Positive Ion Species Research Articles

Observations of ionic species produced in an atmospheric pressure pulse-modulated RF plasma needle

Pulse-modulated generation of atmospheric pressure plasma promises lower operating temperatures and enhanced energy efficiencies compared with those generated using continuous excitation. In this study, the effect of pulse modulation on the ionic content of a radio-frequency helium plasma needle was explored using molecular beam mass spectrometry (MBMS). In all cases reported the pulse repetition frequency was held constant at 6 kHz while the duty cycle and distance between the plasma needle and MBMS were varied from 30% to 90% and 4 mm to 10 mm, respectively. Decreasing the duty cycle was seen to enhance the negative ion flux to the MBMS and yields a clear shift to higher masses for positive ion species. Time-resolved ion intensity measurements (5 µs resolution) demonstrated that negative ions were created almost exclusively in the pulse off-time while positive ions were generated mainly in the on-time. The ions detected were also seen to change as the distance between the sampling orifice and tip of the plasma needle increased, highlighting the effect of different ion mobilities in the flowing gas stream and the presence of an extended cooler afterglow region. The results suggest that there is a possibility of tailoring the ionic plasma chemistry through pulsed modulation, increasing the versatility of the already widely studied plasma needle device.

Charged particle density distributions in multi-component plasmas with two species of warm positive ions

The behavior of charged particle density distributions and the electrostatic potential in the sheath region of a multi-component plasma consisting of electrons and two species of positive ions are investigated by using a fluid description. It is assumed that the ion species are singly charged and have different masses and temperatures. To determine the ion species velocities at the plasma-sheath boundary, we used the recent work of Baalrud et al. [S. Baalrud and C. Hegna, Phys. Plasmas 18, 023505 (2011)]. It is shown that the density distributions of electrons and lighter ion species increase in the sheath region by increasing the temperature of ion species while the density distribution of the heavier ion species decreases. The results also show that by increasing the temperature of the ion species, the electrostatic potential in the sheath region decreases and at the same time the sheath thickness increases.

Mass spectrometric study of discharges produced by a large-area dual-frequency–dual-antenna inductively coupled plasma source

An energy-resolved quadrupole mass spectrometer is used to investigate the time-averaged ion energy distribution (IED) of positive ionic species in an Ar/CF4 (90%/10%) discharge produced by dual-frequency–dual-antenna, next-generation large-area inductively coupled plasma source. The operating pressure is 10 mTorr. Two radio frequencies of 2 MHz (low frequency) and 13.56 MHz (high frequency) are used to initiate and sustain the discharge. The orifice of the mass spectrometer was 100 µm in diameter and placed at 30 mm below the ICP source and 20 mm outside the discharge volume.It is observed that both of the frequencies have significant effect on IEDs of all prominent discharge species. The evolution of IEDs with power shows that the discharge undergoes a mode transition (E to H) as the applied power is increased. At a fixed value of P13.56 MHz (250 and 500 W), the energy spread and the energy separation between two peaks of IEDs increase illustrating enhanced E-mode. Above P13.56 MHz = 500 W, the IEDs show opposite trends, i.e. decreasing energy spread and energy separation between two peaks, showing the strengthening of H-mode. Increasing P13.56 MHz at a fixed value of P2 MHz has similar effects. A comparison of IEDs sampled at a fixed total power (P13.56 MHz + P2 MHz) demonstrates that an IED can be tailored by changing the power ratio (P13.56 MHz/P2 MHz).

Consequences of negative ions for Titan's plasma interaction

A hybrid particle code has been used to examine how Titan's interaction with Saturn's magnetosphere is affected by the presence of negative ions in Titan's ionosphere. The simulations self‐consistently include a version of Titan's ionosphere represented by 8 generic positive ion species, over 40 ion‐neutral chemical reactions, ion‐neutral collisions and Hall and Pederson conductivities. A model consisting of 6 generic negative ion species is also included. The presence of negative ions is found to alter the conductivity of Titan's ionosphere, changing the loss rates of the ionospheric species and modifying the topology of Titan's ion tail.

The positive ion temperature effect in magnetized electronegative plasma sheath with two species of positive ions

The properties of a magnetized multi-component (two species of positive ions, negative ions and electrons) plasma sheath with finite positive ion temperature are studied. By using three fluid hydrodynamic model and some dimensionless variables, the ion (both lighter and heavier positive ions, and negative ions) densities, the ion (only for positive ions) velocities, and electric potential inside the sheath are investigated. In addition, the absence and presence of magnetic field and the orientation of magnetic field are considered. It is noticed that, with increase of positive ion temperature, the lighter positive ion density peaks increase only at the sheath edge and shift towards the sheath edge for both absence and presence of magnetic field. For heavier positive ions, in the absence of magnetic field, the density peaks increase at the sheath edge. But in the presence of magnetic field, the density fluctuations increase at the sheath edge. For both the cases, the density peaks shift towards the sheath edge.

Head-on collisions of electrostatic solitons in multi-ion plasmas

Head-on collisions between two electrostatic solitons are dealt with by the Poincaré-Lighthill-Kuo method of strained coordinates, for a plasma composed of a number of cold (positive and negative) ion species and Boltzmann electrons. The nonlinear evolution equations for both solitons and their phase shift due to the collision, resulting in time delays, are established. A Korteweg-de Vries description is the generic conclusion, except when the plasma composition is special enough to replace the quadratic by a cubic nonlinearity in the evolution equations, with concomitant repercussions on the phase shifts. Applications include different two-ion plasmas, showing positive or negative polarity solitons in the generic case. At critical composition, a combination of a positive and a negative polarity soliton is possible.

Ambipolar diffusion in strongly electronegative plasma

This paper studies the ambipolar diffusion in an electronegative plasma containing electrons, a single species of negative ions and a single species of positive ions. We demonstrate that in a plasma with the concentration of negative ions more than 10 times exceeding that of electrons one has to take into account the mobility of negative and positive ions. We established that in a strongly electronegative plasma (when the ratio of the concentration of negative ions and that of electrons α = n−/ne ≫ 1 and μe ≪ α·μ−, where μe and μ− are the mobilities of electrons and negative ions) the diffusion ceases to be ambipolar, and the coefficients of ambipolar diffusion of the charged particles become approximately equal to their coefficients of free diffusion.

Ion-acoustic solitons in negative ion plasma with two-electron temperature distributions

Ion–acoustic solitons in a warm positive and negative ion species with different masses, concentrations, and charge states with two electron temperature distributions are studied. Using reductive perturbation method, Korteweg de-Vries (KdV) and modified-KdV (m-KdV) equations are derived for the system. The soliton solution of the KdV and m-KdV equations is discussed in detail. It is found that if the ions have finite temperatures, then there exist two types of modes, namely slow and fast ion-acoustic modes. It is also investigated that the parameter determining the nature of soliton (i.e., whether the system will support compressive or rarefactive solitons) is different for slow and fast modes. For the slow mode, the parameter is the relative temperature of the two ion species; whereas for the fast mode, it is the relative concentration of the two ion species. At a critical concentration of negative ions, both compressive and rarefactive solitons coexist. The amplitude and width of the solitons are discussed in detail at critical concentration for m-KdV solitons. The effect of the relative temperature of the two-electron and cold-electron concentration on the characteristics of the solitons are also discussed.

Magnetized electronegative plasma presheath-sheath containing two positive ion species

The structure of the presheath-sheath for a magnetized electronegative discharge in the presence of two species of positive ions is investigated. In the absence of collision, for different magnetic field strength, the densities of the charges and the profiles of the positive ions velocities and electric potentials are obtained. It is shown that a slightly magnetized uniform structure discharge converts to a stratified multiple layer oscillatory structure for an intermediate magnetic field. But, in contrast to a single positive ion species up to high values of magnetic field, oscillatory behavior continues to exist. We also observe oscillations in the profile of electron densities which are hidden in the single positive ion species case. A competition between the two positive ions species exists which depends on their different masses. The density profiles of the species follow each other along the sheath and the heavier component always occupies higher density values. The influence of the elastic collisions between positive ions and neutrals on the structure of a magnetized electronegative discharge is also investigated. Finally, the positive ion flux at the plasma-sheath edge for each species is obtained as a function of magnetic field strength, for different values of the collision frequency.

Effect of dust non-linear charge and size-distribution on dust-acoustic double-layers in dusty plasmas

The investigation of the existence of arbitrarily large amplitude electrostatic dust-acoustic double layers is conducted in a four-component plasma consisting of electrons, two distinct positive ion species of different temperatures, and massive negatively-charged dust particles that are assumed spheres of different radii distributed according to a power-law. The dependence of the dust grain charge on its size is considered to be nonlinear. The number densities of electrons and ions are assumed to follow a Boltzmann distribution, whereas the dynamics of charged dust grains is described by fluid equations. Comparison is conducted between plasmas containing size-distributed dust grains and those containing monosize dust grains, while examining the criteria for the existence of dust-acoustic double layers along with the dependence of their amplitudes and Mach numbers on plasma parameters.

Arbitrary amplitude ion-acoustic soliton coexistence and polarity in a plasma with two ion species

Using a Sagdeev pseudopotential formalism, large ion-acoustic waves are investigated in a multispecies plasma model consisting of two cold positive ion species in the presence of nonthermal Cairns electrons. Positive solitons are limited in amplitude by infinite compression of the lighter ion species. Negative solitons become possible at a sufficient degree of electron nonthermality, and their amplitudes are limited by the occurrence of double layers. What determines coexistence is the possibility of having parameter ranges for which there are solitons at the acoustic speed, which is a necessary and sufficient condition but unfortunately not an easy criterion to check analytically. For the first time a parameter range has been found where both negative and positive polarity solutions coexist, with the solitons at the acoustic speed always having a positive polarity, contrary to earlier results which seemed to indicate that polarity switches and coexistence ranges were linked. Finally, some useful proofs are presented in an Appendix, concerning the Sagdeev pseudopotential behavior at the acoustic speed, results of which are generic to a range of physical plasma models.

Numerical investigation of the ion temperature effect in magnetized plasma sheath with two species of positive ions

The effect of ion temperature, magnitude of magnetic field and its orientation on a magnetized plasma sheath consisting of electrons and two species of positive ions are investigated. Using three-fluid hydrodynamic model and some dimensionless variables, the dimensionless equations are obtained and solved numerically. It is found that with the increase of the ion temperature and magnetic field strength there is a significant change in ion densities and energies in the sheath. It is also noticed that increase of magnetic field angle enhances the ion density near the sheath edge for a constant ion temperature. With increase in ion temperature and magnetic field angle, the lighter ion density near the sheath edge enhances and reverses for the heavier ion species.

Investigation of ion–ion-recombination at atmospheric pressure with a pulsed electron gun

For future development of simple miniaturized sensors based on pulsed atmospheric pressure ionization as known from ion mobility spectrometry, we investigated the reaction kinetics of ion-ion-recombination to establish selective ion suppression as an easy to apply separation technique for otherwise non-selective ion detectors. Therefore, the recombination rates of different positive ion species, such as protonated water clusters H(+)(H(2)O)(n) (positive reactant ions), acetone, ammonia and dimethyl-methylphosphonate ions, all recombining with negative oxygen clusters O(2)(-)(H(2)O)(n) (negative reactant ions) in a field-free reaction region, are measured and compared. For all experiments, we use a drift tube ion mobility spectrometer equipped with a non-radioactive electron gun for pulsed atmospheric pressure ionization of the analytes. Both, ionization and recombination times are controlled by the duty cycle and repetition rate of the electron emission from the electron gun. Thus, it is possible to investigate the ion loss caused by ion-ion-recombination depending on the recombination time defined as the time delay between the end of the electron emission and the ion injection into the drift tube. Furthermore, the effect of the initial total ion density in the reaction region on the ion-ion-recombination rate is investigated by varying the density of the emitted electrons.

Influence of radio-frequency power on the state of H2/C4H8 glowing discharge plasma

Taking advantage of the glowing discharge technology and plasma mass spectrometry diagnosis technology, the variation rules of positive ion species and energy with power in H2/C4H8 mixture gas are investigated. The characteristic ions are measured and their forming process is discussed. The results show that the intensity of the C-H segment ions in the H2/C4H8 plasma increases with radio-frequency (RF) power increasing, the intensity reaches a maximum at an RF power of 20 W, and then reduces after the RF power has reached 25 W. The relative concentration of m/e 57 (C4H9+) is highest when the power is less than 10 W, and when the power is more than 10 W, the relative concentration of m/e 39 (C3H3+) reaches a maximum. The energy of the C-H segment ions increases gradually with the increase of RF power. The qualitative analyses of H2/C4H8 plasma composition and energy will be beneficial to improving the fabrication technology of glowing discharge polymer coating.

1-D fluid model of atmospheric-pressure rf He+O2 cold plasmas: Parametric study and critical evaluation

In this paper atmospheric-pressure rf He+O2 cold plasmas are studied by means of a 1-D fluid model. 17 species and 60 key reactions selected from a study of 250+ reactions are incorporated in the model. O2+, O3-, and O are the dominant positive ion, negative ion, and reactive oxygen species, respectively. Ground state O is mainly generated by electron induced reactions and quenching of atomic and molecular oxygen metastables, while three-body reactions leading to the formation of O2 and O3 are the main mechanisms responsible for O destruction. The fraction of input power dissipated by ions is ∼20%. For the conditions considered in the study ∼6% of the input power is coupled to ions in the bulk and this amount will increase with increasing electronegativity. Radial and electrode losses of neutral species are in most cases negligible when compared to gas phase processes as these losses are diffusion limited due to the large collisionality of the plasma. The electrode loss rate of neutral species is found to be nearly independent of the surface adsorption probability p for p > 0.001 and therefore plasma dosage can be quantified even if p is not known precisely.

Quantitation of amyloid beta peptides Aβ 1–38, Aβ 1–40, and Aβ 1–42 in human cerebrospinal fluid by ultra-performance liquid chromatography–tandem mass spectrometry

Critical events in Alzheimer’s disease (AD) involve an imbalance between the production and clearance of amyloid beta (Aβ) peptides from the brain. Current methods for Aβ quantitation rely heavily on immuno-based techniques. However, these assays require highly specific antibodies and reagents that are time-consuming and expensive to develop. Immuno-based assays are also characterized by poor dynamic ranges, cross-reactivity, matrix interferences, and dilution linearity problems. In particular, noncommercial immunoassays are especially subject to high intra- and interassay variability because they are not subject to more stringent manufacturing controls. Combinations of these factors make immunoassays more labor-intensive and often challenging to validate in support of clinical studies. Here we describe a mixed-mode solid-phase extraction method and an ultra-performance liquid chromatography tandem mass spectrometry (SPE UPLC–MS/MS) assay for the simultaneous quantitation of Aβ 1–38, Aβ 1–40, and Aβ 1–42 from human cerebrospinal fluid (CSF). Negative ion versus positive ion species were compared using their corresponding multiple reaction monitoring (MRM) transitions, and negative ions were approximately 1.6-fold greater in intensity but lacked selectivity in matrix. The positive ion MRM assay was more than sufficient to quantify endogenous Aβ peptides. Aβ standards were prepared in artificial CSF containing 5% rat plasma, and quality control samples were prepared in three pooled CSF sources. Extraction efficiency was greater than 80% for all three peptides, and the coefficient of variation during analysis was less than 15% for all species. Mean basal levels of Aβ species from three CSF pools were 1.64, 2.17, and 1.26 ng/ml for Aβ 1–38; 3.24, 3.63, and 2.55 ng/ml for Aβ 1–40; and 0.50, 0.63, and 0.46 ng/ml for Aβ 1–42.

Bohm Criterion for Collisionless Sheaths in Two-Ion-Species Plasmas

The sheaths of a plasma containing two species of positive ion generated in a low pressure discharge (∼ mTorr) are studied using a fluid model. It is shown analytically that for the weakly collisional presheath, as long as the ion-ion streaming instability is absent, each ion species enters the sheath satisfying each individual Bohm criterion. The fluid equations of electrons and ions, including the time derivative terms, are solved numerically to follow the temporal evolution of the plasma and sheath. Our numerical results show that in the parameter range explored, the ion-ion streaming instability is not observed, and each ion species has its own Bohm criterion independent of the relative concentrations of the two ion species. In addition, the RF sheath is studied numerically and the similar conclusion is drawn.

Numerical analysis of plasma evolution on dielectric barrier discharge plasma actuator

Time evolution of the discharge plasma in the dielectric barrier discharge (DBD) plasma actuator was simulated by the simple fluid model in which the electron and single positive ion species were considered. The characteristics of the discharge plasma evolution were investigated in detail, and the following results were obtained. When the positive-going voltage is applied, the streamer discharge is formed periodically. The periodically formed streamer expands from the exposed electrode, and its length becomes longer than the previous one. Periodic breakdown of the gas and step-by-step plasma expansion are also observed during the negative-going voltage; however, the streamer is not formed and the breakdown frequency is much higher. The simulation results with a triangular applied voltage waveform show the same characteristics as observed in the experiment; large discharge current spikes are observed during both the positive- and negative-going voltage phase, and the plasma in the negative-going voltage phase expands more smoothly than that in the positive phase because of its higher breakdown frequency. It was shown that even the simple numerical model could provide valuable insights into the physics of DBD plasma actuator; this indicates that the positive ions and electrons play a prominent role in determining the general characteristics of the plasma evolution.

Large acoustic solitons and double layers in plasmas with two positive ion species

Large nonlinear acoustic waves are discussed in a plasma made up of cold supersonic and adiabatic subsonic positive ions, in the presence of hot isothermal electrons, with the help of Sagdeev pseudopotential theory. In this model, no solitons are found at the acoustic speed, and no compositional parameter ranges exist where solutions of opposite polarities can coexist. All nonlinear modes are thus super-acoustic, but polarity changes are possible. The upper limits on admissible structure velocities come from different physical arguments, in a strict order when the fractional cool ion density is increased: infinite cold ion compression, warm ion sonic point, positive double layers, negative double layers, and finally, positive double layers again. However, not all ranges exist for all mass and temperature ratios. Whereas the cold and warm ion sonic point limitations are always present over a wide range of mass and temperature ratios, and thus positive polarity solutions can easily be obtained, double layers have a more restricted existence range, specially if polarity changes are sought.

Fluid simulation of an electrostatic plasma sheath with two species of positive ions and charged nanoparticles

One-dimensional fluid simulations are used to study the dynamics of an electrostatic plasma sheath containing nanosized dust grains and two species of positive ions, i.e., He+ and Ar+. The impacts of the concentration of each species, the velocity at the sheath edge of the ions, and the bias voltage of the substrate, on the spatial distribution of the velocity and number density of the plasma particles, and the incident fluxes of the ions on the substrate, are investigated. The numerical results show that the sheath thickness increases with increasing σ, the density ratio of He+ ions to Ar+ ions at the sheath edge. For nanosized dust particles considered in this work, the dominant forces are the ion drag and the electric force and the effects of the neutral drag and gravity are negligible. Due to enhancement of the ion drag force and the electric force, the dust velocity increases and, consequently, the dust number density decreases as the concentration at the sheath edge of Ar+ ions is increased. For the same velocity and number density at the sheath edge, the number density of Ar+ ions near the wall is larger than that of He+ ions, but their incident fluxes are the same. The maximum in the dust number density increases with the velocity of Ar+ ions at the sheath edge, but it weakly changes with the Mach number of He+ ions, except for large values of σ. The position of the maximum dust number density initially decreases very quickly with increasing the velocity at the sheath edge of the ions from small values, but then at larger values it changes quite slowly. The differences in the sheath parameters for different values of σ disappear at some values of the velocity at the sheath edge of the ions and dust particles. The incident flux of the ions are independent of the bias voltage of the substrate, but their kinetic energy is equal to the bias potential.