Formation Of Negative Hydrogen Ions Research Articles

The Increase of Negative Hydrogen Ions Production in ECR Source by use of the Additional Low-Temperature Emitters of Electrons

Analysis of the mechanisms of formation of negative hydrogen ions in the plasma source, operating at the electron cyclotron resonance, leads to the conclusion about the fundamentally important role played in this process by low-temperature electrons. In the source the negative ion production is realized in two stages. First, hydrogen molecules colliding in a plasma with energetic electrons, are exited to high-laying Rydberg electron states and to high vibration levels in the plasma volume. Further, pulling the low-energy electrons, excited molecules acquire a negative charge. Negative atomic ions result from dissociation of excited negatively charged hydrogen molecules. Necessary for this process, the electrons of low energies are the result of collisions of fast plasma electrons with plasma electrodes. In the presented experiments to further increase the number of low-energy electrons were used electrons, emitted from the heated tungsten filaments and ceramic electrodes LaB6 placed in the chamber of the source. The experiments found that emission of electrons from tungsten heaters have improved stability of the discharge and expanded the range of pressure under which there was a discharge, without changing substantially the magnitude of current of negative ions. The emission of electrons from the LaB6 electrodes increased the current of negative ions from a source more than 3 times.

Studies on Hydrogen Plasma in a Penning Ion Source by Optical Emission Spectroscopy

A Penning ion source with self-heated cathodes has been applied in a miniature cyclotron. Optimizing the formation of negative hydrogen ions in the ion source needs the knowledge of plasma parameters. Optical emission spectroscopy is introduced as a noninvasive and in situ diagnostic tool for line-of-sight averaged plasma parameters in this paper. The relative intensities of neutral atomic hydrogen emission spectral lines are used to evaluate the electron temperature of hydrogen plasma. The experimental results show that the relative intensities of Balmer lines (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">α</sub> , H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">β</sub> , and H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">γ</sub> ) display significant dependence on gas flow rate, not sensible to arc current and magnetic field. The calculated electron temperature is in the range of 4000-7000 K in the experiments. The correlations of electron temperature with arc current, gas flow rate, and magnetic field are discussed, respectively.

Formation of negative hydrogen ions in 7-keV OH++Arand OH++acetone collisions: A general process for H-bearing molecular species

We demonstrate that the formation of negative hydrogen ions (H-) occurs in a wide class of atomic and molecular collisions. In our experiments, H- emission from hydroxyl cations and acetone molecules was observed in keV-energy collisions. We show that hydride (H-) anions are formed via direct collisional fragmentation of molecules, followed by electron grabbing by fast hydrogen fragments. Such general mechanism in hydrogen-containing molecules may significantly influence reaction networks in planetary atmospheres and astrophysical media and new reaction pathways may have to be added in radiolysis studies.

Cesium dynamics in long pulse operation of negative hydrogen ion sources for fusion

Large scale negative hydrogen ion sources operating stable for 1 h (cw mode) are required for the neutral beam heating system of the fusion experiment ITER. The formation of negative hydrogen ions relies on the surface effect for which cesium is evaporated into the source. In order to monitor the cesium dynamics the laser absorption spectroscopy technique is applied to the long pulse test facility MANITU. In the vacuum phase, without plasma operation the evaporation of cesium and the built-up of the cesium in the source are measured. Typical neutral cesium densities are 10(15) m(-3). During plasma operation and after the plasma phase a high cesium dynamics is observed, showing also depletion of cesium during long pulses with low cesium amount. The co-extracted electron current decreases with the cesium amount to a certain level whereas the ion current indicates an optimum density range.

Optimizing the laser absorption technique for quantification of caesium densities in negative hydrogen ion sources

The performance of negative hydrogen ion sources, which rely on the formation of negative hydrogen ions on a surface with low work function, depends strongly on the caesium dynamics in the source. A quantitative measurement of the amount of caesium in the source during plasma-on and plasma-off (vacuum phase) is highly desirable. The laser absorption technique is optimized for the diagnostics of neutral caesium densities close to the extraction surface on which the negative hydrogen ions are generated. The setup is simplified as much as possible utilizing also an automatic data evaluation for online measurements at high power rf sources. The setup is tested and calibrated in a small scale laboratory experiment. The system and the analysis of the D2 caesium line at 852.1 nm are described in detail, including effects of line saturation and density depletion. The system is sensitive in the density range 1013–1017 m−3 (path length of about 15 cm), allowing also for a temporal resolution of 40 ms. First very promising results from the negative hydrogen ion source are presented, such as the increase in the caesium density due to the caesium evaporation and time traces before, during, and after the discharge indicating a strong caesium redistribution.

Electronic excitations during grazing scattering of hydrogen atoms on KI(001) and LiF(001) surfaces

The energy loss of hydrogen atoms with energies of 400 eV and 1 keV is studied in coincidence with the number of emitted electrons during grazing scattering from atomically clean and flat KI(001) and LiF(001) surfaces. The energy loss spectra for specific numbers of emitted electrons are analyzed in terms of a binary interaction model based on the formation of transient negative ions via local capture of valence band electrons from anion sites. Based on computer simulations we derive for this interaction scenario probabilities for the production of surface excitons, for electron loss to the conduction band of KI, for emission of electrons, and for formation of negative hydrogen ions. The pronounced differences of data obtained for the two surfaces are attributed to the different electronic structures of KI and LiF.

Population dynamics of molecular hydrogen and formation of negative hydrogen ions in a magnetically confined low temperature plasma

An attempt is made to unravel the influence of vibrational population distributions of molecular hydrogen in the electronic ground state on the formation of negative hydrogen ions in the volume of a magnetically confined low temperature plasma. The densities of rovibronically excited molecules are determined by laser-induced fluorescence spectroscopy and optical emission spectroscopy. Energy distributions of electrons are measured with a Langmuir probe and densities of negative ions are found by laser-induced photodetachment. A global model is presented that describes the stationary state of the molecular plasma. An overall view of the discharge is obtained on the basis of this model. This allows us to cross-check the obtained results for coherence. It also allows us to assess the relevance of specific processes and their dependences on the rovibronic molecular states and the discharge current. The influence of associative recombination of a hydrogen atom of the plasma bulk with a hydrogen atom adsorbed at the wall is discussed with respect to the influence of this process on the densities of other plasma particles, in particular the density of the negative ions.

Effects of foreign gases on H− formation in a magnetic multipole hydrogen plasma source

The effects of admixtures of argon and xenon and of nitrogen (for the purpose of comparison between atomic and molecular additives) to a given H2 base pressure are investigated with respect to the vibrational populations of hydrogen molecules in the electronic ground state, to the density of negative ions and to the electron energy distribution function (EEDF). This work aims to unravel the influence of the vibrational population distribution and the EEDF on the formation of negative hydrogen ions in the volume of a magnetic multipole plasma source. The admixtures of these foreign gases lead to a measurable state-specific decrease in the population of the high vibrational states of the H2 molecule. Higher states exhibit a clearly stronger decrease with increasing foreign gas partial pressure. The measured density of the negative ions decreases with increasing noble gas partial pressure, despite the fact that the low-energy fraction of the measured EEDF is modified such that the efficiency of ion formation by dissociative attachment is more favourable. The various measurements are compared for the case of the H2–Ar discharge, with a global model developed for the stationary plasma state. The decrease in the density of the negative ions with increasing argon admixture can be reproduced by the model with high accuracy on the basis of measured population distributions of the vibrationally excited H2 molecules and the measured EEDF.

Contribution of wall material to the vibrational excitation and negative ion formation in hydrogen negative ion sources (invited)

The wall production contribution to the negative hydrogen ion formation in multicusp ion sources has been investigated using the photodetachment diagnostic (for determining the negative ion density and temperature), negative ion and electron extraction, and vacuum ultraviolet (VUV) emission spectroscopy. The wall material was modified either by depositing thin films from filaments made of different material or by depositing fresh material of the same filament. Thus we show that a fresh tantalum film leads to enhanced negative ion density and enhanced temperature of the hot negative ion population. The slow poisoning effect due to argon additive also indicates the presence of the wall contribution to H− formation. The study of the VUV spectra with different wall materials indicates the presence of vibrationally excited states of H2.

Electronic processes during impact of fast hydrogen atoms on a LiF( [formula omitted]) surface

We present an analysis of data obtained in coincident studies on projectile energy loss and number distributions of emitted electrons for scattering of hydrogen atoms from an atomically clean and flat LiF(0 0 1) surface. The emission of electrons, production of surface excitons and formation of negative ions is explained by a model of electron capture by projectiles at fluorine lattice sites where the formation of negative hydrogen ions is the common precursor for the electronic processes of relevance here. We describe the interactions in terms of concepts for gas phase collisions and derive probabilities for the detachment of negative ions during the collision with the insulator surface.

Formation of surface excitons and electron emission during grazing impact of hydrogen atoms on LiF( [formula omitted

Neutral hydrogen atoms with energies ranging from 300 eV to 1.5 keV are scattered from a clean and flat LiF(0 0 1) surface under grazing angles of incidence. By detection of time-of-flight spectra in coincidence with the number of emitted electrons, we study electron emission and excitation phenomena near their respective thresholds. We find in this energy regime population of surface excitons as dominant inelastic interaction channel. The data are consistently interpreted in the framework of a binary collision model where the formation of negative hydrogen ions is the common precursor of electronic excitation of the target and electron emission.

Formation of negative hydrogen ions in a Ne-H2 hollow cathode discharge

The formation of negative hydrogen ions in a conventional hollow cathode discharge has been investigated. A mixture of Ne and H2 proved to be more advantageous compared to pure hydrogen. The study has been performed by solving the electron Boltzmann equation, coupled with a system of balance equations for neon and hydrogen neutral and charged particles. The vibrational distribution function of hydrogen has been calculated. Our calculations show unusually high population of vibrationally excited hydrogen molecules in a Ne–H2 mixture, which explains the high density of negative hydrogen ions under optimal conditions (total gas pressure of few Torr, hydrogen number mole fraction of 1–10% and discharge current of 10–100 mA). Line intensities originating from highly excited neon states vs. hydrogen pressure have been calculated and a comparison with existing experimental results has been made.

Magnetic Susceptibility of Powders of Hydrogen Intercalates of Niobium Diselenide

We have studied the structurally sensitive magnetic susceptibility of intercalates HxNbSe2 (0 ≤ x ≤ 5.2) in the temperature range 4.2 to 300 K (the compound is obtained by intercalation by hydrogen from the gas phase in niobium diselenide powders). We have shown that intercalation leads to a reverse structural transition of the type 2H-TaS2 → 2H-MoS2 at x = 0.92, which is accompanied by an abrupt decrease (by more than two orders of magnitude) in the Pauli paramagnetism all the way to the diamagnetic state. Such behavior of the magnetic susceptibility is explained by formation of negative hydrogen ions with a large ionic radius in the interval 0 < x < 0.9, and a decrease in the charge carrier concentration and the density of electron states on the Fermi surface with an increase in hydrogen amount intercalated into quasi-two-dimensional layers.

Negative Hydrogen Ions Produced by Electron Temperature Control in an RF Plasma

The formation of negative hydrogen ions is investigated in a pure hydrogen RF plasma using a grid-method for electron temperature control. Using the grid method we produce both high and low electron temperature plasmas in the separated regions in the chamber, in which the electron temperature in the downstream region is controlled by the grid potential. The production rate of negative ions depends strongly on the grid potential. We observe efficient negative hydrogen ion production in the downstream region when the grid is negatively biased in the hydrogen pressure range of 2–5 mTorr.

Formation of H − on thin aluminum films

We report on a theoretical study on the formation of negative hydrogen ions in the scattering from a thin aluminum film. In contrast to a semi-infinite metal electronic states of the film show pronounced effects of quantization along the direction of the surface normal. We predict that this quantization has specific effects on the formation of H −-ions in the scattering from thin films.

Study of H − formation in grazing surface collisions

By combining the transfer Hamiltonian (TH) formalism with a rate-equation approach we have studied the final formation of negative hydrogen ions in grazing scattering of fast protons at the surface of a free-electron metal. In particular, the influence of the image potential on the dynamics of H − formation has been investigated. Good overall agreement of our model calculations with recent experimental data for an Al(111) surface is found. Furthermore, we have derived a simple analytical relation of the H − fraction in the scattered beam with the particle velocity parallel to the surface, the surface work function and the effective binding energy of the H − ion, respectively. This formula may be used as a tool to analyze experimental data. In particular, it allows a reliable estimate of the effective atom-surface distance of final H − formation along the outgoing path of the projectile in grazing ion-surface scattering experiments. The sensitivity of resonant transfer rates and H − formation probabilities, respectively, on the shape of the surface-potential barrier is also studied.

Theory of H− formation in a weakly collisional plasma

A kinetic plasma model for the formation of negative hydrogen ions is developed, with particular emphasis on conditions prevailing in the NIBLI experiment [Nucl. Instrum. Methods A 243, 255 (1986)]. The overall picture of the plasma in the discharge and values of various parameters have been taken from experimental data, notably a series of electrostatic probe measurements. Physically reasonable electron and ion distributions have been assumed. The magnetic field has a mirror inhomogeneity, and analysis of the quasineutral steady states of isotropic ions with electrons having high perpendicular energy, due to gyroresonance, shows that the plasma is divided into two compartments, one with energetic electrons where vibrationally excited hydrogen molecules can be produced, the other with low-energy electrons where negative ions can be produced and maintained. The mutual interaction between the electric field necessary for quasineutrality and the H− formation from electrons and vibrationally excited hydrogen molecules has been studied. The position is determined as a function of the potential by solving an integrodifferential equation. Quasineutral solutions exist only for a limited range of position and potential, which may be related to transitions between subsonic and supersonic motion. Substantial produced fluxes of 10–100 mA/cm2 of H− are found. The influence of various factors on the flux, the magnitude of the potential, and the size of the structure is discussed, as well as a possible autocatalytic effect from the reaction-induced electric field, and the extraction of the negative ions without simultaneous depletion of the plasma in which they are formed.

Negative ion formation at a barium surface exposed to an intense positive-hydrogen ion beam

A fundamental study of the formation of negative hydrogen ions via surface conversion is presented. Employed is a novel type of converter, namely a pure barium metal surface. In spite of the high work function of barium compared to more conventional cesiated converters, considerable yields of negative ions were produced. Conversion efficiencies up of 4% are obtained, which is of the same order as for cesiated converters. The high negative-ion yield is probably related to the electron density of barium, which is almost twice that of cesiuim. This is confirmed by model calculations and by UHV scattering experiments under well-defined conditions. Furthermore, calculations showed that the hydrogen coverage of the converter increases with increasing flux of positive hydrogen ions to the surface. This behavior is confirmed experimentally. Seeding the hydrogen plasma with argon has no significant effect on the conversion efficiency. This is believed to be related to the competition between the lowering of the surface hydrogen coverage and the increase of the hydrogen desorption rate, both due to the higher sputter coefficient of argon compared to hydrogen.

The role of cesium-ion bombardment in the formation of negative hydrogen ions on a converter surface

A fundamental study of the formation of negative hydrogen ions via surface conversion is presented. Employed is a novel type of converter, namely a porous tungsten button with liquid cesium flowing through it towards the side which is in contact with the plasma. A high cesium coverage, i.e., a small work function, can easily be maintained with this approach. This is related to the high flux of neutral cesium atoms to which the converter is exposed and to the small cesium density in the discharge. Despite the small work function, we obtain negative-ion yields which are an order of magnitude smaller than is usually found in more conventional experiments, in which the converter is cesiated via injection of cesium vapor into the discharge. Furthermore, our energy distributions show that no negative ions are formed via desorption by cesium-ion impact. This gives a strong indication that the extracted negative hydrogen ions are primarily formed via this process in cesium seeded discharges. Our view is confirmed by the observation that the negative-ion yield increases with an order of magnitude when a small amount of argon gas is injected into the discharge.

Detection of low-energy hydrogen atoms from a tokamak plasma by means of H-formation on tungsten surfaces

Formation of negative hydrogen ions by scattering of protons from a cesiated tungsten (110) surface has been studied in the incident energy range from 15 to 500 eV. From the results a new type of diagnostic for studying the plasma boundry is proposed.