Negative Ion Fractions Research Articles

Negative hydrogen ion production in a helicon plasma source

In order to develop very high energy (>1 MeV) neutral beam injection systems for applications, such as plasma heating in fusion devices, it is necessary first to develop high throughput negative ion sources. For the ITER reference source, this will be realised using caesiated inductively coupled plasma devices, containing either hydrogen or deuterium discharges, operated with high rf input powers (up to 90 kW per driver). It has been suggested that due to their high power coupling efficiency, helicon devices may be able to reduce power requirements and potentially obviate the need for caesiation due to the high plasma densities achievable. Here, we present measurements of negative ion densities in a hydrogen discharge produced by a helicon device, with externally applied DC magnetic fields ranging from 0 to 8.5 mT at 5 and 10 mTorr fill pressures. These measurements were taken in the magnetised plasma interaction experiment at the Australian National University and were performed using the probe-based laser photodetachment technique, modified for the use in the afterglow of the plasma discharge. A peak in the electron density is observed at ∼3 mT and is correlated with changes in the rf power transfer efficiency. With increasing magnetic field, an increase in the negative ion fraction from 0.04 to 0.10 and negative ion densities from 8 × 1014 m−3 to 7 × 1015 m−3 is observed. It is also shown that the negative ion densities can be increased by a factor of 8 with the application of an external DC magnetic field.

Low-frequency electrostatic waves in a magnetized, current-free, heavy negative ion plasma

AbstractWe report experimental observations of a low-frequency (≪ ion gyrofrequency) electrostatic wave mode in a magnetized cylindrical (Q machine) plasma containing positive ions, very few electrons and a relatively large fraction (n−/ne > 103) of heavy negative ions (m−/m+ ≈ 10), and no magnetic field-aligned current. The waves propagate nearly perpendicular to B with a multiharmonic spectrum. The maximum wave amplitude coincided spatially with the region of largest density gradient suggesting that the waves were excited by a drift instability in a nearly electron-free positive ion–negative ion plasma

Evidence for the incoming-velocity effect on negative fluorine ions scattering from a highly-oriented-pyrolytic-graphite surface

Negative-ion fractions depending on incident energies and angles have been measured for keV-energy fluorine negative ions scattering on a graphite surface at a scattering angle of 38\ifmmode^\circ\else\textdegree\fi{}. For specular scattering, the fraction increases monotonously with increasing incident energy. A specific feature is the nonmonotonic angular dependence of the fraction with the variation of incident angles. It strongly indicates the interaction-time-dependent electron transfer process. The incident-velocity effect has been taken into account to analyze the experimental results, in which an exponential scaling is found.

Emission of Negative Ions From Surfaces With High Emissivity

Spectra of the negative ion fraction in the low field emission from MPG-8 graphite with rough surface and tungsten surface with nano fuzz as a function of the residual gas composition and pressure are measured and analyzed. The intensity of the emitted negative ions increases linearly with the pressure of the working gas in the range of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$10^{-5}$</tex></formula> to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$10^{-7}~{\rm Torr}$</tex></formula> and depends on the atomic mass of the residual gas. A simple model based on the secondary negative ion emission of adsorbed particles under positive ion bombardment of the cathode is proposed.

Electron transfer and energy loss processes in fluorine scattering on oxygen covered Ag(1 1 0) – Crystal azimuthal dependence

Measurements of energy-losses and electron transfer processes in 1–4keV fluorine ions and atoms scattering on clean and oxygen covered Ag(110) were studied. We report data for surface channeling conditions for different surface azimuths for the clean and oxygen covered O(2×1)–Ag(110) (2×1) reconstructed surface. Adsorption of oxygen results in strong changes in energy losses and a decrease in the negative ion fraction. Energy loss results are discussed in the light of calculations using non-linear models for stopping power, ab initio crystal structure calculations of the electronic density and semi-classical deterministic simulations.

Complex wakes behind objects in multispecies plasmas

Many space, astrophysical, technological and laboratory plasmas are multispecies. Using particle-in-cell (PIC) numerical simulations, the novel features of the wake structures behind finite-sized objects in multispecies collisionless plasma flows are studied. In electropositive plasmas, different ion species can form separate wakes with distinct ion focus regions, and give rise to polarization of the plasma. In electronegative plasmas, with a large fraction of heavy negative ions, the polarization can be stronger due to substantial ion density reduction in the wake.

Charge Fraction Measurements for Heavy Particle Beams Generated by the Tandem Accelerator with MCP System

In order to expand the applied range of the heavy ion beam probe measurement on LHD, it is important to increase the probing beam current. A method of increasing the current is the improvement of charge exchange efficiency of a gas cell. The experiments were carried out on a tandem accelerator at Kobe University, 5SDH-2, because various conditions of experiments can be selected for gas species, gas pressure, and beam energy. Charge fractions of negative ions, neutral atoms and the positive ions were measured by a micro channel plate or Faraday cup. Dependence of the fractions of Au - , Au 0 , Au + and Au 2+ on gas thickness was measured. Results were compared with prior calculations, and a solution of rate equations. The cross sections for ionizing and electron capture were calculated, and some cross sections were obtained from experiments. The dependence of charge fractions for Au 0 , Au + and Au 2+ on the gas

Discharge kinetics of inductively coupled oxygen plasmas: experiment and model

In this paper, neutral and charged particle dynamics in both the capacitive and inductive modes of an inductively coupled oxygen discharge are presented. Langmuir probes, laser-assisted photodetachment and two-photon laser-induced fluorescence are employed to measure plasma parameters in the 13.56 MHz system for a range of plasma powers and gas pressures. It is found that the capacitive mode is more electronegative with lower molecular dissociation compared with the inductive mode. However, the negative ion density in each mode is comparable. A maximum is observed in the negative ion density and fraction with pressure for both modes. The experimental measurements are supplemented by a global model, which includes capacitive and inductive coupling effects. The model and experiments demonstrate that negative ion loss is dominated by ion–ion recombination and electron detachment at low pressures (<10 mTorr), while it is dominated by detachment processes with the metastable molecule O2(a 1Δg) and oxygen atoms at higher pressures. These findings support recent global model predictions of oxygen discharges.

Ion acoustic waves and double-layers in electronegative expanding plasmas

Ion acoustic waves and double-layers are observed in expanding plasmas in electronegative gases, i.e., plasmas containing an appreciable fraction of negative ions. The reported experiments are performed in argon gas with a variable amount of SF6. When varying the amount of SF6, the negative ion fraction increases and three main regimes were identified previously: (i) the plasma smoothly expands at low negative ion fraction, (ii) a static double-layer (associated with an abrupt potential drop and ion acceleration) forms at intermediate negative ion fraction, (iii) double-layers periodically form and propagate (in the plasma expansion direction) at high negative ion fraction. In this paper, we show that transition phases exist in between these regimes, where fluctuations are observed. These fluctuations are unstable slow ion acoustic waves, propagating in the direction opposite to the plasma expansion. These fluctuations are excited by the most unstable eigenmodes and display turbulent features. It is suggested that the static double layer forms when the ion acoustic fluctuations become non-linearly unstable: the double layer regime being a bifurcated state of the smoothly expanding regime. For the highest negative ion fraction, a coexistence of (upstream propagating) slow ion acoustic fluctuations and (downstream) propagating double layers was observed.

Response of an ion–ion plasma to dc biased electrodes

Electronegative plasmas are plasmas containing a significant fraction of negative ions, when magnetized they are very often segregated: the core is electropositive or weakly electronegative whereas a highly electronegative plasma forms at the periphery. At strong magnetic fields this segregation can lead to the formation of ion–ion plasmas almost free of electrons close to the walls or extraction surfaces and allows access to both positive and negative ions. The PEGASES thruster aims at alternately extracting and accelerating positive and negative ions from the ion–ion plasma region to provide thrust by both types of ions. The acceleration schemes depend on the possible control of the potential in an ion–ion plasma relative to the acceleration grids. In this paper continuous extraction and acceleration of positive ions from the PEGASES thruster is investigated by a retarding field energy analyser. It is shown from the measured ion energy distribution functions that the continuous acceleration potential can be controlled by biasing bare electrodes in contact with the region of the plasma with high electron density (i.e. the weakly electronegative plasma core). A grounded grid placed in the ion–ion region allows consequently the acceleration of positive ions, where the ion velocity is controlled by the bias applied to the electrodes in the plasma core. In contrast, when the grid in the ion–ion region is biased, positive ion beams are not detected downstream of the grid. The results indicate that biasing a grid positively in the ion–ion region may result in an electronegative space-charge sheath in front of the grid, which traps the positive ions inside the thruster.

Resonance hairpin and Langmuir probe-assisted laser photodetachment measurements of the negative ion density in a pulsed dc magnetron discharge

The time-resolved negative oxygen ion density n− close to the center line in a reactive pulsed dc magnetron discharge (10 kHz and 50% duty cycle) has been determined for the first time using a combination of laser photodetachment and resonance hairpin probing. The discharge was operated at a power of 50 W in 70% argon and 30% oxygen gas mixtures at 1.3 Pa pressure. The results show that the O− density remains pretty constant during the driven phase of the discharge at values typically below 5×1014 m−3; however, in the off-time, the O− density grows reaching values several times those in the on-time. This leads to the negative ion fraction (or degree of electronegativity) α=n−/ne being higher in the off phase (maximum value α∼1) than in the on phase (α=0.05–0.3). The authors also see higher values of α at positions close to the magnetic null than in the more magnetized region of the plasma. This fractional increase in negative ion density during the off-phase is attributed to the enhanced dissociative electron attachment of highly excited oxygen molecules in the cooling plasma. The results show that close to the magnetic null the photodetached electron density decays quickly after the laser pulse, followed by a slow decay over a few microseconds governed by the negative ion temperature. However, in the magnetized regions of the plasma, this decay is more gradual. This is attributed to the different cross-field transport rates for electrons in these two regions. The resonance hairpin probe measurements of the photoelectron densities are compared directly to photoelectron currents obtained using a conventional Langmuir probe. There is good agreement in the general trends, particularly in the off-time.

Negative ion formation during scattering of fast ions from diamond-like carbon surfaces

Fractions of negative ions after grazing scattering of fast hydrogen and oxygen atoms and ions from diamond-like carbon (DLC) surfaces are studied as function of projectile velocity. We reveal a pronounced kinematic resonance behaviour for the negative ion fractions as function of projectile velocity which provides important information on the charge transfer mechanism. The conversion of neutral atoms into negative ions during scattering from DLC surfaces is used for the detection of energetic neutral atoms in space.

Negative ions, energy loss, and electron emission during grazing scattering of fast H and He atoms from a clean and oxidized NiAl(1 1 0) surface

Negative ion fractions, projectile energy loss, and the emission of electrons is studied for grazing scattering of hydrogen and helium atoms/ions from a clean and oxidized NiAl(1 1 0) surface. Making use of translation energy spectroscopy and the coincident detection of the number of emitted electrons we have studied the electronic interaction mechanisms for the change from a clean metal target to an insulator surface via the preparation of a well defined ultrathin alumina film on top of the metal substrate. We find that already for a monolayer thick oxide film the characteristic different features of electronic processes for the surface of an insulator crystal are present.

Electron capture and loss in the scattering of H + from HOPG graphite

In this paper we report measurements of the positive (H +) and negative (H −) ion fractions after H + scattering from a highly oriented pyrolytic graphite (HOPG) surface. The experimental results show that for a fixed (large) exit angle the negative fraction is almost independent of the primary ion energy, while the positive fraction increases steadily. For a fixed incident energy, the H + and H − fractions show a complementary behavior with exit angle. A dynamical quantum mechanical calculation, based on the localized and extended features of the atom–surface interaction, allows us to understand many of the features of the experimental results.

Active species characterization in RF remote oxygen plasma using actinometry OES and electrical probes

Actinometry optical emission spectroscopy (AOES), single cylindrical Langmuir probe and electrostatic planar probe were used to investigate the active species (electrons, ions and atomic oxygen) of remote oxygen plasma in an RF (13.56 MHz) hollow cathode discharge system, as a function of applied power and gas pressure. The electron density and electron temperature were determined from Langmuir probe. The atomic oxygen (AO) density was measured using argon actinometry method. Positive ion flux rate was estimated from the positive ion saturation current of the planar probe, and the combination of the planar and Langmuir probes enabled the determination of the negative ion fraction in the discharge. The explanation of the behavior of the different active plasma species was performed on the basis of the main kinetic reactions of oxygen plasma.

Diagnostic and processing in SF6 RF remote plasma for silicon etching

Plasma diagnostics and Si etching were carried out in 13.56 MHz SF6 remote plasma generated in a hollow cathode discharge system. The plasma diagnostics were performed in the remote zone at a constant pressure of 40 Pa, at a constant applied RF power of 300 W and as a function of the SF6 flow rate (40–2000 sccm), where absolute concentrations of fluorine atoms were measured using actinometry optical emission spectroscopy; electron density, electron temperature and plasma potential were determined using single Langmuir probe, positive ion flux and negative ion fraction were determined using an electrostatic planar probe. The silicon etching process was studied at two distant values of flow rate, 80 and 1800 sccm and for three conditions of the substrate holder, namely the substrate is grounded, the substrate is negatively biased and the substrate is positively biased. The etched silicon was characterized for etch rate, optical reflectance and photoluminescence properties. It was found that the etch rate is relatively high (about 15 mg cm−2 min−1) and it is controlled mainly by the ratio of the ion flux over the reactive atomic fluorine flux, the highest etch rate was obtained at the higher flow rate (1800 sccm) and for positively biased substrate. The reflectance of the silicon surface was significantly reduced after etching and a reflectance as low as 0.2% was measured. A visible photoluminescence from the etched surface was recorded; it is centred at about 600 nm, and its intensity is inversely proportional to the measured reflectance.

Negative Ion Density Measurements in Reactive Magnetron Sputtering

AbstractA combination of laser photo‐detachment and conventional Langmuir probing has been used to obtain the bulk negative ion density in both a DC and radio frequency (RF) sputter magnetron. The argon and oxygen discharges were operated at low powers and over a range of pressures. The photo‐detachment signal is expected to reach a limiting value; however, the signal continues to increase with laser energy density and this can be attributed to a laser ablation effect. In the RF magnetron the electron temperature (Te) in oxygen decreased with increasing pressure, whereas the electron density (Ne) increased from 0.53 to 8.6 × 1014 m−3. At around 12 mTorr, a sudden increase in Ne by 3.7 is accompanied by a small drop in Te. The negative ion density (N−) also increases with pressure reaching a maximum of 1.7 × 1014 m−3 between 5 and 10 mTorr. Under similar conditions, the DC magnetron negative ion fraction (N−/Ne) is estimated to be ∼0.01, being significantly lower than in the RF magnetron where N−/Ne ≈ 1.

Scattering of light molecules from thin Al 2O 3 films

Molecular oxygen and hydrogen ions were scattered at grazing incidence from various thin Al 2O 3 films. The energy of incident particles was varied from 390 to 1000 eV. For scattered positive oxygen ions, negative ion fractions of up to 17% were recorded. For scattered positive hydrogen ions, the negative ion fractions reached up to 2%. These findings qualify thin films of Al 2O 3 as possible candidates for use as charge state conversion surfaces in neutral particle sensing instruments, which will work in space.

Relativistic modulational instability of electron-acoustic waves in an electron-pair ion plasma

The modulational instability of finite amplitude electron-acoustic waves (EAWs) along the external magnetic field is studied in an electron-pair ion plasma. Accounting for the relativistic electron mass variation nonlinearity and the Boltzmann distribution of both positive and negative ions, new regimes for the relativistic modulational instability (MI) for the low frequency (below the electron gyrofrequency) short-wavelength (in comparison with the ion gyroradius) modes are obtained numerically. It is found that the presence of a significant fraction of negative ions suppresses the MI growth/decay rate for the modulated EAW packets. The results could be of important for understanding the origin of amplitude modulated EAW packets in space (e.g., Earth’s magnetotail) as well as in laboratory plasmas.

Comparison between fluid simulations and experiments in inductively coupled argon/chlorine plasmas

Comparisons of 2D fluid simulations with experimental measurements of Ar/Cl2 plasmas in a low-pressure inductively coupled reactor are reported. Simulations show that the wall recombination coefficient of Cl atom (γ) is a crucial parameter of the model and that neutral densities are very sensitive to its variations. The best agreement between model and experiment is obtained for γ = 0.02, which is much lower than the value predicted for stainless steel walls (γ = 0.6). This is consistent with reactor wall contaminations classically observed in such discharges. The electron density, negative ion fraction and Cl atom density have been investigated under various conditions of chlorine and argon concentrations, gas pressure and applied rf input power. The plasma electronegativity decreases with rf power and increases with chlorine concentration. At high pressure, the power absorption and distribution of charged particles become more localized below the quartz window. Although the experimental trends are well reproduced by the simulations, the calculated charged particle densities are systematically overestimated by a factor of 3–5. The reasons for this discrepancy are discussed in the paper.