Destruction Of Negative Ions Research Articles

Detection of negative ions in streamer discharge in air by transient cavity ringdown spectroscopy

Atmospheric-pressure discharges generated in air are expected to be electronegative, but experiments that examine negative ion densities are limited to date. In this work, we measured the temporal variation of the negative ion density in a streamer discharge generated in air. We adopted cavity ringdown spectroscopy, where negative ions were detected via weak optical absorption caused by laser photodetachment. The temporal variation of the absolute negative ion density was deduced by the transient analysis of the ringdown curve. Negative ions were detected after the disappearance of the discharge voltage and current. The negative ion density started the increase at 0.4 µs after the initiation of the discharge. The increase means the enhancement of the electron attachment frequency in the late phase of the secondary streamer with electron cooling. The survival of electrons until 0.4 µs is understood by the steep decrease in the cross section of dissociative recombination with the electron energy. The maximum negative ion density was observed at 1 µs, and it was around the noise level at 1.5 µs. The rapid decay is consistent with the destruction of negative ions by mutual neutralization with positive ions.

Influence of high energy electrons on negative ion density in a hot cathode discharge

This study investigates the influence of high energy electrons on the negative ion density along with the effect of filament current, discharge voltage, pressure, and magnetic filter field strength by carrying out measurements of negative ion density in a hot cathode discharge system in the double plasma device using a Langmuir probe technique. In the volume production technique, negative ions are formed by the dissociative attachment of low energy electrons to metastable hydrogen molecules and among the negative ion loss mechanisms; the presence of high energy electrons in the plasma is one of the important factors. Although the magnetic filter could reduce the temperature of the electrons in the target region considerably, high energy electrons were still present in the diffusion region, which could reduce the negative ion density. Negative hydrogen ion density measured by using Langmuir probe in the target region is also compared with the particle balance model considering the set of reactions involved in the production and destruction of negative ions, and a reasonably good match between them has been observed.

Negative-ion formation by H0/H– projectiles grazing scattering from the MgO(100) surface

A simple theoretical model used to treat the formation of negative ions during ${\mathrm{H}}^{--}/{\mathrm{H}}^{0}$ grazing scattering from the MgO(100) surface is presented. It is revealed that the resonant transfer from projectiles to the conduction band and the detachment of tunneling Coulomb repulsive barrier to the vacuum of the affinity electron of ${\mathrm{H}}^{--}$ ions both play an important role in the electron-loss processes. The negative-ion destruction is dominated by a nearly resonant electron loss in the low projectile energy region of $E\ensuremath{\le}0.7\phantom{\rule{0.16em}{0ex}}\mathrm{keV}$ (velocities $v\ensuremath{\le}0.17\phantom{\rule{0.16em}{0ex}}\mathrm{a}.\phantom{\rule{0.16em}{0ex}}\mathrm{u}.$), whereas the detachment by Coulomb repulsive barrier tunneling during the interaction between projectiles and surface anion sites becomes efficient in the high-energy region of $E\ensuremath{\ge}1.2\phantom{\rule{0.16em}{0ex}}\mathrm{keV}$ ($v\ensuremath{\ge}0.22\phantom{\rule{0.16em}{0ex}}\mathrm{a}.\phantom{\rule{0.16em}{0ex}}\mathrm{u}$). Combined with the valence-band electron capture, our present calculation results of the negative ion yields are in good agreement with the available experimental data in the whole velocity region.

Production of hydrogen negative ions in an ECR volume source: balance between vibrational excitation and ionization

The operation of an ECR-driven (2.45 GHz) hydrogen negative ion source is studied. Electron densities and temperatures are investigated with electrostatic probes and negative ion densities are measured with laser photodetachment. Vacuum ultraviolet irradiance measurements are focused on molecular transitions to the ground state while high-resolution visible emission spectroscopy is used to study the transitions between excited states for both molecules and atoms. The standalone operation of the source is found to be more efficient in higher pressures (12 mTorr) where negative ion densities are as high as 4 × 109 cm−3. Further investigation on the operation of the source reveals a rich vibrational spectrum. On the other hand, a limitation on the production of negative ions which is attributed to a lack of low-energy electrons becomes apparent. The underlying mechanisms that lead to this behavior are discussed along with possible solutions to this issue. Finally, the rates of different negative ion destruction processes are estimated and compared.

Experimental and simulation study of a capacitively coupled oxygen discharge driven by tailored voltage waveforms

We report experimental and particle-based kinetic simulation studies of low-pressure capacitively coupled oxygen plasmas driven by tailored voltage waveforms that consist of up to four harmonics of base frequency 13.56 MHz. Experimentally determined values of DC self-bias and electrical power deposition, as well as flux density and flux-energy distribution of the positive ions at the grounded electrode are compared with simulation data for a wide range of operating conditions. Very good agreement is found for self-bias and flux-energy distribution of the positive ions at the electrodes, while a fair agreement is reached for discharge power and ion flux data. The simulated spatial and temporal behaviour of the electric field, electron density, electron power absorption, ionization rate and mean electron energy shows a transition between sheath expansion heating and drift-ambipolar discharge modes, induced by changing either the number of harmonics comprising the excitation waveform or the gas pressure. The simulations indicate that under our experimental conditions the plasma operates at high electronegativity, and also reveal the crucial role of singlet metastable molecules in establishing discharge behavior via the fast destruction of negative ions within the bulk plasma.

Extracted H− ion current enhancement due to caesium seeding at different plasma grid bias

This paper discusses the possibility that there is an enhancement of volume production due to gettering of atomic hydrogen by the caesium deposited on the walls, which reduces the negative ion destruction by associative and non-associative detachment. Such an enhancement would take place for all plasma grid bias voltages. Furthermore at plasma grid bias voltages higher than the plasma potential there is flow of the negative ions from the bulk plasma.

Magnetic filter field for ELISE––Concepts and design

Negative ion neutral beam injection heating systems as planned for ITER need efficient precautions in the plasma source to minimize the co-extraction of electrons and destruction of negative ions. One solution is to apply a magnetic filter field of several mT, which reduces the electron temperature and the amount of electrons in the extraction region in front of the plasma grid. For the small IPP prototype sources it has been found, that both, the absolute value of the magnetic flux density in the extraction region as well as its integral along the distance from plasma driver to plasma grid has an important influence on the performance of the source. In the ITER ion sources, a strong current of several kA driven through the plasma grid is used to create the transversal magnetic field. The test bed ELISE (Extraction from a Large Ion Source Experiment) at IPP Garching houses the first negative ion source with the full width of the ITER source, with a similar aperture arrangement of the extraction system and with a magnetic filter field formed by a plasma grid current. One issue of the research at this test facility will be to explore and optimize the magnetic filter field. The paper summarizes experiences and results of previous filter field test campaigns and presents the magnetic filter field design for ELISE.

Physics of a magnetic filter for negative ion sources. I. Collisional transport across the filter in an ideal, 1D filter

Magnetic filters are used in negative ion sources for neutral beam injection in fusion devices to reduce the electron temperature in the extraction region in order to limit negative ion destruction by fast electrons. The drop in electron temperature through the filter is due to the enhanced residence time and collisional energy losses of electrons trapped in the magnetic field. The mechanisms controlling particle and energy transport through the magnetic filter in negative ion sources of the ITER type are still not clear and the aim of this paper is to clarify and quantify these mechanisms. A particle-in-cell Monte Carlo simulation is used to revisit and analyze the role and operation of the magnetic filter in an “ideal” one-dimensional configuration and to study the stability of the one-dimensional solution in a two-dimensional configuration with periodic boundary conditions. The roles of collisions and instabilities on electron transport through the filter are discussed. The influence of a more realistic geometry on electron transport through the filter is analyzed in the companion paper [Boeuf et al., Phys. Plasmas 19, 113510 (2012)].

The destruction of negative ions in slow collisions with atoms: Taking into account the quantum-mechanical character of nuclear motions

The collisional destruction of negative ions close to the reaction threshold is considered using the delta-function model. The dynamics of an electron and atoms is explicitly included in the Schrodinger equation. The use of an integral transformation allowed the problem of solving the partial differential equation to be reduced to solving an ordinary second-order differential equation. The forms of the spectrum of free electrons were found to be similar for two possible arrangements of quasi-molecule terms. A generalization of the suggested approach was used in the problem of the interaction of one tilted term with a group of parallel tilted terms in a quantum description of the motion of atoms. A simple equation was obtained which led to a correct electron distribution exponential function.

Detachment instability of self-sustained volume discharge in active media of non-chain HF(DF) lasers

The development of detachment instability in active media of electric-discharge non-chain HF(DF) lasers due to the electron-impact detachment of electrons from negative ions is considered. This instability is initiated in large volumes of SF6-based gas mixtures, spatially separated from electrodes and heated by a pulsed CO2 laser. The self-organisation of self-sustained volume discharge upon laser heating, which results in the formation of quasi-periodic plasma structures within the discharge gap, is experimentally investigated. The evolution of these structures, depending on the gas temperature and specific deposition of electric energy, is analysed. The possible relationship between the self-organisation and detachment instability is discussed. A mechanism of development of single plasma channels in the working media of HF(DF) lasers, based on electron-impact destruction of negative ions is proposed.

Global models of electronegative discharges: critical evaluation and practical recommendations

A first step towards understanding a complex plasma is usually to develop a zero-dimensional or global model. This is difficult when the plasma is electronegative, because the literature contains many proposed models with different and sometimes contradictory detailed assumptions, and different domains of applicability. The appropriateness of such models in a given context is often hard to assess. In this paper, we present a set of detailed kinetic simulations spanning a wide of range of parameters, especially with respect to electronegativity, collisionality and dominant negative ion destruction mechanism. We use these simulations as a benchmark to investigate the validity of the fundamental global model assumptions when used to model electronegative discharges. We reach two important conclusions: (1) that an accurate electron kinetics model is more important than detailed considerations relating to plasma dynamics in the presence of negative ions and (2) that there exists a simple and robust transport model that is in reasonable agreement with all of our benchmark simulations, when the electrons are treated properly.

R&D on a high energy accelerator and a large negative ion source for ITER

R&D pertaining to a 1 MeV accelerator and a large negative ion source has been carried out at the Japan Atomic Energy Research Institute for the ITER neutral beam system. The R&D is, at present, progressing towards: (1) 1 MeV acceleration of H− ion beams at the ITER relevant current density of 200 A m−2 and (2) improvement of uniform negative ion production over wide extraction area in large negative ion sources. Recently, H− ion beams of 1 MeV and 140 mA level have been generated with a substantial beam current density (100 A m−2). In the uniformity study, it was clarified that the electron temperature in the ion extraction region was locally high (>1 eV), which resulted in the destruction of negative ions at a high reaction rate. Interception of fast electrons leaking through a transverse magnetic field called ‘magnetic filter’ has been found to be effective in lowering the local electron temperature, followed by an improvement of negative ion beam profile.

Negative ion destruction by O(3P) atoms and O2(a 1Δg) molecules in an oxygen plasma

Laser photodetachment was used to investigate the dynamics of negative ion density in the positive column of a pure oxygen dc glow discharge over a range of pressure (0.1–5 Torr) and current density (2–40 mA cm−2). Upon discharge current modulation, the negative ion O− concentration decayed with the characteristic loss times of oxygen O(3P) atom and metastable oxygen O2(a 1Δg) molecule concentrations over a wide range of discharge parameters. To determine the rate constants of negative ion loss by reaction with these species, the dynamics of O(3P) atoms and O2(a 1Δg) molecules was investigated using time-resolved actinometry and IR spectroscopy at 1.27 µm, respectively. At pressures greater than ∼0.5 Torr the attachment–detachment dominated regime of the oxygen discharge was realized and the main negative ion was O−. Under these conditions, electron attachment to O2 molecules to produce O− was compensated by detachment of O− with O(3P) and O2(a 1Δg). The rate constants of O− detachment with O(3P) atoms ) and singlet O2(a 1Δg) molecules ), were determined in the plasma itself for the first time.

NBI:核融合炉に向けた物理•工学の進展 5.核融合炉用負イオン源•加速器の開発の現状

This article reviews R&D progress of a MeV accelerator and a large negative ion source at the Japan Atomic Energy Research Institute (JAERI). The JAERI MeV accelerator at present aims at acceleration of high current density (200 A/m2) H- ion beams up to MeV range energy. According to the progress of vacuum insulation technology, the MeV accelerator sustained 1 MV for more than two hours stably, followed by beam acceleration experiments of high current density. So far, H- ions beams of 146 A/m2, 836 keV have been obtained, which is the world's first demonstration of “ITER class” high power density beams. Recent investigation of negative ion uniformity in a large negative ion source revealed fast electron leakage through a magnetic filter, followed by destruction of negative ions in the local extraction region during pure volume (without Cs) operation. On the other hand, the leaked fast electrons enhance surface production of negative ions locally under Cs seeded operation. This implies possible high current density and uniform negative ion production by a high-density plasma in the magnetic configuration like positive ion sources, to be coupled with RF driven plasma production for an NB system of a fusion DEMO reactor.

Compact surface plasma sources for heavy negative ion production

Features of a compact surface plasma source designed for heavy negative ion beam production are considered. Discharges in crossed E×B fields with hollow cathodes were used for efficient plasma generation. Production of heavy negative ion beams by plasma interaction with the surface of spherical focusing emitters made of different materials, such as copper and lanthanum hexaboride, with cesium catalysis are presented. The dynamics of emitter activation is discussed. dc ion beams with current up 0.9 mA have been extracted from the compact surface plasma sources with a 12-mm-diam LaB6 emitter and a emission aperture of 3 mm diam. Minimizing plasma and gas thickness between emitter and emission aperture is essential for minimizing negative ion destruction and maximizing high intensity beam production. A small mass analyzer and secondary ion mass spectroscopy were used to quantify boron ion yield. Presence of atomic and molecular boron negative ions is confirmed. The estimated emittance of the 0.5 mA heavy negative ion beam is ε=13 π mm mrad(MeV)1/2.

Isotope effect and cesium dependence of negative ion production in volume H− and D− ion sources

Isotope effect of H− and D− productions with and without cesium vapor injection is studied theoretically as a function of plasma parameters. Model calculation in a tandem volume source is performed by solving a set of particle balance equations for steady-state hydrogen and deuterium discharge plasmas. In a pure volume case, as a whole, H− production is higher than D− production for various plasma conditions. This isotope effect is caused by mainly atomic collision processes, i.e., collisional cooling of vibrational molecules and collisional destruction of negative ions. Therefore, density of atoms plays an important role for deciding negative ion density. On the other hand, in a cesium case, there is no remarkable isotope effect on H−/D− production. Considering H−/D− surface production caused by both atoms and positive ions, H−/D− production is enhanced by a large factor and then isotope effect observed in pure volume case seems to be almost masked.

The role of negative ions in high-frequency discharges

The paper discusses the onset and development of high-frequency discharges under homogeneous field conditions when electron oscillation peak-to-peak amplitude is smaller than the length of the discharge gap, in the frequency range 106-108 Hz. According to the well-known diffusion theory, self-sustained high-frequency discharge can arise under the conditions mentioned above, when the increase in number of electrons due to ionization equals or exceeds losses of electrons due to attachment and diffusion. The paper shows that some experimental data on high-frequency breakdown voltages of air at atmospheric pressure as well as those on breakdown formative time are contradictory to the above theory, and that destruction of negative ions must be taken into account to overcome the contradictions. It is shown that attachment-detachment processes together lead to an extra diffusion of electrons, with rate up to several orders of magnitude greater than that of the normal diffusion.

The role of the perturbed plasma potential in the dynamic plasma response to photodetachment

The destruction of negative ions by laser photodetachment suddenly modifies the electron and negative ion densities in a localized region. The subsequent recovery of negative ion density n−(t) may be measured with a second photodetaching laser pulse. These measurements, compared with a ballistic model for n−(t), are the basis of a negative ion temperature diagnostic, which is being developed and employed in H− and D− sources. In this paper the accompanying plasma potential perturbation and its effect on the negative ion response is discussed. The perturbation can introduce an error in negative ion temperature measurements. Late in time, the negative ion density is observed to recover more slowly than indicated by the ballistic theory. An intriguing mechanism is proposed for the late slow recovery of negative ion density, based on a small lingering potential perturbation and a hole in negative ion velocity space. These different theoretical approaches are illustrated with a detailed analysis of an example case from a hydrogen discharge.

Electric dissociation of negative ions

As an alternative to naturally occurring negative ion discrimination, we consider the possibility of electric dissociation to discriminate between isobars, by destroying the weaker negative ion of a pair. The theoretical basis of negative ion dissociation using high electric gradients and an appropriate experimental setup are discussed. The possibility of negative ion destruction by electric dissociation during the first stage of tandem acceleration and at the ion source is also considered. In consequence, it is possible to place a limit on the electron affinity of some elements considering their survival in the AMS system. Mn −, Ca − and C −∗ among others are considered.

The properties of high-intensity heavy-negative-ion source, NIABNIS

The production and destruction of negative ions, which are very important factors for designing high-intensity heavy-negative-ion sources, have been experimentally investigated. For the optimum production of negative ions by sputtering, the control of both the cesium supply and the target temperature is essential. The production probability of carbon negative ions was obtained to be 18.3% at the maximum of sputtered particles. Single- and double-electron detachment cross sections are found in the order of 10−15 and 10−16 cm2 , respectively, in a velocity range of 105 m/s. The heavy-negative-ion source NIABNIS, developed by the authors, can deliver continuous high negative-ion currents of 735 μA for C−, 200 μA for C−2 , 320 μA for Cu−, and 170 μA for Si−.