applying a numerical D region ion chemistry model, we evaluated efficiencies for the channels forming hydrated cluster ions from the NO+ and O2+ primary ions (i.e. NO+.H2O and O2+.H2O, respectively), and the channel forming H+(H2O)n proton hydrates from water clusters at different altitudes using profiles with low and high water vapour concentrations. Profiles for positive ions, effective recombination coefficients and electrons were modelled for three particular cases using electron density measurements obtained during rocket campaigns. It was found that the water vapour concentration variations in the mesosphere affect the position of both the Cl2+ proton hydrate layer upper border, comprising the NO+(H2O)n and O2+(H2O)n hydrated cluster ions, and the Cl1+ hydrate cluster layer lower border, comprising the H+(H2O)n pure proton hydrates, as well as the numerical cluster densities. The water variations caused large changes in the effective recombination coefficient and electron density between altitudes of 75 and 87 km. However, the effective recombination coefficient, αeff, and electron number density did not respond even to large water vapour concentration variations occurring at other altitudes in the mesosphere. We determined the water vapour concentration upper limit at altitudes between 75 and 87 km, beyond which the water vapour concentration ceases to influence the numerical densities of Cl2+ and Cl1+, the effective recombination coefficient and the electron number density in the summer ionosphere. This water vapour concentration limit corresponds to values found in the H2O-1 profile that was observed in the summer mesosphere by the Upper Atmosphere Research Satellite (UARS). The electron density modelled using the H2O-1 profile agreed well with the electron density measured in the summer ionosphere when the measured profiles did not have sharp gradients. For sharp gradients in electron and positive ion number densities, a water profile that can reproduce the characteristic behaviour of the ionospheric parameters should have an inhomogeneous height distribution of water vapour.

Effects of the magnetic field strength on the modulated pulsed power magnetron sputtering of metallic films

The influence of the magnetic field strength (B) on the deposition rate and plasma properties for Ta, Cr, Ti, Al, Cu materials using the modulated pulsed power (MPP) magnetron sputtering technique in a closed field unbalanced magnetron sputtering system was investigated. The MPP deposition rates were compared to those obtained from the films deposited by direct current magnetron sputtering (DCMS) under similar experimental conditions. The time averaged ion energy and mass distributions of positive ions in the MPP plasmas at different magnetic field strengths were compared, using a Hiden electrostatic quadrupole plasma mass spectrometer. The effects of the repetition frequency and pulse length on the MPP deposition rate were investigated. For a given target power, the MPP deposition rate increased when the repetition frequency was increased. It also increased as the pulse length was increased at a constant repetition frequency and target power. The MPP deposition rate is strongly material dependent. The MPP deposition rate increased as B decreased for a given target power. For a B of 550 G, the RMPP/RDCMS ratio for Cu was in a range of 0.81–1.02, for Al it was 0.84–1.01, for Cr it was 0.64–1.01, for Ti it was 0.52–0.89, and for Ta it was 0.47–0.84. For a B of 350 G, the RMPP/RDCMS ratio for Cu was increased to 1.03–1.07, for Al it was 0.94–1.04, for Cr it was 0.8–1.03, for Ti it was 0.79–0.94, and for Ta it was 0.72–0.88. However, a decrease in the ionization of metal and gas species was observed as B was decreased, which affected the microstructure and mechanical properties of the deposited Cr films.

Ion distribution measurements to probe target and plasma processes in electronegative magnetron discharges. II. Positive ions

Spectra of the ion mass and energy distributions of positive ions in reactive (Ar/O2) and nonreactive (Ar) dc magnetron sputtering discharges have been investigated by energy-resolved mass spectrometry. The results of three sputter target materials, i.e., Cu, In, and W are compared to each other. Besides the main gas constituents, mass spectra reveal a variety of molecular ions which are dependent on the target material. In reactive mode, ArO+ is always observed in Ar/O2 but molecules containing Ar and the metal were exclusively found for the Cu target. The occurrence of the different ions is explained in the context of their bond strengths obtained from density functional theory calculations. The energy spectra generally contain the known low-energy peak corresponding to the plasma potential. Differently extended high-energy tails due to sputtered material were observed for the different targets. Besides these, high-energetic ions were detected with up to several 100 eV. Their energies are significantly different for Ar+ and O+ with Ar+ strongly depending on the target material. The spectra are discussed together with results from transport of ions in matter (TRIM) calculation to elucidate the origin of these energetic ions.

Nonequilibrium radio frequency plasma interacting with a surface

In this paper, we discuss the successive results of our series of investigations into a two-frequency capacitively coupled plasma in CF4(5%)/Ar, sustained by a very high frequency source and biased by a low frequency source in a collision-dominated region. The nonequilibrium in a collisional radiofrequency plasma is studied in terms of the velocity distribution of charged particles. In the bulk plasma, especially, we discuss the velocity distribution of electrons influenced by the quantum structure of a feed gas molecule, and in the passive ion sheath, we predict the flux velocity distribution of positive ions. We then investigate the active species interacting with a patterned SiO2 wafer surface and predict each species' contribution to the feature profile, and discuss the relationship between the pattern size and the etch rate.

Online aerosol mass spectrometry of single micrometer‐sized particles containing poly(ethylene glycol)

The analysis of poly(ethylene glycol) (PEG)-containing particles by online single particle aerosol mass spectrometers equipped with laser desorption/ionization (LDI) is reported. We demonstrate that PEG-containing particles are useful in the development of aerosol mass spectrometers because of their ease of preparation, low cost, and inherently recognizable mass spectra. Solutions containing millimolar quantities of PEGs were nebulized and, after drying, the resultant micrometer-sized PEG-containing particles were sampled. LDI (266 nm) of particles containing NaCl and PEG molecules of average molecular weight<500 Da generated mass spectra reminiscent of mass spectra of PEG collected by other mass spectrometer platforms including the characteristic distribution of positive ions (Na+ adducts) separated by the 44 m/z units of the ethylene oxide units separating each degree of polymerization. PEGs of average molecular weight>500 Da were detected from particles that also contained the tripeptide tyrosine-tyrosine-tyrosine or 2,5-dihydroxybenzoic acid, which were added to nebulized solutions to act as matrices to assist LDI using pulsed 266 nm and 355 nm lasers, respectively. Experiments were performed on two aerosol mass spectrometers, one reflectron and one linear, that each utilize two time-of-flight mass analyzers to detect positive and negative ions created from a single particle. PEG-containing particles are currently being employed in the optimization of our bioaerosol mass spectrometers for the application of measurements of complex biological samples, including human effluents, and we recommend that the same strategies will be of great utility to the development of any online aerosol LDI mass spectrometer platform.

Comparison of dissociation mechanism between collisionally activated dissociation and charge inversion using alkali metal targets for chlorophenol isomers

Chlorinated aromatic compounds are well-known environmental pollutants whose toxicities depend dramatically on the chlorine substitution pattern, making differentiation of chlorophenol isomers important for environmental analysis. Collisionally activated dissociation (CAD) spectra and charge inversion spectra of ortho-, meta-, and para-chlorophenols (ClC 6H 4OH) and their partially deuterated forms (ClC 6H 4OD) were measured using alkali metal targets. The peaks associated with C 6H 4O + and C 5H 5Cl + ions observed in the CAD spectra result from the loss of HCl and CO fragments, respectively, after the re-arrangement of the hydroxyl hydrogen atom. The peaks associated with C 6H 4OH − and ClC 6H 4O − ions observed in the charge inversion spectra result from Cl loss and from hydroxyl bond dissociation, respectively. Isomeric differentiation is possible based on the clear differences observed in the relative intensities of these pairs of peaks. Although the intensities of the peaks associated with C 6H 4O + relative to those of C 5H 5Cl + in the CAD spectra are independent of the target species, the intensities of the peaks associated with C 6H 4OH − relative to those of ClC 6H 4O − in the charge inversion spectra are target dependent. The isomeric dependence of the positive ion distribution patterns in the CAD spectra is proposed to be due to the differences in the rate of the hydrogen atom re-arrangement process. In contrast, the isomeric dependence of the negative ion distribution patterns in the charge inversion spectra is attributed to differences in the bond strength involved in the direct dissociation process in the neutral intermediate species.

Increase of Ar/sup +/ ion density in argon RF magnetron discharges caused by the addition of a small amount of oxygen

Particle-in-cell/Monte Carlo (PIC/MC) simulations were performed for radio frequency (RF) planar magnetron discharges of pure argon gas and an O/sub 2/-Ar gas mixture. The number densities of Ar/sup +/ and O/sup -/ ions drastically increased when oxygen was added to the Ar discharge gas. Moreover, the spatial distributions of positive ions, Ar/sup +/, O/sub 2//sup +/, and O/sup +/, had two peaks in the O/sub 2//Ar mixture discharge. The positive ions were distributed in such a way that the charge neutrality was kept in the plasma bulk, indicating that the behavior of Ar/sup +/ ions is influenced by the existence of O/sup -/ions. The mechanism of this phenomenon is clearly explained by visualizing results of the simulation.

Numerical simulation of streamer propagation in N2

For the improvement of performance in various applications, it is necessary to understand gas discharge dynamics experimentally and numerically. A Monte Carlo simulation is carried out in N2 in uniform electric fields. Streamer propagation, electron and positive ion distributions and space charge fields are studied in detail as time increases.

Ion energy distributions and the density of CH3 radicals in a low pressure inductively coupled CH4/H2 plasma used for nanocrystalline diamond deposition

Ion energy distributions (IEDs) and the density of CH3 radicals (n) in a 13.56 MHz radio frequency (rf) low pressure inductively coupled CH4/H2 plasma used for nanocrystalline diamond deposition have been investigated with a quadrupole mass spectrometer. The energy distributions of positive ions were measured in a CH4/H2 plasma with 50 mTorr of the gas pressure at 500 W of the plasma input power, and were compared with those of an Ar plasma. We have found that the IEDs of Ar+, CH4+, and C2H5+ have a nearly monoenergetic peak, and a hump due to a small degree of capacitive coupling. The plasma potentials obtained from the peaks are consistent with the previously reported values measured with a Langmuir probe. On the other hand, the IEDs of H+, H2+, and H3+ have a clear asymmetric double peak due to the modulation of rf driven glow discharge. The n monotonously increases with increasing pressure. The n indicates that CH3 radicals are main precursors for the growth of nanocrystalline diamond. The estimated sticking coefficient of the CH3 radical is comparable with the reported value.

Intensity dependence of cation kinetic energies from 2,5-dihydroxybenzoic acid near the infrared matrix-assisted laser desorption/ionization threshold.

The mechanisms responsible for matrix-assisted laser desorption/ionization (MALDI) are far from being well understood, particularly where infrared laser irradiation is used to initiate the process. We measured the emission yields and kinetic energy distributions of positive ions emitted from 2,5-dihydroxybenzoic acid loaded with angiotensin II in a standard MALDI preparation during irradiation with an infrared free-electron laser tuned to 2.94 microm. As the laser intensity is scanned through the MALDI threshold, we see a marked change in the energy distributions of the matrix ion. Above threshold, the energy distributions of both analyte and matrix cations are constant over a broad range of laser intensities. This behavior does not appear to be consistent with any extant model of the MALDI mechanism.

Massive positive and negative ions in the wake of a jet aircraft: Detection by a novel aircraft‐based large ion mass spectrometer (LIOMAS)

Negative and positive chemiions (CI) were measured by an aircraft‐based large ion mass spectrometer (LIOMAS) in the wake of a jet aircraft (ATTAS) at an altitude of 8 km and at plume ages between 0.6 and 6.2s. CI mass distributions were measured for mass numbers m up to 8500 atomic mass units, and additionally total fractional CI abundances fM for CI with m &gt; 8500 were obtained. Very massive CI were observed even when nearly sulfur free jet fuel was burnt in the ATTAS engines (fuel sulfur content FSC = 2mg/kg). This indicates that a CI growth process was operative which did not involve sulfur, but more likely low volatility organic compounds (LVOC). However, when fuel with an FSC = 118mg/kg was used a significant additional negative CI growth was observed which must be due to sulfur‐bearing molecules, probably sulfuric acid which is formed by oxidation of fuel sulfur. Use of the fuel with higher FSC did not change the size distribution of positive ions significantly. For both FSC the negative ions had a larger mean m compared to the positive ions.

On distribution over the cathode surface of the current due to positive ions in discharge with oscillating electrons

The mechanisms of processes determining distribution of positive ions on the cathode surface in a discharge with oscillating electrons are studied. At low pressures P≤5×10−5 torr over the entire range of anode length variation la=0.5–11 cm, the ion current distribution over the cathode radius Ji(r) features a single maximum in the center and drops steeply with distance from the axis. At pressures P=1–4×10−4 torr, the distribution Ji(r) for short anodes (la<6 cm) is similar to the previous one but, for long anodes (la=6–10 cm), new maxima at higher Ji values have been detected. A physical explanation for the obtained results is proposed.

Desorption of positive ions from ionic crystals accompanying 248 nm laser irradiation

We present a study of the energy distributions of positive ions (Na+, Li+, Ca+, and Mg+) photodesorbed from cleaved NaCl, LiF, MgO, NaNO3, and CaCO3 surfaces during 248 nm excimer laser irradiation at fluences well below the damage thresholds. The observed ion energies are significantly higher than those predicted by already existing models that allow ion rearrangement and relaxation during the electrostatic ion repulsion by the nearby photoionized sites. In contrast to what one would expect, we find that treating the ions as fixed charges and neglecting any ion rearrangement during the emission of the adions describes best the experimentally observed ion energies from all five ionic crystals.