Negative Ion Fractions Research Articles

Experimental investigation of an electronegative cylindrical capacitively coupled geometrically asymmetric plasma discharge with an axisymmetric magnetic field

In this study, we have investigated the production of negative ions by mixing electronegative oxygen gas with electropositive argon gas in a geometrically asymmetric cylindrical capacitively coupled radio frequency plasma discharge. The plasma parameters such as density (electron, positive, and negative ion), negative ion fraction, and electron temperature are investigated for fixed gas pressure and increasing axial magnetic field strength. The axisymmetric magnetic field creates an E × B drift in the azimuthal direction, leading to the confinement of high-energy electrons at the radial edge of the chamber, resulting in decreased species density and negative ion fraction in the plasma bulk. However, the electron temperature increases with the magnetic field. It is concluded that low magnetic fields are better suited for negative ion production in such devices. Furthermore, in addition to the percentage ratio of the two gases, the applied axial magnetic field also plays a vital role in controlling negative ion fraction.

Angle-dependent negative ion formation in H+ ions grazing scattering on Cu(100)

0.5–5 keV-energy H+ ions are scattered from Cu(100) in grazing incidences. The angle dependent positive and negative ion fractions have been measured in the non-specular scattering configuration. The positive-ion fractions for 0.5 and 1.0 keV incident energies are about 0% during the variation of the incident angle from 1° to 7°. Moreover, the non-monotonic trends of negative-ion fractions are observed for these two low energies, which may be ascribed to the sufficient Auger neutralization at the non-specular configuration.

Charge transfer of keV-energy H+ ions in grazing scattering on Cu(100)

An experimental study of 0.5-5.0 keV-energy H+ ion scattering on Cu(100) has been presented at a grazing scattering angle of 7°. The velocity dependent ion fractions have been measured in this work. The positive-ion fraction increases with the increase of incident velocity, which can be described by the exponential scaling. In particular, the threshold velocity for the positive-ion fraction is observed at about 0.25 a.u., and explained by the Auger ionization process. The negative-ion fraction increases monotonically with the impact velocity, which can be explained by the first type of the parallel velocity effect. Considering the formation of positive ions and the contribution of the parallel velocity effect, the negative-ion fraction is calculated by the modified theoretical model and the general trend is consistent with the experimental data.

Sensitivity of the negative ion beam extraction to initial plasma parameters by 3D particle modelling

The ITER neutral beam injection system for heating and current drive requires Negative Ions (NI) produced in large ceasiated ion sources. Self-consistent 3D Particle-In-Cell Monte-Carlo Collision (PIC-MCC) modelling is necessary for simulating extraction of NI and the co-extraction of electrons in ITER-like ion sources. In the RF driven negative ion sources at the ELISE and BUG test facilities, the filter field is generated by a current drawn through the Plasma Grid (PG). The 3D PIC-MCC code ONIX has been used to study the role of electron temperature, plasma density and emission rate on the extracted currents, as well as the negative ion transport close to an extraction aperture of the PG in ELISE. The results are compared to a case with higher electron density, lower electron temperature and higher emission rate, corresponding to the typical working point of a filter field configuration generated by permanent magnets. It is shown that the current of co-extracted electrons is strongly influenced by the electron temperature and plasma density. Increasing T e from 1 eV to 2 eV and decreasing the plasma density lead to an increase of co-extracted electrons by a factor 4 due to the increased electron flux towards the aperture and a deeper penetration of the extraction potential into the plasma. With reduced plasma density, the fraction of NI extracted directly after production on the PG increases. An additional parameter variation was conducted to study the effect of the emission rate of NI on the extracted currents. A reduced emission rate of NI from the PG from 550 Am−2 to 200 Am−2 reduces the extracted negative ion current by Am−2 due to a reduction of the number of directly extracted NI.

Unexpected Negative-Ion Conversion in Grazing Scattering of Negative Ions on HOPG

An experimental study of 8.5-22.5 keV-energy C-, O-, and F- ion scattering from an HOPG surface has been performed at a grazing scattering angle. For a given incident angle or impact velocity, the F- fraction is the highest and the C- fraction is the smallest among these three projectiles, which is explained with the order of the electron affinity level of the projectile. Moreover, the unexpected observations are that the F- ion fraction increases monotonically with impact velocity and that the negative-ion fraction increases with the decrease in exit angle. It can be explained by the influence of the effective high work function of HOPG and the production of positive ions at short distances from the surface.

Effective Temperatures of Olivine Dust Impact Plasmas

The effective temperatures of the positive and negative charge carriers in impact plasma are measured experimentally. The measurements are performed using a dust accelerator using polypyrrole (PPy)-coated olivine dust particles impacting onto a tungsten (W) target in four velocity bins: 2–3.6, 5–6.6, 8–12, and 12–18 km/s. The setup measures the retained impact charge as a function of applied bias potential, and the temperatures are calculated by fitting the data. The effective temperatures of the cations are approximately 7 eV and independent of the impact speed. The effective temperature of the negative charge carriers changes from 1 eV for the lowest speed to values comparable to that of the cations at higher speeds. The temperature measurements presented here are significantly different from prior studies using Fe dust particles. The discrepancy is possibly due to a larger fraction of negative ions in the impact plasma that likely originates from the PPy coating.

Experiments of keV negative ions transmitted through straight and tapered glass capillaries: tilt angle dependence

In this study, experiments are performed to study the transmission of 15 keV C− ions through straight and tapered borosilicate glass capillaries. The tilt angle is varied from 0° to 0.8°. In a straight capillary, the transmitted ions produced only one spot on the detector, and its intensity declined with increasing tilt angle. In this case, almost 98% of the transmitted particles maintained their initial charge. However, in the tapered capillary, the transmitted particles formed a different pattern composed of a core and a halo. The negative ion fractions of the core and the halo were 97.5% and 42.5% at a 0° tilt angle, respectively. Therefore, the particles formed the halo by scattering after colliding with the inner surface of the capillary, and most of them were neutralized. As the tilt angle increased, the intensity and negative ion fraction of the transmitted particles declined, and the halo gradually became quite asymmetric. These results indicate that the scattering process plays a role in the transmission.

Discharge properties of helicon oxygen plasma in the source and expansion chambers

In this paper, a study of a helicon discharge with O2 gas, operating at 13.56 MHz frequency and powers between 0–1500 W and pressure between 3 × 10−3 and 8 × 10−3 mbar is presented. Measurements of plasma parameters were made in the plasma production chamber as well as in the downstream expansion chamber. In the source chamber, transition from E to H and finally to W mode appears by increasing the input radio frequency (RF) power. To the best of our knowledge, this is the first detailed report of Helicon mode transition in Oxygen discharge. Mode transition is further confirmed by studying the radial density profile and by external matching circuit parameters measurement. Comparison of Langmuir probe I-V measurements and electron energy distribution function (EEDF) in source and expansion chambers confirms formation of negative ions. Negative ion fraction is measured in the expansion chamber by using two-probe method and densities of charged particles (electrons, positive and negative ions) are calculated using standard Langmuir probe theory and quasi-neutrality condition.

Charge Transfer in Low-Energy Collisions of Protons with a Thick C60 Film

Charge-exchange processes in collisions between H+ projectiles and a three-monolayer C60 film grown on Cu(111) are experimentally and theoretically analyzed for a wide range of energies (2–8 keV) in the low-energy regime. The negative, positive, and total projectile scattered ion fractions are experimentally determined by using the low-energy ion scattering technique. Two different collisional setups are studied for a fixed backscattering angle of 135°: 45/90° and 67.5/67.5° incoming/exit angles, determined with respect to the target surface plane. Total ion fractions between 10 and 20% are experimentally found in the whole energy range analyzed, with a slight but sustained increase with the projectile incoming energy. Positive ions were measured to be the majority of the total scattered charged projectiles in almost the whole energy range analyzed. Only for very low incoming energies (2–3 keV), the positive and negative ion fractions become comparable. A first-principle-based theoretical model is used to describe the dynamic charge-exchange processes taking place in the experimental collisional situation. A good description of the experimental results is obtained in the studied energy range. The theoretical and experimental results are also analyzed in comparative terms with previous measurements in the H+/highly oriented pyrolytic graphite (HOPG) system. The differences in the measured ion fractions can be directly related to differences in the electronic structures of C60 and HOPG.

Hydrogen ion scattering from a potassium impurity adsorbed on graphene

In this work we study the charge exchange process in the scattering of protons by potassium atoms adsorbed on a graphene surface in a low coverage limit. Both, the projected density of states on the alkaline atom site and the final charge states of the hydrogen projectile are calculated by considering the electronic Coulomb repulsion in the $s$-valence orbital. The inner $3p$ and $3s$ states of potassium are included and the local perturbations of the density matrix on the surrounding C atoms are also considered. The interacting systems are described by an Anderson Hamiltonian whose terms are calculated from the chemical properties of the atoms and the extended features of the graphene surface. The positive and negative ion fractions of hydrogen in the collision process are obtained from Keldysh-Green functions, which are calculated by employing the equation of motion method closed up to a second order in the atom-surface coupling term. It is found that the carbon atoms have no possibility of a direct charge exchange process in a frontal collision of the proton with the K adatom, and that the K-$3p$ band, broadened by the interaction with the graphene surface, provides an important source of electrons for the negative ionization of hydrogen, which is also promoted by the presence of a $\mathrm{K}\text{\ensuremath{-}}3s$ core state. The narrow $4s$ and $3p$ bands of the adsorbed potassium lead to an oscillatory dependence with the projectile incoming energy, of the probability for the three correlated charge states of hydrogen.

Observation of significant electron loss in grazing scattering of negative ions off a LiF(100) surface

LiF has potential applications in developing intensive current negative-ion sources and neutral particle detection in space due to its wide band gap. Charge transfer processes on a LiF(100) surface have been studied for 10.5–22.5 keV negative oxygen and chlorine ions under a grazing angle of incidence. In contrast with previous studies, the electron loss of the projectile has been obviously observed. The most striking experimental result is that the positive and negative ion fractions nonmonotonically vary with increasing incident angle in the scattered beam. The positive-ion production is explained by the molecular orbital promotion mechanism. The nonmonotonic positive-ion fraction is well described by the Landau-Zener formula. Besides the complete destruction from the anion sites of the insulator surface, the channel of negative-ion destruction from the cation sites has been suggested and considered in the model calculation to understand the electron loss process. It demonstrates that incident angle is a sensitive parameter for the negative ion conversion during grazing ion surface scattering.

Influence of magnetic filter and magnetic cage in negative ion production in helicon oxygen plasma

Negative ion rich oxygen plasma at low pressure is produced in a Helicon Plasma Source setup, which is primarily designed to perform electronegative gas plasma experiments including the studies of ion-ion plasma. The negative ion fraction and hence the negative ion density are obtained by using a two probe technique in which the electron current is obtained by using an RF compensated cylindrical Langmuir probe and positive ion saturation current is obtained by using an RF compensated planar probe. By measuring the negative ion fraction, both with and without a magnetic filter, the importance of the magnetic filter field in the production of negative ions in oxygen plasma is investigated. The maximum value of negative ion fraction α (n−/ne) is calculated to be approximately around 9 when the value of temperature ratio γ (Te/T−) is taken as 10. The observed negative ion fraction and other plasma parameters are explained by considering the set of reactions that are involved in the production and loss of negative ions.

Charge transfer of oxygen negative ions scattering on dissociated H2O-covered Cu(1 1 0) surface

Charge transfer during oxygen negative ion surface scattering is performed to investigate the electronic interaction for the water/Cu system as a function of incident velocity. We find that both positive and negative ion fractions increase with incident velocity for the specular scattering condition. The positive ions produced close to the surface can be understood by the mechanisms of collision-induced ionization and Auger ionization, and they finally survive from the resonant and/or Auger neutralization, which are well described by the exponential scaling. The negative ions are formed through the resonant charge transfer, and the velocity dependence has been explained by the kinematically parallel velocity effect and the surface work function. The modified rate equation taking into account these two factors is in rough agreement with the experimental result.

Role of projectile energy and surface work function on charge transfer of negative ions grazing scattering on dissociated H2O-covered Cu(110)

Abstract Shifted Fermi sphere model and parallel velocity effects on clean metals have been verified by a number of previous studies. However, for fast F − and Cl − ions scattering on dissociated water covered Cu(110) at grazing incidence, the negative-ion fraction is projectile dependent and monotonically increases with the increase of impact velocity, which cannot be explained by the conventional parallel velocity effect. The positive-ion fraction increases with the increase of perpendicular velocity, which can be well described by the exponential scaling. Positive ions efficiently formed at large perpendicular velocities strongly suppress the negative ion yield at short distances from the surface, so that negative ions are only formed at large distances. In addition, the large surface work function further promotes a transition between two kinds of parallel velocity effects. The calculation taking into account these contributions is in rough agreement with the experimental data.

Negative ion production in energetic O− and C− ions scattering on HOPG

In this work, we present negative-ion fractions as a function of exit velocity and angle after keV-energy negative oxygen and carbon ions scattering on a highly oriented pyrolytic graphite (HOPG) surface, at a scattering angle of 38°. The negative-ion fractions increase monotonously with the increase of exit velocity for specular scattering, while negative-ion fractions present a bell-shaped dependence on exit angle. In order to gain further insight into the nature of charge transfer on HOPG, a modified charge transfer model involving the contribution of positive ions has been proposed and roughly reproduced the velocity and angle dependences of negative-ion fractions.

Dynamical resonant charge transfer of fast C−, O−, F− ions and water covered Si(111) surface

The experiment of 6.5–22.5 keV C−, O− and F− ions scattering on water covered Si(111) surface has been performed. It is found that the positive-ion fraction is very low and increases monotonically as a function of perpendicular exit velocity and exit angle. In particular, the negative-ion fraction increases monotonically with perpendicular exit velocity for specular scattering, and for a given incident energy the angle dependence of the fraction is nonmonotonic. We interpret the observed positive ions in terms of inelastic binary collision, and adopt a modified resonant charge transfer model to calculate the bell-shaped negative-ion fraction. We find that the neutral yield at short ion-surface distance is nonzero and obeys well an exponential dependence. It strongly indicates that a dynamical equilibrium for negative ion formation is never achieved at short distances, and the band gap effect on charge transfer can be neglected to a large extent in this relatively high energy region.

Dissociation and Charge Transfer of H2O on Cu(110) Probed in Real Time Using Ion Scattering Spectroscopy.

Water-Cu(110) interaction is of particular importance during the routine use of graphene-based devices. In this work, water adsorption, dissociation, and desorption at elevated temperatures have been well studied using the time-of-flight ion scattering technique. It is found that water adsorption meets the first-order Langmuir adsorption model at room temperature. The variation of the ratio between residual O and H on the surface with temperature has been well determined, which profoundly reveals the dynamical process of surface composition. Furthermore, the change in the surface electronic properties has been probed by measuring negative-ion fractions as a function of the annealing temperature for fast ion scattering. It suggests that charge transfer is a very sensitive method for studying specific electronic processes in real time.

Landau–Zener model for electron loss of low-energy negative fluorine ions to surface cations during grazing scattering on a LiF(001) surface**Project supported by the National Natural Science Foundation of China (Grant Nos. 11175075, 11405078, 11474140, 11404152, and 11305083).

There is no available theoretical description of electron transfer from negative projectiles at a velocity below 0.1 a.u. during grazing scattering on insulating surfaces. In this low-velocity range, electron-capture and electron-loss processes coexist. For electron capture, the Demkov model has been successfully used to explain the velocity dependence of the negative-ion fraction formed from fast atoms during grazing scattering on insulating surfaces. For electron loss, we consider that an electron may be transferred from the formed ionic diabatic quasi-molecular state to the formed covalent diabatic quasi-molecular state by the crossing of the potential curves of negative projectiles approaching the surface cations, which can be described by the Landau–Zener two-energy-level crossing model. Combining these two models, we obtain good agreement between the experimental and calculated data for the F−–LiF(001) collision system, which is briefly discussed.

The description of charge transfer in fast negative ions scattering on water covered Si(100) surfaces

Doping has significantly affected the characteristics and performance of semiconductor electronic devices. In this work, we study the charge transfer processes for 8.5–22.5keV C− and F− ions scattering on H2O-terminated p-type Si(100) surfaces with two different doping concentrations. We find that doping has no influence on negative-ion formation for fast collisions in this relatively high energy range. Moreover, we build a model to calculate negative ion fractions including the contribution from positive ions. The calculations support the nonadiabatic feature of charge transfer.

Nonadiabatic dynamics in energetic negative fluorine ions scattering from a Si(100) surface.

The dependence of the negative-ion fractions on incident energy and angle is reported for 8.5-22.5 keV F(-) ions scattered from a Si(100) surface at a fixed scattering angle of 38°. The negative-ion fraction increases monotonically with incident velocity for specular scattering. In particular, the variation of the fraction with incident angle is bell shaped for a given incident energy. We interpret this variation using the incident-velocity effect at short distances where the yield of negative ions depends on the number of initial neutrals. It strongly indicates that at short distances, a dynamical equilibrium population is never achieved. This nonadiabatic feature is supported by simple calculations using modified rate equations.